LDSS-3 Users Guide
with help from Mark Phillips (LCO/OCIW), Alan Uomoto (Magellan), Ian Thompson (OCIW), Christoph Birk (OCIW) and David Osip (Magellan)
This document provides an outline of how to use LDSS-3 on the Clay telescope (Magellan II). It was initially written based on experience during engineering runs in February 2005 and May 2006. We have tried to be as complete as possible (at the risk of boring many of you). Please let us know if you encounter any issues not contained in this manual or associated materials.
Table of Contents
1.2 LDSS-3 Filters
1.3 Filter Ghosting
1.4 LDSS-3 CCD
1.7 LDSS-3 Throughput
3.1 Error Recovery
1. Instrument Overview
LDSS-3 is the upgraded version of the LDSS-2 spectrograph that was in operation on Magellan from 2001-2004. The upgrade includes a new collimator, camera, CCD device, filters and grisms. LDSS-3 was designed to be very red sensitive - allowing efficient multi-object spectroscopy out to 1 micron.
LDSS-3 operates as follows. The telescope is focussed onto a multi-aperture mask held in an 8-position wheel. The light then passes through various apertures cut in the mask and enters the collimator which converts the input f/11 beam into parallel light, before passing it through either a filter and/or a grism. The light is then focussed by the camera onto an external detector with a final focal ratio of f/2.5. Removable Hartmann masks are provided in the filter and grism wheels to aid in focusing.
Three grisms are provided and mounted in the grism wheel at any one time. These cover a range of spectral resolution of several hundred to several thousand. Up to 7 filters can be mounted in the filter wheel at once.
By using clear positions in the aperture and grism wheels, LDSS-3 can be used to give direct images in the chosen filter passband over a wide field of view. It thus doubles as a wide-field imager.
On Magellan, LDSS-3 reimages approximately a 8.3 arc-minute diameter field onto the CCD camera, with a scale of 0.189 arcsec/pixel.
LDSS-3 currently has four grisms available for general use. The new VPH grisms were designed by Mike Gladders and assembled by Ivan Baldry, Karl Glazebrook and the JHU Instrument Design Group. These grisms offer significantly higher throughput and resolution than the old LDSS grisms and are probably preferred for most programs. A feature of these grisms is that the wavelength coverage and blaze varies substantially over the detector. The throughputs and wavelength ranges of the VPH-RED and VPH-BLUE grisms are shown in Figures 1a and 1b for three different positions on the CCD. The 0 degree curves correspond to a position near the center of the CCD and the 4 degree curves correspond to a slit about 2 arcminutes from the optical axis.
A third VPH grism called VPH-All has a lower dispersion than the other VPH grisms, allowing one to cover the entire wavelength range available with LDSS-3. A comparison of the throughputs for the VPH-All grism to the Medium-Red grism is shown in Figure 1c for three different CCD positions. Several things to note: Near the CCD center, the VPH-All has a slightly higher throughput than the Medium-Red grism (with similar 2nd order contamination) . On the blue side, VPH-All is an excellent broadband choice - with good throughput over a large wavelength range with very little second order contamination.
The throughput values for the old LDSS-2 grisms are also shown in Figure 2.
The grisms properties are given in Table 1. There are several other grisms that were available with the old version of LDSS (i.e. "medium-blue", "high-blue" and "low"). For nearly all science programs, the currently available grisms provide similar wavelength coverage with higher throughput and better spectral resolution.
Note that for the VPH-red and Medium Red grisms the OG590 filter should be used to eliminate contamination from second order (which can be substantial redward of about 7000A for objects with significant blue flux).
Users are responsible for designing their own multi-object masks. Software exists to make slitmasks and this software should already be installed on the computers systems at all Magellan partner institutions. If the software is not available on your system, please contact Ken Clardy (clardy "at" obs.carnegiescience.edu). There are a series of standard longslits available for LDSS-3. A list of the available longslits can be foundPlease note that observers must submit their mask files at least 6 weeks prior to the first night of their observing run.
|(lines/mm)||(A/pixel)||(4 pixel slit)|
|Medium Red||300||2.26 (@8000A)||850||0.6|
|VPH Red||660||1.175 (@8500A)||1810||0.92|
|VPH Blue||1090||0.682 (@5200A)||1900||0.85|
|VPH ALL||400||1.890 (@6500A)||860||????|
Fig. 1a -- Efficiency curve for the VPH-Red grism.
Fig. 1b -- Efficiency curve for the VPH-Blue grism.
Fig. 1c -- Efficiency curves for the VPH-All grism.
Fig. 2 -- Efficiency curve for the old LDSS-2 grisms. Note that the medium-red grism is the only one that is still generally available.
