Welcome to IMACS, the Inamori-Magellan Areal Camera & Spectrograph.  IMACS offers a wide range of imaging and spectroscopic options, and as such lends itself to both standard and novel observational programs.  In respect to large number of modes in which IMACS can be used, it is unlike any imaging spectrograph, including GMOS, DEIMOS, and VIMOS, which are limited to a a few spectral and imaging modes -- often only one each. As a result of the versatility of IMACS, this User Manual is itself wide ranging and contains much more information than many observers will need to know. At the same time, it will not cover important details and methods that will be appropriate to more custom observational programs, including the specific "cookbooks" that available to lead an observer through important tasks like object aquisition and mutlislit mask alignment.  However,  If a specific mode of operation is not discussed here or in the cookbooks, observers should not assume that the IMACS software, or the interface software for the Baade telescope, can be readily modified to accommodate the desired observations.  In such cases, observers should query the IMACS Team and/or the Magellan Support Staff before showing up at the telescope.  Many non-standard requests will require weeks or even months of advance notice, and, because the Magellan operation is a lean one, some things that IMACS could do will not be supported by software and/or the Magellan Support Staff. Again. if you read about it here, it can be done without special requests of the Team or the Staff.  Requests for clarification of the standard modes are encouraged -- after a careful reading of the available documentation. 

The User Manual is divided into four parts -- use the collapsible "outline" menu at right to navigate.  The first describes IMACS, in brief and then in detail, the second covers capabilities, characteristics (including performance), and considerations that are relevant to planning programs, the third is about preparing for IMACS observations, and the fourth is a brief tutorial in making common observations.

As the IMACS PI, I have added some recommendations in this new version of the User Manual, denoted by my initials.  These are my opinions, of course, and others may see things differently.

     - Alan Dressler

A Description of IMACS

A Thumbnail Sketch

The Inamori-Magellan Areal Camera and Spectrograph, IMACS, was built at Carnegie Observatories to provide a wide-field imaging and spectroscopic capability for the Magellan Baade Telescope.  A principle design goal for IMACS was to provide a versatile spectrograph that would support many types of scientific investigations with minimal compromise in the quality and sensitivity of any of these diverse modes. Pictures of the development work can be seen at the IMACS project website http://www.ociw.edu/instrumentation/imacs.

IMACS offers a choice of two cameras -- f/2 and f/4 -- each with two different scales and spectral resolutions covering the range 500 < R < 20,000.  As of March 2008 there are now two 8K x 8K CCD mosaic cameras, one at each foci. In addition to wide-field imaging and a range of low- to medium-resolution spectroscopic modes, IMACS has a 2 x 1000 fiber-fed integral field unit (IFU) built by Durham University, a mutli-object echelle mode (MOE), and a full-field tunable filter (MMTF).  An image-slicing reformator for dense mutlislit coverage over a 4 x 4 arcminute field (GISMO) is available as of 2008B.

IMACS: a more detailed description

IMACS is a reimaging spectrograph mounted permanently on the Magellan Baade Telescope. It is fed by the telescope's 6.5m primary, f/11 Gregorian secondary, and flat tertiary mirrors, through an ADC/corrector.  This optical train delivers an unvignetted field out to R = 12 arcmin with 9% vignetting at R = 15 arcmin. The all-transmitting (all-spherical-optics) collimator in IMACS contributes very little in the way of additional aberrations (FWHM < 0.10 arcsec). The Gregorian secondary was specifically designed to feed the IMACS collimator to produce this excellent performance --- in effect, the Gregorian secondary is the first optical element in the IMACS collimator.

The rotating tertiary mirror in the telescope directs the beam to IMACS on the west Nasmyth platform. (It is possible to switch between IMACS and the instrument at the east Nasmyth platform  --- presently PANIC, in the future FourStar --- or to an auxiliary folded port, e.g. that with MagIC. It takes 10-15 minutes to make one of these changes.) Following the tertiary mirror, but still mounted on its rotator in the center of the telescope, is an integral ADC + corrector, which was installed in May 2004.  (Prior to that time, IMACS was used without a corrector, which means that data from the first 10 months of operation showed significant aberrations outside of a field radius of 5 arcminutes, and also non-negligible spherical aberration on-axis.)

To calculate the spectral resolution in angstroms of a given observation, divide the slit width you intend to use by 0.11 arcsec and multiply by the dispersion for the chosen grating.

The f/2 camera in IMACS -- imaging and spectroscopy

The all-transmitting, double-asphere, glass-and-oil-lens f/2.5 "short" camera (known as f/2) works with the collimator "straight through" (no reflections) for both imaging and spectroscopy.  The f/2 focus delivers an image 27.4' diameter field at a (central) scale of 0.200 arcsec per pixel. The field is vignetted in the corners (by the tertiary and its mounting assembly), changing from 0% at R = 12' to ~10% at R = 15'. Beyond this, baffling inside the camera cuts light off completely --- the corners of the square field of the CCD mosaic camera are missing --- which is a consideration for 'tiling' a field. This vignetted area is also subject to "shadowing" by the two IMACS outboard guiders, which move over 40 degree arcs over the top and bottom of the f/2 field.  The Epps design provides for wavelength coverage of 3900 Å < Lambda < 10500 Å without  refocus. The Maryland-Magellan Tunable Filter (MMTF) is a transmissive device and therefore only used with the f/2 camera.

Up until June 2006 the f/2 camera field suffered from a tilted field that limited image quality to 0.45 arcsec in the field center and 0.60 arcsec at the field edges.  The Mosaic1 CCD camera was tilted to compensate this; following this correction, a signficant amount of coma and astigmatism was also found, in excess of the design.  As of March 2008, these have also been substantially reduced, by translation and piston of the last element in the camera. The combination of camera tilt --- now the Mosaic2 camera --- is producing excellent images over most of the f/2 field, with a small degradation  of elliptical (but still very tight) images at some parts of the field's edge.  In the best seeing (FWHM = 0.25") the f/2 camera will now produce images of 0.40 arcsec (2 pixels) or better over its entire field.  In more common but still excellent seeing of FWHM = 0.50 it will deliver 0.55" or better images over most of the field, with a few patches of 0.60" at the edge.  Median seeing of FWHM = 0.65" will be minimally degraded by the camera, consistent with original design specification.

At IMACS commissioning (10/2003) through 2006 necessary baffling in the telescope for the f/2 field was not in place.  This has been corrected and, although baffling of the f/2 field may not be not perfect, it appears now to be adequate for general imaging programs, including accurate photometry --- flat fielding is effective to the few percent or better level.

Spectroscopy with the f/2 camera is done by grisms, a transmission grating replicated on a prism in order to pass the 1st order at zero deviation. Three 150 x 200 mm grisms are available for f/2 spectroscopy,  and one additional moderate-dispersion-red grism will be available in mid-2008. The grisms produce spectral resolutions of 7-10 Å FWHM for a slit width of 1.0 arcsec, corresponding to spectral resolutions of R=500-1200 at the blaze wavelength. In spectroscopic mode the full-area of the CCD mosaic is illuminated; that is, after accounting for the loss due to vignetting at the position of the object, the spectrum is uniformly diminished by that amount from the center out to the corners. 

Either camera, f/2 or f/4, can be used for single object spectroscopy using the slit-viewing feature of the centerfield guider, for multislit spectroscopy using laser-cut masks, and with the Durham-IMACS Integral Field Unit (IFU). These modes are described in more detail below.  It is not necessary to use a slit mask when observing a single object over an area of 10 arcsec or less! This provides a particularly efficient way of making flux calibrations with standard stars. (Note, however, as is explained below, if the f/2 camera is used in with grisms rotated to the N&S orientation, the slits will not be perpendicular to the dispersion and therefore cannot be used for spectroscopy.)

Observers with a program of f/4 single-slit spectroscopy have noted that once they position an object or objects along the slit, the image seems to shift from this position once a spectrum is taken. That is, the pixel positions in the area move to the right or left from where they were in the direct image.  (The position of objects relative to the slit itself does not change.) This is an issue of the precise alignment of the mirror used for imaging and that of each spectral grating  --- technically, the roll angle of these elements.  A misalignment of only 0.02 deg of the grating produces a shift of about 10 pixels with respect to imaging.  Anything larger that than 10 pixels indicates that the grating has moved in its cell with respect to roll and must be re-aligned.  Although shifts of up to 100 pixels have been reported, even such large shift does not effect the quality of the data -- the angle from normal is still very small.  However, a sizeable shift is a problem for setup, for example, making sure that objects do not disappear in a CCD chip gap.