Up to 7 filters can be mounted in the filter wheel at once. Four Sloan broad-band filters (griz) and a Harris B-band filter are supplied for direct imaging. An OG590 blocking filter is also availble for spectroscopic observation. In addition, a number of filters have recently been purchased by observers including a broad VR filter and a series of bandpass filters (KG650,KG750,KG850,KG950 and BPF3800-5900, BPF4800-7800). These user filters are kept at the Observatory and are available for general use. To assure that these filters are availble for your run, please specify them in the Instrument Setup Request Form at least 1 month prior to your run. Observers may also supply their own filters. However, the use of such filters must be coordinated with Alan Uomoto (Carnegie Technical Manager in Pasadena: au at obs.carnegiesciene.edu) and David Osip (Magellan Instrumentation Scientist: dosip at lco dot cl) at least 2 months prior to the observing run. The standard filters have diameters of 100mm and thickness less than about 10 mm.
Fig. 3a -- Sloan Filter Transmissions
Fig. 3b -- OG590 Filter Transmission. This filter should be used with the VPH Red and Medium Red grisms.
Fig. 3c -- Harvard VR Filter Transmission.
Fig. 3d -- Bandpass Filter Transmissions.
Imaging observations of bright targets using LDSS-3 will clearly show ghosts. The location of these ghosts relative to their parent objects shows that they arise from light which bounces of the detector, passes back through the camera (and hence gets recollimated to a parallel beam), reflects off the filter, and returns once again through the camera to be imaged on the opposite side of the field. The intensity of the ghosts is principally a function of the reflectivity of the CCD surface, and the steepness of the filter cut-on/off (over the wavelength range over which the filter drops from transmitting to reflecting, it acts as a ~50% mirror). We have measured the ghost intensity and behaviour using direct images of masks (which provide a recognisable pattern of bright slits) and find the the following:
The ghost amplitude is less than 1% at almost all field locations for all four of the SDSS filters (griz). The ghosts are strongest for parent objects near the center of the field and decline approximately linearly with radius. This is likely due principally to the wavelength shift of the filter bandpass induced by the steepening incidence angle of the reflected light from far field locations, which reduces the effectiveness of the filter as a reflective surface. Figure 4 show the measured ghost intensity versus field radius for the four SDSS filters. Ghost amplitudes of other filters are likely comparable to those of the SDSS filters, except for color-glass blocking filters (such as OG590) which should not experience ghosting.
Figure 5 shows the apparent center of reflection for the 4 different SDSS filters. The lines in the plot connect parent-ghost pairs as measured from a single image; the crossing point of the lines indicates the centerpoint. It is unclear at this point whether these values are repeatable in detail; the slight difference betwen filters probably reflects slight tips of the filters relative to the optical axis, which may not repeat as filters are taken in and out of the instrument from run to run. Regardless, an observer may measure this directly from any image in which at least two parent-ghost pairs may be identified (if this is not the case you don't need to be worrying about this!); once this center point is known, ghost pairs are easily identified.
|Band||FWHM at R=0 (pixels)||Slope (pixels/arcmin)|
The LDSS-3 CCD is a STA0500A 4064 x 4064 device from Mike Lesser at the Imaging Technology Laboratory, University of Arizona. The CCD cosmetics are excellent. The device uses a two-amplifier read mode. Each amplifier produces its own fits file, identified by ccdxxxxc1.fits and ccdxxxxc2.fits. There is an iraf script ("lstitch" in the ldss package) that reconstructions the two frames into a single image. The stitched image is named ccdxxxx.fits.
The linearity of the CCD has not been analyzed in detail. However, preliminary tests suggest it is linear to better than 1 percent up to 40,000 ADU.
The CCD has three readout speeds. The "fast" speed readout is appropriate for most spectroscopy, and certainly for imaging. The "slow" speed achieves the lowest noise at the cost of a ~72 sec read time. The 'turbo' readout is intended for setup frames and tests. There is low level pattern noise with both the "fast" and "turbo" mode. We recommend that observers doing faint object spectroscopy use the "slow" mode.
The rotation of the CCD is fixed and will not be adjusted for observers. This maintains consistency between the data reduction software and the orientation of the spectra. This also ensures that the alignment software will work. Users should expect that the spectra will not align perfectly with the cardinal directions of the CCD. This is true regardless of the rotation of the CCD because of distortions in the instrument.
Low frequency fringing is seen in the z'-band. 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 two-amplifier readout of the LDSS-3 CCD invariably produces a small amount of crosstalk between the two halves of the image. This has been measured using both saturated and unsaturated direct images of masks (which provide a recognizable pattern of bright slits). In unsaturated images the crosstalk in both directions is found to have an amplitude in slow readout mode of 0.0162 percent (with an uncertainty of 0.0007 percent). This single factor (1.6 parts in 10,000!) is sufficient correct both sides; simply remove a scaled image of 'chip 1' from 'chip 2' and vice versa as a first processing step. Note this proceedure will not work properly for saturated pixels.