The IMACS Mosaic CCD Cameras

The IMACS Mosaic1 detector is an 8192 x 8192 Mosaic CCD camera at f/4 which uses eight thinned 2K x 4K x 15-micron SITe detectors with broad wavelength sensitivity (http://www.noao.edu/kpno/mosaic/chips.html -- note: these are room temperate QE values). Mosaic2 also has 2K x 4K x 15-micron CCDs, manufactured by E2V, and is placed at f/2.  The configuration of Mosaic1 and Mosaic2 is the same.   With two available CCD cameras, the observer is free to switch between foci choose during observing --- the change is simple and rapid. The observational consequences of this are discussed below. For Mosaic1 the chips are parallel within ~5 pixels, with of gaps of about 0.93 mm = 12.4" (62 pixels); the gaps are about the same between both the short axes and long axes of the CCDs.  The Mosaic2 camera CCDs are aligned to within 1-2 pixels. The gap between the short sides of the CCDs are 0.86 mm = 11.4 arcsec (57 pixels).  The gaps between the longer sides are larger, about 1.38 mm = 18.4 arcsec (92 pixels).

More information about the Mosaic1 CCD camera can be found at http://www.ociw.edu/instrumentation/ccd/imacs.html.

More information about the Mosaic2 CCD camera can be found here.

The IMACS Guiders

IMACS has one insertable "centerfield guider" and two outboard guiders -- Shack-Hartmann (SHG) and Principal (PG) -- which are at the edge of the f/2 science field, but well outside the f/4 field.  The two outboard guiders have field diameters of 105" and they sweep in ~40-deg arcs (at a field radius of 30 arcmin), in total covering about 30 sq armin of sky each. 

The Centerfield Guider (CFG) can be inserted, by either the observer or the Telescope Operator (TO), to a fixed position at the center of the IMACS field (on the optical axis of the telescope). There is also a second off-axis position which allows the centerfield guider to be used for slit-viewing spectroscopy, described below.

Capabilities, Characteristics, and Considerations: Planning IMACS Observations

Imaging with IMACS

The choice of cameras

With two Mosaic CCD cameras, both the f/2 and f/4 focal positions are available to the observer at any time.  There are many considerations that fold into the choice of f/2 or f/4, but field (and spectral resolution) are the two major drivers. The f/4 camera has a field of 236 sq arcmin (0.066 sq deg) and uniformly excellent image quality (down to 0.30"-0.35" FWHM in the best seeing). Its fine sampling takes advantage of the best seeing for both imaging and spectroscopy. The fully-illuminated, unvignetted, square field is well-suited for tiling an area of sky. (Note: because of the gaps between the chips, the "active area" is actually 230 sq arcmin).

The f/2 camera accesses the largest possible field, 670 (656 "active") sq arcmin (0.186 sq deg), although the fact that the corners are not illuminated makes it less suitable for tiling an area of sky. Furthermore, at some positions the Principal and SH guider encroach on the science field and shadow a 1-2 arcminutes arc top and/or bottom. (Since their positions are not fixed with respect to camera, this will make flatfielding difficult at the camera edges, top and bottom.  The E2V CCDs are "flat" enough to recover important data, however.) The image quality of the f/2 camera is also excellent for its field size: with seeing of 0.30 FWHM the f/2 camera delivers images of 0.45 arcsec FWHM or better.  In particular, the central region R < 8 arcmin, can produce images of 0.30" FWHM in the best seeing, however, these images are undersampled by the 0.2 arcsec pixels. See summary in image below.

The choice of filters

IMACS has two identical filter servers, one for each camera. They are "jukebox style" with 15 black-anodized aluminum frames designed to hold 6.5-inch (165 mm) square filters up to 0.47-inch (12 mm) thick. The basic complement of filters includes Bessel B, V, R, & CTIO-I and a spectroscopic filter (full band -- necessary to equalize optical path length) for each camera, and one Sloan g,r,i,z set that can be used in either. Many wide-band blocking filters, and several narrow-band filters, are available as well --- there are plenty of vacant positions for these or for user-provided filters.

Pixel Binning Options and Archiving data

The readout of the full 8K x 8K mosaic camera (in 1x1 binning) generates 128 Mbytes 16-bit unsigned integers and is stored as integer FITS files. Disk storage on the two available observer workstations -- Llama and Burro -- is a Terabyte, more than sufficient for several nights of observations, but data acquisition is done only with Llama. Data can be transferred directly to the observer's laptop computer via sftp, stored on DVDs using Llama (see this), or transferred to a personal USB or FireWire disk.

Because of the fine sampling of IMACS, particularly the f/4 camera, many observations can and should be made using 2x2 rebinning. (The binning options are 1,2,3,4... in any x,y combination.)  This obviously reduces data quantity considerably. Furthermore, some applications, such as centerfield slit-viewing spectroscopy, only illuminate two of the eight CCDs, and many setup frames will be done in subraster mode where data volume is greatly reduced. Subraster setups can be created to store only the parts of the array that are illuminated in slit viewing mode.

Performance: Throughputs in Imaging Mode

Photometric Calibration -- The IMACS Shutters

Many imaging programs require absolute photometric calibration. There are many Landolt Standard Fields that will provide suitable brightness and color standards for most programs (http://www.cfht.hawaii.edu/ObsInfo/Standards/Landolt/). It is convenient to take in-focus images of these fields, however, in excellent seeing it may not be possible to avoid saturating the CCD with the brighter standard stars in the Landolt fields without some modest spreading of the image. An intentional defocus of 500 microns (the units of the focus readout) will generally produce unsaturated donut-shaped images a few arcseconds in diameter. Generally it is a good idea to keep the standard stars at 12th mag or fainter for f/2, 10th mag or fainter for f/4.

The IMACS shutters provide help in this regard -- they are capable of very short, very accurate exposures. The IMACS shutters are driven by linear motors, each of which have closed-loop feedback. Each shutter has two blades that alternatively open and close the exposure. Because the time trajectories of each blade closely match, even short exposures are guaranteed to have equal and accurate exposure length over the field. In pre-commissioning tests IMACS exposures of 1 sec were shown to be accurate to 1% over the entire field of the CCD mosaic camera, although this has yet to be confirmed at the telescope.  For good practice, exposures should be 2 seconds or longer.

Spectroscopy with IMACS

The Choice of Cameras -- f/2 or f/4

For spectroscopy, the choice between f/2 and f/4 is driven by field, spectral dispersion, and (less often) spatial resolution.  The field of the f/2 camera is approximately 2.5 times larger, and its spatial resolution is a factor of two less (in each direction).  The spectral resolution is typically a factor of 3-5 higher in f/4 compared to f/2, although there is overlap -- the 150 l/mm grating produces about the same spectral dispersion per pixel as the 300 l/mm grism, but note that to actually achieve comparable resolution requires a slit approximately a factor of two less wide in the f/4 mode. 

Programs targeting faint galaxies or stars will benefit from high multiplexing and large field and employ rather low spectral dispersions, so for these the f/2 will generally be the choice. This is now enhanced by the presence of the Mosaic2 CCD camera with its ~50% Q.E. improvement over Mosaic1 in the visible to infrared.  Often multiple tiers of spectra are used to increase the multiplexing factor to 500-1000 objects per field, or higher (the LDP has been used to observe 2000-8000 in a single exposure).

For a point source or single object extended as much as 15 arcmin, single-slit spectroscopy is a natural choice for f/4, although multislit spectroscopy of objects, particularly for brighter objects (m < 20) where higher spectral dispersion is an option, can be a powerful use of IMACS.  Unless the spectral range can be narrowed down substantially, these are likely to be single tier --- one spectra only along the dispersion direction.

Mosaic2 CCD camera is operated in  "normal" orientation (spectra along the long-axis of the CCD detectors) at the f/2 focus.  "Nod & Shuffle" observations --- spectral dispersion along the short-axis of the CCDs --- are made at f/2 by rotating the grism of choice --- this request must be specified on the Setup Form and submitted well in advance, and the orientation cannot be changed during the night.  The LDP and the 150-l grism and 300-l grism at f2 have been used in N&S exclusively, although the latter could be used effectively in normal orientation. 

The f/4 Mosaic1 CCD camera is used only in N&S orientation.  The wavelength coverage of the f/4 extends several hundred angstroms further into the ultraviolet than the f/2. The spectral dispersions for f/4 can be much higher with the f/4 camera, all the way up to the Multi-Object Echelle mode of R ~ 20,000.

Multislit spectroscopy, long-slit spectroscopy, or centerfield slit-viewing spectroscopy?

IMACS has been designed principally for wide-field multislit spectroscopy. This mode is intended for faint objects where multiplexing is a necessity, and/or for objects of sufficiently high spatial density (> 50 per sq deg) that program efficiency can be substantially increased. Multislit spectroscopy requires considerable advanced preparation, and the production cost --- $200 per IMACS slitmask -=- may be a consideration.

Long-slit spectroscopy of up to 27 arcmin can be carried out with IMACS, simply by laser cutting the appropriate width slit into one of the slitmask blanks. This needs to be done in several sections to maintain the structural integrity of the mask.  The program for generating long slits puts a small (subarsecond) bridge about every 2 arcminutes.  Observers will be charged for any first-time, cut-to-spec long slit, but a collection of such long-slit slitmasks with widths ranging from 0.7" to 1.5" is kept on hand.

Traditional slit-viewing spectroscopy, typically for single objects, is also supported by IMACS. This is accomplished by sending the IMACS Centerfield Guider to a location 2 arcmin off the optical axis.  In  Slit-View mode (as opposed to Shack-Hartmann mode of the Centerfield Guider) an arcminute-long multi-width slit, and a 7" hole, are viewed by the TV guider. (A field mask which occults the remaining IMACS field will be deployed automatically.)  In "slit-viewing" spectroscopic mode a grating or grism can be left in as the observer proceeds from object to object, without using one of the Mosaic cameras for acquisition, and without reconfiguring IMACS (except for optional grating or grism changes).  The slit-viewing mode is particularly useful for flux or radial velocity calibrations of standard stars. Details of operating in this mode are described below.

Performance: in Spectroscopic Mode

The anti-reflection coatings of the IMACS lenses are typically 1-2% per surface: through the optical train of either camera, the losses are about ~15% for the collimator and either camera, amounting to a total loss of about 30% for either optical train.  Losses from absorption in the glasses are small, less than 3%. The S filter and the otheres used for spectroscopy typically have losses of 1-2%.   The quantum efficiency of the SITe CCDs is thought to peak at about 75%, with an average of ~65% over most of the accessible wavelength range, but the sensitivity falls off rapidly beyond 7500 angstroms. The E2V CCDs have peak quantum efficiency near 90%, and they stay high out into the near-IR.  The gratings and grisms have efficiencies ranging from 60-80% peak  Throughput in spectroscopic mode for IMACS alone therefore ranges between 25-50%. Individual measured performance is tabulated below.  Including losses from the three reflective surfaces, primary, secondard, and tertiary, and ADC/Corrector - about 50-70% in total, depending on the freshness of the coatings - the spectroscopic efficiency of the Magellan+IMACS system is typically 15-25% at f/4, and 15-35% at f/2, not including atmospheric losses. 

Efficiencies with the f/2 with its 3 available grisms

Click here for data tables

Efficiencies with the f/4 with its 6 available gratings

Click here for data tables

The choice of reflection gratings and grisms

IMACS carries an assortment of 3 reflection gratings and 2 grisms for a night's observing, chosen from a present complement of 6 gratings and 3 grisms (Link). These are mounted on the disperser server, a large wheel that also carries the reflecting mirror for f/4 imaging. The IMACS Echelle (MOE) and tunable filter (MMTF) are also carried by the disperser server, when requested.

A filter mount is also available for the disperser wheel: this places a filter in the collimated beam (at what would be the position of a grism).  The fixture takes a standard IMACS 6.5-inch square filter and mounts in the "disperser server" (wheel), in place of one of the dispersing units or the mirror. The beam is the preferred location for a relatively narrow-band (< 100 Å FWHM) filter. It is also possible to mount a filter in front of a grism, however, this is by special arrangement with the Magellan staff.  The MMTF -- a six-inch aperture tunable etalon, is also inserted into the collimated beam, but its use precludes the use of additional dispersing elements. MMTF is available for general use, but programs are at present block-scheduled to facilitate setup and verify performance by the MMTF group, led by PI Sylvain Veilleux (see IMACS Accessories, below).

Observers specify their choice of these units before the beginning of observations, that is, they will not be swapped during a night's observing. Grating exchanges can usually be made without removing the grating-tilt-mechanisms from the disperser wheel, which has made it easier to support grating changes between observing nights.

Operating features of the disperser server

Within the constraints listed above, once the gratings or grisms are chosen are chosen, and the disperser server is loaded, the observer's control of these devices is rapid and efficient. Exchanging gratings or grisms takes 30-60 seconds. The tilts of the gratings are encoded: the central wavelength and wavelength intervals of each grating as a function of encoder position are easily calculated (Link). Setting a new grating tilt usually takes about 30 seconds. (All IMACS mechanical functions can be exercised simultaneously.) Grating tilts positions are repeatable to +/- 2 pixels, but this refers to the accuracy of setting the tilt once a grating tilt mechanism is inserted into the beam. Once grating tilts are established, they will remain fixed -- it is not necessary to zero the tilt in order to install or retract a grating.  The caveat to this is that the mechanisms themselves are not inserted with perfect repeatability; shifts of 5 pixels are common even in proper operation, and may be considerably larger if there is a problem with the latching of the mechanism.  If a specific pixel location is required for a particular wavelength, this can only be achieved after the grating is installed, by adjusting the tilt.

The wavelength coverage and central wavelength of a grism is fixed. Because the grisms and their mounts have no moving parts and are lighter than the grating mounts, they usually re-insert with good repeatability, typically 1-2 pixels.

Guiding is done differently with N&S.  In order to accomodate the nods from one telescope position to another, the target (a software cross) is moved back and forth on the IMACS Principal Guider image, and the telescope is moved the same amount.  The SH guider itself does not move.  The Shack-Hartmann guider is turned off "every-other-exposure" because it would need to move to keep the SH star centered in its (physical) aperture.  Long nods > 50 arcsec will not work because the star must stay on the field of the Principal Guide camera (105" diameter).

A more detailed description of nod-and-shuffle spectroscopy can be found in Glazebrook and Bland-Hawthorn (2001, PASP, 113, 197).

Recommendation: Although Nod-and-Shuffle has been shown to be extremely effective over the small field of some spectrographs, for example, the Gemini GMOS, the very wide field has made the critical alignment and precision of subsequent shifts challenging.  The PI recommends standard mode over N&S for most observing programs. An exception is Low-Dispersion-Prism observations, for which the benefits of accurate sky subtraction greatly outweigh loss of flat-fielding in the N&S method.  --- AD

To take account of the imperfect repeatability of the slit masks and mirrors, the alignment procedure includes a program Ifalign that adjusts the target positions to reflect the final position of the mask and/or mirror at the time of observation.  The full alignment procedure is discussed below.

Flexure between the guider cameras and the focal surface (slit)

A design goal of IMACS was to provide negligible flexure between the guide cameras and the focal plane.  At present, the amount of this flexure is measured to be less than 0.5 arcsec for all orientations.  This being the case, one would expect that an multislit-mask alignment, for example, would hold with < 2 pixel drift over many hours of observation, in other words, the alignment of the objects and slits would "hold." However, such  a drift is observed, intermittantly, of the order of 0.5 arcseconds that appears when IMACS is rotating relatively rapidly, specifically, passing through the meridian and especially approaching the zenith. The present understanding is that the problem arises from a failure of the "rotation guiding" in which both guide probes are used to "close the loop" of tracking with the west Nasmyth rotator. Work is presently underway to isolate and fix the problem, which appears to be in the telescope control system (TCS).  For observations away from the meridian, when IMACS is rotating slowly, there is little or no drift -- a little advanced planning can usually assure that this is the case and the meridian is avoided.

Note that the issue here is keeping objects square on the slits, or otherwise aligned.  Possible shifts between the focal plane to the Mosaic CCD camera, through flexure in the spectrograph, is another matter but, as discussed below, it is negligibly small.

The rotation guiding has no effect on broad-band imaging or spectroscopy of brighter objects, where exposure times are typically 1/2-hour or less and there is no alignment to be maintained over long periods.  Narrow-band imaging or high-dispersion spectroscopy -- for which exposures of one hour or more in order to overcome the detector read noise -- might show some elongation of images if the telescope crosses the meridian, although this has not been reported.

The most serious consequence relates to holding alignment for long-slit and multislit observations over hours of exposure.  Present experience is that, again, sequences of observations on one side of the meridian or the other can proceed for 2-3 exposures of 30-40 minutes without realignment.  Unless and until rotation guiding is made completely reliable,  it is wise to re-align every one to two hours of tracking, and probably before each exposure when following a field when the rotator angle is changing rapidly.  This would also apply to IFU observations if the observer wants to keep objects falling on exactly the same detector pixels over hours of integration.

Flexure between the focal surface (slit) and the Mosaic CCD camera

All spectrographs exhibit flexure between the focal plane and the detector, but the flexure in IMACS is exceptionally low for such a large device. For a full rotation of the spectrograph, images move 2-4 pixels for the f/4 camera and 1-2 pixel for the f/2 camera. Most of this is repeatable (elastic), so it is taken out with one of two sets of lookup tables (direct, spectroscopy) that are used to position a piezo-driven x.y stage in the Mosaic CCD Camera.  After correction, the complete range of residual flexure for a full rotation of the instrument is +/- 0.5 pixels for the f/4 camera and +/-0.25 pixels for the f/2 camera, equivalent to +/-0.05 arcsec and +/- 0.3 A (typical) for both cameras. (Most observations use only a sector of the possible rotation, so these numbers can be considerably smaller.) The bottom line is that flexure from the focal plane to the Mosaic CCD camera is remarkably small and should not be an issue for the majority of programs. For direct imaging, in particular, errors in guiding caused by the mechanical separation (load path) of the IMACS components (guiders, optics, CCD camera, etc.) should be small even compared to the best seeing.  As of April, 2008, Mosaic1 at the f/4 focus is using an open-loop flexure control, but this has not yet been implemented on Mosaic2 at the f/2 focus.  The full amplitude of the latter is approximately 3 pixels for a full rotation of the instrument.

Recent observations with the IFU, however, have shown a steady drift of about 0.2 arcsec/hr.  Of course, the IFU does not experience "slit losses," so this should be a relatively benign effect, but some observers would prefer to hold the exposures to the same pixel patterns for several hours.  The IFU is several times heavier than a slit mask (which hold position very well - see next section), so the likely explanation for this is that the IFU itself is shifting slightly as it rotates with respect to gravity.  Again, the way to minimize this is pick regions of the sky for observation where the rotator angle is moving slowly.  A new latching mechanism hould eliminate this problem.

The pieces are in place to implement closed-loop flexure control, which would reduce "focal plane-to-detector flexure" to a small fraction of a pixel.  This feature has not been implemented for lack of motivation, but could be if it were found to be essential to one or more highly-rated science programs.



The IMACS "Accessories" --- IFU, MOE, MMTF, & GISMO

The following accessories will not normally be installed in IMACS unless requested in the observer's setup form:

The Durham-IMACS IFU

The IFU displaces three slots in the slitmask cassette, so observational programs can combine both mutlislit and IFU observations. During observations the IFU is inserted like any other slitmask. At the present time, installation of the IFU is a daytime changeover. A detailed description of the IFU and its use can be found here.

The Multi-Object Echelle (MOE)

MOE is an "accessory" that transforms IMACS into a spectrograph with properties similar to ESI on Keck, however with a significant multiplexing capability.  MOE on IMACS permits crossed-dispersed echelle spectra to be obtained over the 15x15 arc minute field of the f/4 camera, accomplished by the use of a grating and cross-dispersing prism mounted on the IMACS grating wheel.

The optical layout of MOE (top) and a schematic representation of the spectral format (bottom).

MOE installs in the IMACS disperser-server in place of one of the grating tilt mechanisms. Because of the large number of detector pixels in the 8K x 8K array, IMACS can simultaneously record, with full wavelength coverage, ~10 such spectra, each chosen from anywhere within a 1' x 10' (spatial direction x dispersion direction) area of sky. By using MOE in combination with GISMO (see below), 16 objecxts --- one from each 0.5' by 2' rectangle selected from the centeral 4' of the IMACS field --- can be used to take high-resolution spectra in dense fields or for small objects.  By limiting the wavelength interval with a bandpass filter, an even greater number of objects can be included. A detailed description of the Echellette and its use can be found in Sutin & McWilliam (2003, SPIE 4841, 1357), http://ads2/astro.puc.cl/cgi-bin/nph-bib_query?bibcode=2003SPIE.4841.1357S&db_key=AST&high=3f7a6c574f22263 More documentation on the use of MOE will be forthcoming. 

For a 0.6 arcsec-wide slit MOE provides a 2.5 pixel resolving power of R=21,000 in the center of the field.  Each echelle spectrum is composed of 9 orders, from roughly 3400 < lambda < 9500 Ã….  The wavelength coverage is complete near the center of the field up to l~8200 Ã….  An estimate of the speed is that for a V=17.5 RGB star a S/N ~ 50 per extracted pixel can be obtained in about 3 hours in good seeing.

There is enough CCD "pixel space" to acquire full-coverage echelle spectra for as many as 15 objects.  In practice, however, objects are not ideally spaced, and at least 3 alignment holes are required, so one typically can obtain full coverage for only 7 or 8 objects.  (However, GISMO allows a more efficient packing for objects selected from a smaller field.) Blocking filters, employed to restrict the number of orders, substantially increase the number of objects observed per exposure. For example, a test exposure by McWilliam in  December 2004 with the 4800--7800A IMACS blocking filter was used to obtain spectra of 20 red giant stars (plus 3 alignment stars) in the Carina dwarf spheroidal galaxy.  In this case each object spectrum was composed of 4 orders.  If a single-order blocking filter were employed then about 100 objects could be observed simultaneously. Note also that, with the new f/2 MosaicCam2, it will be possible to locate alignment holes outside the f/4 field of view and use the f/2 camera to accomplish the alignment.  Alignment stars in the f/4  area covered by MOE will not be necessary, which will increase the number of feasible targets per exposure.

The grating supplied by the manufacturer for MOE turns out not have the intended blaze angle, so it has been necessary to tilt the grating and additional 4 degrees, which further necessitates a translation of the grating to fit into the tight space constraint.  As a result, vignetting loss for a central slit has increased from the designed 20% to ~40%  (a loss of efficiency equivalent to 0.3 mag).  The ~20% vignetting has moved to the lower part of the field, however these slits now lose some spectral coverage since parts of the orders fall off the detector.  Programs that push the faintness limit will fare better accepting this loss of wavelength coverage in order to recover the higher throughput, while those for whom multiplexing is the main goal will simply accept the lower throughput.  Andy McWilliam and the IMACS team are now addressing this problem to see if the planned performance can be reached.

Also, due to the change in grating tile, the wavelengths of the order centers have changed:

The maskgen software package contains options for making the special masks for MOE.  Conventional IMACS masks will result in some spectral overlap, as will slits longer than 5 arcsec.  It is therefore important to use the options specifically for MOE.  Judicious choice of the slit position angle set in maskgen can significantly improve the packing efficiency of spectra.

Reduction of MOE spectra has been successfully performed using standard IRAF routines, but this route can be laborious.  Modifications have been made to the COSMOS package that will incorporate MOE reductions in a straighforward manner.

The Marland-Magellan Tunable Filter (MMTF)

The MMTF is a narrow-band filter which is tunable in both central wavelength and transmission bandpass.  The MMTF is  based on a Fabry-Perot etalon that operates in low orders (close plate settings) to provide a large "monochromatic spot" (region of constant wavelength) across the field of view.  Its tunability, sensitivity, narrow bandpass, and wide field make this a unique capability applicable to a wide range of programs.  The MMTF Principal Investigator is Sylvain Veilleux (U. of Maryland).

The MMTF is mounted in the IMACS diperser server (see above) and is used with the f/2 camera.  The basic characteristics of the etalon are: (1) a monochromatic spot diameter ranging from 7 to 11 acrminutes (larger for wider bandwidths); (2) a bandpass tunable from 6-25 angstroms; and (3) wavelength coverage 5000 < lambda < 9200 -- the actual wavelengths depend on blocking filters.  The measured 6σ sensitivity to a emission line pont source at 6600 angstroms is 3 x 10^-17 ergs s^-1 cm^-2 for a 1 hour exposure with a 25-angstrom bandpass.  This sensitivity can be improved by fequency switching the etalon in synchronization with charge  shuffling in the  CCDs of the mosaic array.  This observing mode averages over temporal variations of the atmosphere and the instrument. 

The MMTF achieved first light in 2006 and was comissioned during the 2007A observing semester.  Observers can now propose to use the MMTF: for some time to come, observing runs with MMTF will be block-scheduled and coordinated with observering by Sylvain Veilleux and the MMTF team.  There is an MMTF website: http://www.astro.umd.edu/~veilleux/mmtf/. This site is still under construction but will be relatively complete in advance of the deadline for 2008A observing proposals.

Gladders Image-Slicing Mutlislit Option for IMACS (GISMO)

GISMO is an image-slicing reformatter that installs into the slitmask area of IMACS -- at the focal surface of the Baade telescope.  Its purpose is to slice up into 16 sectors an approximately 4' x 4' area at the center of the IMACS f/4 field and reimage them back in the focal plane at the same scale and f-ratio, but covering the entire f/4 field. Its principal purpose is to allow multislit spectroscopy of densely packed fields, more than 5 targets per sq arcmin whose spectra would necessarily interfere in the central area, but will not once the slices have been separated.  This results in a multiplexing gain of a factor of 5-8. This unique capability makes GISMO idea for such fields as the centers of galaxy clusters, including --- for example --- faint arcs in distant clusters, MW globular clusters and LG dwarf galaxies, and for globulars and planetary nebulae around nearby galaxies.  GISMO can also be used in a combination long-slit & multislit mode for mapping extended objects, and in combination with MOE to increase the resolution to R ~ 20,000.  Planning observations with GISMO, and more discussion of these and other  additional features and characteristics that are described at the website:

Preparing for IMACS Observations

Calculating Observing Times

The sensitivities for direct imaging in the standard IMACS filters are given above. An exposure time calculator for imaging observations can be found here. 

Designing a Multislit Mask

Multislit masks are cut by a computer-numerically-controlled laser-milling machine that resides in the Astronomer Support Building at Las Campanas Observatory. The Technical Support Group at Magellan is responsible for producing the multislit masks and installing them in IMACS. Generally, up to 6 masks will be loaded into IMACS at the beginning of a night's observing, with a second box of up to 6 additional masks placed on the platform (top of the stairs) for a swap during the night.  This must be requested in advance.

A complete software package maskgen is now available for designing multislit masks. The observer needs to provide a list of objects and setup stars with sub-arcsecond astrometric accuracy. Relative positional accuracy is more important than absolute, which will be tweaked at the time of observation, but a precise field position will reduce setup time for a multislit-mask to 5-10 minutes. The list of potential objects can be definitive or can be an overpopulated list from which the software will choose to maximize the number of targets that do not conflict in the spatial or dispersion direction.  Priority can be specified in the list, for example, by object magnitude, but if no priority is given, the maskgen will simply prioritize from the top of the list down, elminating conflicts below as it goes.  Several lists can be entered for the program to use: they will be stacked in order of entry to make one long list.

Priorities can be set in the list in order to guarantee that certain objects will appear on the mask. Observers with their own design for a slit mask can simply enter a list of objects for which all the objects are assigned this priority of "must include."  In either case, observers should recognize that no conflict resolution is done between "must have" objects  -- the user must make sure that these do not conflict with each other.  One way to do this is to run the program with just the "must have" list without the -2 flag that makes them obligatory and see if any of the objects are tossed out because of conflicts.  It is also possible to view the conflicts on the display of the mask design the program produces.

Recommendation: The observing catalog input to maskgen must also contain a list of stars that will be used for aligning the mask.  Good results have been obatined by choosing stars between 17th and 19th R magnitude. The number is specified in the maskgen GUI; 10-15 has been shown to give excellent results, but the alignment procedure will work well with as few as 6.  Note, however, that some stars are lost in "chip gaps" and some turn out to be galaxies, so preparing a mask with only 6 alignment stars is risky. If you have access to photometry and astrometry for your field, choose your own guide stars and alignment stars, rather than relying on the USNO catalog, which includes a significant fraction of galaxies and stars with significant proper motions. --- AD

In theory, a perfectly accurate slit mask requires not only perfect object positions but also knowledge of the field's position on the sky at the time of observation (to account for atmospheric dispersion) and ambient temperature during observation compared to the temperature at which the mask was cut. Generally, if observations are at an airmass of less than 1.5 and the slits are 0.7 arcsec or wider, these errors will be acceptably small. The atmospheric dispersion corrector makes it unnecessary to align slits along the paralactic angle, but the ADC does not compensate for the overall scale changes as a field moves from the zenith to the horizon.  Multislit observations with the f/2 camera at more than 2.0 airmasses will exhibit alignment errors because of the scale change and should be avoided.

Maskgen produces the file FILENAME.SMF, which is a description of the mask in ASCII form that is used to generate the "maskcut" file (done at Magellan). These are the files that the observer uploads to the website slit mask form in order to have the masks cut. Note that mask files must be received in Chile 6 weeks prior to the observing run to guarantee that the masks will be cut.  Note that the SMF file contains a line which begins !oc that is the proper entry for the observing catalog that is given to the Telescope Operator.  This ensures that the position and the angle of the rotator are communicated properly.

Also created by maskgen is FILENAME.obw. This contains an abridged version of the catalog file that is used to keep track of the objects that have been targeted. This file can be used to make subsequent masks that avoid duplication.  This can be overcome, for example, to repeat an observation, by hand-editing the list.

It is crucial that observers, when creating slitmasks, verify that there are suitable stars for the Principal Guider and the Shack-Hartmann Guider.  This is accomplished by choosing the option skywin during the running of intgui.  To successfully complete the  mask design process requires the SH guider field must contain at least one star 15th magnitude or brighter.  The Principal guide star can be considerably fainter.  It is important to check the guide stars to make sure they are not galaxies or stars with high proper motion --- a link is now available in the software to an imaging survey.  For most field centers and field rotations there will not be a problem finding adequate stars, however, it will be sometimes be necessary to either offset the field center or rotate the field around the original center.  Most observers choose to capture the image of skywin to take to the telescope.  This picture will directly match the GMAP monitor display that the Telescope Operator will use to set the telescope.

The proper way to acquire multi-slit fields is to use the accuracy of the IMACS guiders to correct pointing inaccuracies of the telescope.  This is accomplished by moving the telescope to meet the guide stars (known as "centering" in the GMAP program) rather than moving the guiders to the stars once the telescope has settled at its position.  Verify that the TO is using this procedure, which is not the common mode of operation for the other Baade or Clay instruments.

Full documentation of the many ways to use maskgen are given in the link above.

Observing with IMACS

Some things you really need to know

IMACS is permanently mounted on Magellan Baade.  Generally, it is always "powered up" and the detectors are kept at operating temperature by Cryotiger pumps. Under normal conditions there are no systems to power on, however, before the observer arrives, the on-duty Instrument Specialist will usually have started the software, initialized the mechanical devices, and taken test frames with the Mosaic Camera.

Before the observer has come to the telescope, the on-duty Instrument Specialist will have already received the observer's choices for (1) gratings or grisms, (2) special filters, and (3) an IMACS accessory unit (see above). If multislit masks are part of the program for the upcoming night, the observer will inform the Instrument Specialist which of the already prepared masks will be used; the masks are usually installed by 3:00 p.m., and often before lunch. The Instrument Specialist will edit the setup files that identify the slitmasks, filters, gratings, and/or grisms that will appear in the pull-down menu of the IMACS-GUI -- these files cannot be edited by the observer. 

For direct imaging programs, observers will usually take a series of flat field exposures with the appropriate filters, using the pupil-mask screen (see description immediately below).  Exposures will range from a few seconds to several minutes for blue filters, f/4 1x1 binning -- even with the relatively "hot" quartz-halogen lamp.

Recommendation: It is sufficient to take a few flat fields for each filter, with count levels that are comparable to the sky level of the observations.  Except for rare programs (for example, surface-brightness-fluctuation measurements), nothing is accomplished by taking flat fields with an order-of-magnitude or more in total counts compared to the actual observations.  Also, the overscan region on the frames is, for almost all purposes, sufficient for bias-level subtraction -- bias frames are generally unnecessary.  Likewise, dark frames --- which accumulate typically a few counts per hour --- will not necessary for high-quality IMACS imaging.  --- AD

 
For spectroscopic programs, the observer will usually want to take images of the slitmasks (multislit or longslit) at the same binning as is intended for the nighttime observations.  This should be 1x1 for the f/2 camera but 2x2 for the f/4 camera; there is no point in using the full resolution of the f/4 camera for alignment purposes, because errors in the process (for example, telescope tracking and seeing during exposures) are considerably bigger than the erros associated with centroiding the positions of alignment stars with 0.2" pixels.  The mask exposures and alignment pictures should all be taken through the Bessel-R filter. The mask images are usually exposures of 1-2 seconds taken with the lower-light-level quartz lamp controlled by a power supply at the base of the telescope -- ask the Instrument Specialist. These are used in the first step of the alignment procedure, to produce an xxx.align file with the program icbox. This step is also a double-check of the match-up of masks and locations in the cassette.

Recommendation: As has just been emphasized, alignment images should be 2x2 for the f/4 camera.  In the same vein, when the seeing is FWHM > 1.2"  (f/2) or FWHM > 0.6" (f/4), nothing is gained by taking full 1x1 binned frames for the actual observations-- the data are at this point well oversampled.  Using 2x2 binning saves disk pace (not so important) but also readout time, and it helps minimize read noise in proportion to signal. It is a good idea to keep calibration and observation frames at the same binning, however.  This advice is particularly appropriate for maging, where the readout time will become a substantial fraction of the "shutter-open" time, but is less important for long spectroscopic observations. --- AD

Gratings or grisms can be expected to be where the GUI pulldown menus say they are: a few test exposures at the start of an observing will calm the obsessive among us. Grating tilts are defined as angles from the zero-order angle, which is defined as 0 degree. They can be calculated by using the IMACS Grating Tilt Calculator, where the user enters the grating information and desired central wavelength. In addition to the tilt, the program also returns the wavelength range for a slit centered on the field or mosaic array.
 
Tests of any kind can be accomplished with the internal arc-line and continuum (quartz-halogen) lamps in IMACS. These are located in the forward compartment of IMACS; they illuminate the screen that forms the IMACS hatch. Control of the lamps is from the IMACS Observing GUI. Light from the lamps hits the screen at a 45 degree angle and scatters to some extent over the rest of the forward compartment, so these are intended for tests and test calibrations rather than for application to actual observations. The lamp choices are quartz-halogen, neon, neon + mercury, krypton, argon, and xenon.  The krypton and xenon lamps are relatively faint.

A rapidly-deployed screen at the pupil of the telescope is well suited for science calibrations. In addition to the quartz-halogen lamps (settings are QL and QH), this system includes helium, neon, and argon, and a direct view of hollow-cathode sources in the center of the screen (the area obscured by the secondary in normal operation).  The QL setting is appropriate for direct-imaging flats. A few seconds for I flats, and up to a few tens of seconds for B-band, are sufficient to reach half-saturation (recommended) of 32k counts per pixel (unbinned).  Sky flats may be a good alternative for any blue or flats, and a necessity for any ultraviolet (user-supplied) filter.

The QL lamp setting and an exposure of 10-20 seconds is appropriate for spectral flats with the grisms, while the gratings, particularly the higher dispersion gratings, require the QH setting. Exposure levels of 20k-30k counts (maximum) is recommended.  Spectral flat fields should be taken immediately after one or a series of spectral observations, before the slitmask, grating, or grism has been de-installed.  An exposure of 10 seconds will produce ~10k counts per pixel for the 200-l grism (unbinned).  Spectral flats for the 1200-l gratings, particularly in the blue and 2nd order, will require several minutes of exposure, but the user should consider the exposure level of the objects themselves when calculating what number of counts is actually required.  Spectral arc calibrations are typically He + Ne + Ar, but it is useful to take He alone for identifying a spectral range or in cases of very low dispersion (e.g. LDP) where the Ne and Ar lines blend together.

Getting Acquainted

The observing station in the Baade control room has twin computers, Llama and Burro, but only Llama is used to run the IMACS instrument and the CCD mosaic camera. Llama and Burro are now both 1.83GHz Intel core duo Mac mini's running Mac OSX. (See our mini-tutorial if you are unfamiliar with the Mac OSX environment). Each has 2GB of RAM and a 500GB external disk drive. Data can be burned onto 4GB DVDs (see the tutorial), transferred to a personal laptop, or onto a personal USB or FireWire external hard drive. Each computer has a 24-inch widescreen flat-panel display and supports multiple workspaces, so there is room to display the GUIs and the Quick Look tools and a window to run a data reduction package such as IRAF.  The IMACS operation software runs on the Llama computer only. 

The Startup Window is started by a script called 'imacs'. (It is in the observer's path and no parameters need to be given.)  The user learns from the Startup Window the parameters of the configuration. The observer can change the Observer name but is unlikely to modify any of the other entries.  When satisfied, click 'OK'.

How to use of the IMACS-GUI, including details about the fields and pull-down menu choices control the instrument functions, can be found at http://www.ociw.edu/~birk/IMACS/user.html#c2.1. The IMACS-GUI controls all the mechanical functions of IMACS but does not control the outboard guiders or the CCD mosaic camera. The IMACS-GUI displays, in simple format, the present complement of changeable elements: mask, filters, gratings, grisms, and calibration lamps. Each of these has a pull down menu for selecting a new element. 

NOTE: Configuration changes of the Mosaic Camera are locked out while observations are in progress, but not configuration changes of the instrument itself.

Those who are interested in viewing the technical data from IMACS -- temperatures, pressures, guider positions, etc. -- may open "Hardhat" in the Options pull-down menu.

Running the Mosaic CCD Camera

The 8K x 8K Mosaic CCD cameras, Mosaic1 and Mosaic2 (and the guide cameras as well) run from Fedora LINUX PCs -- these computers reside in the Equipment room rather than the Observing room. The Mosaic Camera GUI will look somewhat familiar to those who have used other LCO instruments, although CamGUI has some added features. The GUI for the f/4 camera has a blue tint, and the f/2 GUI a red tint; when the Quick Look tool displays a picture, the right frame area changes color to match the camera that is displayed displayed.  The data paths for the two CamGUI can be set to tsame or to different directories.  Mosaic1 and Mosaic2 are completely independent, so they can be run at any time.

The filenames are specific to Mosaic1 and Mosaic2.  Mosaic1 writes files iffxxxx.fits, where iff stands for IMACS file "four" (f/4), and Mosaic2 writes iftxxxx.fits for IMACS file two.  The difference in the filenames means that a single data directory can be used to store both f/2 and f/4 data, although separate directories are a perfectly good alternative.

A small, bright-green icon of the 8 CCDs is used to select the specific data that will be stored, that is, the observer can "switch off" CCDs (although, since the CCDs are read out in parallel, this will not shorten the readout time.)

There is a IRAF program Mosaic that packs this information into a complete frame that includes all 8 and a coordinate system that takes into account the gaps between CCDs.

The f/2 and f/4 CamGUIs can be launched from the pull down menu "Modules" in the Mechanical GUI, or from each other, also from the "Modules" menu.

ExpTime is the exposure time in seconds for the next observation: the user can alter this time during the exposure by entering a new number and hitting return. Setting up a loop can be a convenient way to take flat fields or make a sequence of timed observations.  The bar to the right of ExpTimes shows the progress of the exposure in graph form -- the bar fills up as the exposure is taken and empties as the exposure is read out.

ExpType includes bias and dark frames.  Observations and calibrations are done by choosing Object or Nod&Shuffle (N&S), which brings up another GUI to set the nod-and-shuffle parameters.

Start begins a science exposure. The exposure can be paused (e.g., for clouds) and resumed by toggling with the "pause" button, but observers should remember that cosmic rays are accumulating while the exposure is suspended. The progress of the exposure can be seen in the graph-bar that fills to the right; as the CCDs read out the bar empties to the right.

Test pictures will typically be taken rebinned by 4 x 4 or 2 x 2 to save readout time. The "Snap" button automatically sets the 4 x 4 mode, takes an exposure, and -- unlike the "start" command -- does not update the frame number at the end of the exposure.  The binning is returned to its former value automatically after snap finishes.

Exposures can be stopped or aborted. At any time a user can stop the exposure and readout will immediately begin. During read out --- at the intended exposure time or as the result of hitting "Stop" --- the user can abort the readout, which results in the data being dumped and the file number not updated. A wipe of the chips follows, but since the user will not generally know when it is finished, it is strongly recommended that a "Snap" of minimal exposure be taken to insure a clean chip, before any science exposure is begun.

The Mosaic1 CCD Camera has three readout speeds. The default "fast" readout takes 93 sec and produces a read-noise of 4-6 e- per pixel.  This will be appropriate for most spectroscopy, and certainly for imaging. The "slow" readout achieves a slightly lower noise - about 0.5 e- less, on average, at the cost of a 142 sec read time -- it's generally not worth it, although observers with very few counts, for example, observations or flats in the blue-ultraviolet, may want to take whatever benefit they can get. The 'turbo' readout is intended for setup frames and tests, but its readout noise of ~10 e- should also be suitable for well exposed direct images, including flats.  However, read time is only marginall faster at 79 seconds read time is marginal for most applications and "Snap" will do a better job for test frames.  To avoid confusion, observers should lin genera leave the speed in the "fast mode" except in the case of very thinly exposed spectroscopic exposures.  As with the binning, the read speed can be changed any time before the readout begins, even during an exposure.

The Mosaic2 CCD camera has two readout speeds, fast and slow.  The default "fast" readout speed takes 82 sec and delivers a read noise of about 4.0 e- per per pixel.  The "slow" readout takes 97 sec and delivers a read noise of about 3 e-.  For 2x2 binning these times are 34 s and 27 s, respectively.  Binning does not change the readout noise, but it does increase the signal, so it should be considered whenever S/N per exposure will be low, especially at f/4, where the data are much oversampled for a reasonable slit width or typical seeing.

The 8 CCDs are read out in parallel. A QuickLook (QL) Tool -- launched from the 'Options' menu of the CamGUI -- will display and image of the 8 CCDs in real-time as the readout proceeds and as the data are stored to disk. The default display fills the screen, but the user can select -- from the "options" menu - a quarter-sized QL-tool.  Due to the limited number of resolution elements on the display, the image of the mosaic is undersampled, a lot or a little, depending on the binning. The panel on the right is tinted red for an f/2 camera image, blue for an f/4 image.

A magnifier window allows the user to zoom in at 1X, 2X, 4X, and 8X to see the data full-scale (at whatever was the rebinning the data was obtained).  The magnifier window can be stretched to a two-times-larger box size.  Placing the cursor in the main window (if the cursor is in the magnifier window it will be automatically returned to the main display) and hitting the left mouse button will sample the exposure level at a given pixel position. Statistical information is given by hitting "return" or the space bar when the cursor is positioned appropriately. The "radius" feature controls the area of the fit for that exposure.  This rms diameter is not closely equivalent to what is calculated in IRAF imexamine, which is more equivalent to FWHM, but is still useful for relative measurements.

There are several autoscale options and the scaling can be set manually - the choices are listed as a pull-down menu on the side of the QuickLook screen.  Note that you can force the chips to be scaled together, or separately, with the two choices below the autoscale menu.

Reading and saving the data from the Mosaic CCD Camera: Rebinning and Subrasters

It is not necessary to save the data from the whole array for each observation. Clicking on the small green moniker on the Camera GUI brings up a menu, which allows the user to specify any subset of CCD chips that are to be recorded as disk file.

The user can also select to rebin the array (or the selected CCDs) separately in X and Y -- this may prove useful in spectroscopic observations where it may be desirable in either the spatial or dispersion dimensions to rebin the data (lower read noise; compacted data).

The software also provides for a very convenient way to set up subrasters. As many as 8 subrasters can be selected for each CCD; the user fills in a table of X0 and Y0 coordinates and the size of each sub-array. The cursor on the QuickLook display can be used to select the center of the subraster: type "a" to transfer the position and set the subraster size. Taking a picture with the subraster feature will save time, since the CCDs are clocked without reading the data between the readout of subrasters (at whatever binning).  However, as the number of subrasters increases, this advantage will diminish. Data for subrasters can be stored separately by choosing "save mode = minimal" or embedded in a full frame, with zeros filling the non-subrastered areas, by choosing "save mode = full" in the Subraster dialog box.  The save mode  = full is the default, and it is essential to use the "full" mode if you are using the mask alignment software, ialign and ifalign.

When viewing an image on the Quick-look tool, typing the number 1 to 8 on the keyboard will move the cursor to that chip image.

Recommendation: Use the QL to check that the frames are what you expected, but use IRAF and DS9 to measure seeing, look for faint objects or features, look for spectral features, etc.  --- AD

Let the Observations Begin!

From long experience, the IMACS team strongly recommends opening the hatch before attempting observations --- use the popup-menu at the top of the IMACS-GUI.  Please remember to close the hatch when you are finished observing.

Direct Imaging

The simplest way to use IMACS is as a direct camera, in either the f/4 (~15 arcmin square) or f/2 (27 arcmin diameter) modes. All that is required of the observer is to select a filter and provide coordinates of the field to the TO, who will set up both the Principal Guider and Shack-Hartmann guiders.

No focus required: A main feature of IMACS is that, in normal operation, focus is automatic. IMACS is internally focused on the surface defined by the slit mask (matching the telescope's focal surface). The optical design, in combination with the focusing stage of the CCD mosaic array, automatically adjusts for temperature to maintain both focus and scale. The Shack-Hartmann guiders (both centerfield and outboard) are set up to maintain the telescope focus at the same surface. The observer is strongly advised to use this autofocus feature of IMACS --- attempts to focus "manually" will have unpredictable results.  The standard filters provided in IMACS have offsets that have already been determined, so part of the autofocus procedure is to automatically compensate when they are inserted. In the case of user-provided filters, a focus zero point at a given IMACS temperature needs to be determined by the Instrument Specialist and entered in the offset table

As mentioned above, and flat fields are best taken with the pupil screen of the telescope or the twilight sky.  Even the internal flat field lamp will be sufficient to measure pixel-to-pixel variations, but for overall illumination patterns the first two options are much preferred. The "loop" feature provided in the camera control may be convenient in taking flatfield exposures. Generally the object is to expose the flatfield to a level several times the count per pixel level of the deepest science exposure. This is sufficient for all but the most critical applications.

The fringing of the SITe CCD Mosaic1 camera in the 'I' band is only a few percent.  Fringing can generally be removed from direct images by making a "fringe frame" out of a dozen or more exposures of relatively empty sky fields and making a median image to eliminate stars and galaxies. The fringe frame, appropriately scaled for the exposure time and/or sky brightness, is subtracted from the science image.  The fringing of the E2V Mosaic2 camera has not yet been measured.

 Flat field exposures (see below) need not be taken at the same telescope (IMACS) position as the observation.

Long-Slit Spectra

IMACS has a set of long-slits that have been cut into multislit-mask blanks, in a range of widths.  One or more of these, installed in the slit-mask server, can be inserted in order to do single object (point or extended) spectroscopy, or spectroscopy for for 2-3 objects along a single line.  The procedure for placing the object(s) at the proper position is described in the IMACS cookbooks -- it involves taking a picture with IMACS to identify the target(s), a first move to get close to the target position using -- toslit -- and then a fine adjustment  that uses the CCD pixels of the guider -- icobject ---  the guider itself is not moved, so the operation should be accurate to a small fraction of an arcsecond. 

After the slitmask itself is inserted, it is a good idea to check that the target position is still where it was on the setup frame --- sometimes the IMACS mask does not return to exactly the same position. Once the observer is satisfied that the object or object are in the right place, the disperser can be inserted (grating or grism), the proper filter inserted (usually Spectroscopic but could be a blocking filter), the binning and exposure time chosen, and the spectroscopic observation can begin.  The procedure is described in careful detail in the IMACS Cookbooks.

Standard-Star Spectral Calibrations

Slit-viewing mode (described in the next section) is the preferred way to make observations of standard stars for flux or radial velocity calibrations.  For radial velocity standards, ask the TO to set to the star and then move it to one of the sections of the multi-width slit -- it is a good idea to choose the slit that best matches the seeing at the time.  For flux calibrations, give the TO the coordinates of, for example, one of the Hamuy standards stars, and ask the TO to move the star into the 7-arcsecond diameter hole.  This assures that nearly all the light passes through to the spectrograph.  Arcs through the hole will be low-resolution, but the ajoining arc from the multi-width slit can be used by the COSMOS software.  If relative flux calibrations are all that is required, the widest slit position is a good choice.

As described below, It is not necessary to save the whole Mosaic array for spectroscopy in Slit-Viewing mode, as the spectrum will only appear in a relatively narrow band, on chips 1-4 in N&S orientation and on chips 2 and 5 in normal mode.  However, if these are the only observations made with the slit-viewing mode (as opposed to a full program of spectroscopy), it is simpler in terms of data reduction to take full-frame exposures (though 2 x 2 or even 4 x 4 binning is sufficient).

One of the slits or alignment boxes in a multislit mask may also be used for spectrophotometric calibrations.  This is the preferred method when the observations are in N&S mode with the f/2 camera, where --- because of the rotation of the grism --- the multi-width slit of Slit-Viewing mode runs in the wrong direction relative to the dispersion (and the slits disappear in the CCD gap). The 7" hole can still be used, of course, and an approximate wavelength calibration can be done with an arc taken through the hole, but for this case it is probably easier to use one of the alignment boxes on the multislit mask,  and then to use the "spectral-map" solution of the COSMOS reduction software to determine a wavelength scale appropriate for the alignment box.

Absolute calibrations to a few percent can be accomplished by trailing flux-calibrated star over a slit at a known rate during an exposure, for example, using a 0.5"/sec non-sidereal rate to move pass a star over a 1" slit, for an effective exposure of 1 sec.   Relative calibrations can be made by simply positioning the star on one of the slits. 

Centerfield Slit-viewing Spectroscopy

A detailed description of how to use the Centerfield Slit-Viewing mode for spectroscopy can be found in the IMACS Cookbooks.

In addition to its two outboard guiders that swing around the ~27 arcmin square field, IMACS has a centerfield guider (CFG) that travels radially into the centerfield (optical axis) of the instrument. By switching between the CFG's two sets of optics (housed in 2 barrels), the observer can view centerfield - a square field 105 arcsec on a side - or perform an on-axis Shack-Hartmann test.

From its fully-inserted position on the optical axis the CFG is pulled back approximately 2 arcmin to turn IMACS into a slit-viewing spectrograph. The utility of SlitView mode is that sufficiently bright objects can be acquired in real time with the TV camera and placed on the desired slit - time is saved because the CCD Mosaic Camera need not be used for the acquisition. The viewing mirror incorporates a multi-width slit at the focal surface of the telescope. The layout is 5 side-by-side slits each about 17 arcseconds long, with widths of 0.25, 0.50, 0.75, 1.0, 1.5 arcseconds, and a displaced 7 arcsec hole for photometric calibrations. When inserting the CFG any installed slit mask will be removed. In its place, a baffle will be automatically inserted behind the CFG to mask out the entire field except for the slit and hole (to eliminate surrounding sky light). The Principal Guider (PG) can also interfere with the CFG. If this conflict exists, the user will also be prompted for confirmation that the PG will be moved (which will result in the loss of any fine adjustment such as might have been accomplished in a multi-slit spectroscopy setup). The PG will be moved to near the end of its range, and is at any rate mostly occulted by the CFG, so its use as a guider is very limited.

Observing in the slit-viewing mode is straightforward - the same as for typical non-imaging spectrographs. The observer provides target coordinates to the TO, who will acquire (and the observer identify) the object at the centerfield position. The TO will then apply an offset of 115 arcseconds west or east (for the two standard settings of the rotator, north up, and north down, respectively).  At this point the obsever should see the object reflected off the slit -- close to the small cross that defines the center of the field. (If a number of objects are to be observed in this way, the observer can ask the TO to skip the Centerfield acquisition step and offset directly to the SlitView position.)  A second procedure is to use the "toCFslit" script in the IMACS IRAF package, which sends the telescope by a coordinated offset to the center crosshairs of the CF guider (LINK).

As yet, the repeatability of the slit-viewing position is not established, but it is expected to be less than 3 pixels. If very accurate flat-field of spectra is required, it is advisable to insert the pupil screen and take flat fields and arcs before returning the CFG to its stowed position. (Another approach is to use a single set of flats but to account for mis-registration by creating a separate "slit flat" -- the one-dimensional sensitivity function that accounts for structure along the slit itself. The shift between the standard flat and the actual observation can be determined by collapsing in the dispersion the sky-continuum and the standard flat and cross-correlating.)

Multislit Spectroscopy

Setting Up

Multislit spectroscopy requires considerable preparation, as described above, but observations are comparatively simple to perform. With the file prepared by the mask-making program, the TO has all the information required to accomplish the initial setup through a program called GMAP that is used to set the telescope. When using the mask preparation program maskgen, the SMF file contains a line (prefaced by !oc) which is intended to be part of the observing catalog that is passed to the TO.  This includes not only the RA & DEC not only of the target, but also the guide stars that have been selected in the process of using the program intgui.  As the telescope slews to the target position, the IMACS guiders are preset to the positions required to acquire the quide stars.  In general, when the target is reached, the two guide stars will be displace by some number of arcseconds (typically 2-10) because of pointing errors in the telescope and errors in the rotator angle.  In order to take advantage of the excellent metrology of the IMACS guiders, it is critically important not to move the guiders to the stars, but to make a coordinated offset of the telescope to bring the stars to the guiders, by adjusting altitude, azimuth, and rotator angle.  The process is called "centering" --- make sure it is being used, as this is not general practice for other Magellan instruments.  In this way, the IMACS-Magellan system corrects the pointing and rotator errors.  This procedure is sufficiently accurate to bring the alignment stars of the slitmask within their target boxes at the first setting, if the astrometry of the field center and alignment stars is done to arcsecond precision.

The observer has already run the IRAF program icbox (part of the IMACS package -- type 'imacs' to load) on the slitmask image (see the Multi-Slitmask Cookbook), which has produced an alignment file and a subraster file.  After the guide stars have been acquired, the next step is to load the subraster file ('full' or 'subraster' -- on the CamGUI) and take a picture.  (Take a full subraster frame, not minimal! -- see the Subraster GUI). Both this picture and the picture of the slitmask taken in preparation for observing should be binned 1x1 for f/2 or 2x2  for f/4.  The exposures should be in the R-band, regardless of the wavelength range of the observations. (If the difference between the R-band and wavelength region of interest is an issue, make sure the mask is designed with the slits at the paralactic angle.)  With the recommended R=18-19 mag for alignment stars, 15-25 sec exposures should be sufficient, (but longer in very poor seeing).  As the picture is read out, the observer checks to see if the alignment  stars are visible in the subrasters --- if not, there is a problem!  A full frame could be taken to look further afield for the stars, but it is likely that the problem is more serious, for example, the wrong rotation angle or problems with the astrometry.

With this subraster picture, the observer then runs the IRAF program ilaign, which will --- one by one --- display the subraster images with the alignment stars, which the observer confirms or re-marks --- see the Cookbook for details.  The result of this process is to provide offsets in RA, DEC, and rotator angle to bring the stars to the centers of the boxes, accomplished through a coordinated offset in which these offsets are sent to the Telscope Control System.  The aligment procedure would be finished at this point if the slitmask inserted in exactly the same position each time it is inserted (a goal for 2008).   For this reason, the next step is to load the multislit mask for the observation and take a second picture.    When this subraster exposure through the mask is finished, the program ifalign (not ilaign!) is used to calculate a coordinated offset, this time taking account of the actual position of the alignment boxes for this insertion of the mask.  The residuals at this point should be less than an arcsecond, or two at the most --- through out bad stars!   The result of this final alignment should be a calculated offset  of < 0.2 arcsec in both RA and DEC and angular misalignment < 0.010 degrees.  A second iteration with ifalign may be required, but generally should not be.\

Once completed, the alignment will hold as the field is tracted,  within the limits of the flexure between focal plane and guiders, as described above.  The telescope must be set for rotation guiding mode, which means that it is tracking in altitude, azimuth, and Nasmyth rotator, to keep the PG star and the SH star centered.

Making and calibrating the mutlislit observations

After alignment is complete, insert a grating (f/4) or grism (f/2). When operating properly, latchup of gratings and grisms results in only small shifts of wavelength.  The grisms in particular repeat well (within 1-2 pixelx at the detector), but at certain gravity orientations the gratings (mounted in heavy grating-tilt-mechanisms) appear to latch up differently from one insertion to another, resulting in a (x,y) zero point shift on the detector of as much as 5 pixels. The reason this is an issue is the degree to which spectral flat fields taken in the afternoon, for example, can be used for the subsequent observations. The safest procedure is to finish each multislit spectral observation with 1-2 flat-field exposures and an arc-line exposure using the pupil screen in the telescope. The guiding needs to be turned off when the puil screen is in, but the guide stars will be re-acquired afterwars with no loss of precision.

If one grating setup is to be used throughout the night, another way to deal with the f/4 latchup uncertainty is to use the grating in 0^th-order (angle = 0 degree) to do the imaging, that is, leave the grating installed and simply move back and forth between 0^th-order and the angle used for observation, usually 1^st or 2^nd order. This procedure also has a built-in uncertainty, but it is thought to be a pixel or less and, at any rate, an internal arc can be taken to reset the tilt exactly, should the program require this unusual precision.

The COSMOS software package (see below) provides a quick-look spectral extraction tool that will subtract the sky and apply a wavelength scale so that the observer can gauge the quality of selected multislit spectra as the data are gathered.   It is perfectly possible to use COSMOS to fully reduce data during the observing, but it generally requires a person dedicated to this effort.

  Summary: A Checklist for Making IMACS Observations

Essentially, there are nine items to double-check before beginning an IMACS exposure, especially a long one:

CamGUIs

1. Exposure time
2. ExpType: Obj or N&S
3. Binning
4. Full readout (or selected chips) or subraster
5. Object name
6. Readout speed

IMACS-GUI: (MechGUI)

1. Calibration lamps off?
2. Slitmask -- proper mask?
   
3. Disperser -- proper grating or grism?
   
4. Correct Filter?

If you are new to the instrument, It's a good idea to keep this checklist handy. There's a lot to remember!

Shutting Down

Observers should close the hatch and turn off all calibration lamps at the end of an observing night. It is not recommended to shut down the GUI displays. As with other Magellan instrumentation, data will normally be kept on disk for only a limited time after the observing run before deletion, so the data should be archived as soon as possible.

Data reduction pipeline

A complete data reduction package for multislit observations - COSMOS - has been developed by Gus Oemler, see http://www.ociw.edu/Code/cosmos. It includes the new sky-subtraction algorithm perfected by Dan Kelson (PASP, 115, 688, 2003). COSMOS includes programs that deal with the complicated data format produced by the Durham-IMACS IFU. In addition to the sophistication of its algorithms, COSMOS processes data fast enough, and produces of high enough quality that extracted, flux calibrated spectra can be provided contemporaneously during observing. Although some programs will benefit from storing raw data and additional reduction procedures,  observers have the opportunity to leave Las Campanas with data that are ready for scientific analysis.