Note that the LDSS-3 CCD is in principle a 4-amplifier device. However, early work with the instrument showed that one of the 4 amps has a charge transfer problem which precludes it from functioning optimally. Hence 4-amp mode, though allowed by the control system (and set in the inital start-up GUI), is NOT recommended and not supported by the Observatory.
Also note that no 1-amp mode is currently offered, nor do we envision offering such a mode in the future. The 2-amp crosstalk is sufficiently small: 2-amp readout is the only supported CCD readout mode for this instrument.
LDSS-3 uses an iris-type shutter that sits in the collimated beam. Because the shutter is in the collimated beam, there are no singificant variations in exposure time across the CCD.
On September 23, 2007 data was taking in order to perform tests on LDSS3 shutter timing. This gave a timing error of approximately 50 milliseconds for a 1 sec exposure. This error is constant over the field of view to within 1%.
The following figure shows an average of columns near the center of the field from amplifier C1.
From the data we plotted the ratio of the actual exposure time to the requested exposure time. This shows that for exposure times longer or equal to 5 seconds, the shutter timing is good to 1% or better.
LDSS shutter error plotted in terms of the ratio of Actual to Requested exposure time versus the requested exposure time in seconds.
Fig. 6a - Total throughputs with the VPH blue and VPH red grisms (for a slit near the center of the field). The OG590 filter was used with the VPH red grism to remove second order contamination.
2. Multiple Aperture Masks
LDSS-3 multi-aperture masks consist of a number of slits cut with a laser machine. The masks are fabricated at LCO prior to the observer's run. Currently, LCO does not support masks making in real-time: Observers must submit mask files at least 6 weeks prior to the observing run.
Observers must prepare the files necessary to generate their aperture masks prior to their observing run. Instructions on how to prepare these files is availableFiles must be emailed to Chile at least 6 weeks before the observing run. Please note that we cannot guarantee your masks will be made if your files are received less than 6 weeks prior to your run. Observers will be charged a fee of $35 US for each aperture mask cut.
Observers should bring to Chile printouts of the postscript files created by the mask generating software and finder charts of each field with the alignment stars labeled.
LDSS-3 can also be used for high-throughput longslit spectroscopy. A series ofalready exist for this purpose. Users can also design specific longslits using the software available for multi-aperture masks.
If you specified the masks you need for the first night on the Instrument Setup Request Form well in advance of your run, your masks will likely be mounted in holders and loaded in the aperture wheel by the day crew before you arrive at the telescope. The mounting and loading process is described below, although most observers will not need to do this themselves.
The mask holder comes in two rings held together with six socket head screws (Allen head; use 2.5 mm hex key). The large ring has three notches that define rotation and translation (see Fig. 7a) through positioning screws that push on three of the six flats. On one side there are two small dowel pins of different sizes (1/16 and 3/32 inches in diameter) that mate to holes in the slit mask (Fig. 7b). The slit mask is sandwiched between the large and small rings with six screws.
Place the large ring in front of you with the pins up and the smallest pin closest to you. Place the slit mask on the ring, concave side up (you can see the lettering at the top) and fit the pins into the two holes (Fig. 7c). Put the small ring on top, taking care to see the pins and six screw holes match the big ring. The small ring will be a sloppy fit over the two positioning pins. There is no easy way to assemble the components incorrectly.
Slide the assembly over the edge of the table, pinch the two rings together with both hands, and flip it over (Fig. 7d). Install the six screws retaining screws using a 2.5 mm hex key (Fig. 7e).
Fig. 7a - The mask (with the appropriate hex key), the large ring (left) and the small ring(right)
Fig. 7b - The mask with the large ring showing the small divel pins in the large ring
Fig. 7c - The mask mounted onto the pivel pins on the large ring
Fig. 7d - Slide the assembly over the edge of the table, pinch the two rings together
Fig. 7e - Installing the screws
When you have all of your masks in holders, you are ready to have them mounted in the aperture wheel. Mounting the masks in the aperture wheel is a bit tricky and should only be attempted by the day crew.
The CCD camera and motion of the various wheels (i.e. filter, grism, apertures) are controlled by the observer using the CCD Camera GUI. The CCD Camera GUI will usually be setup by the day crew and already be running when the observer arrives at the telescope. If not, or if a re-start is needed, the observer can easily start the GUI themselves. First, launch the Configuration Window by typing 'ldss3' in a terminal on the control computer. (It is in the observer's path and no parameters need to be given.) This GUI allows the observer to define the system setup without the need of editing any of the setup-files. In most cases, the only thing the observer needs to change is the "Observer" parameter (see Fig. 8. Click "ok". This will launch the CCD Camera GUI (Fig. 9).
Fig. 8 - LDSS3 CCD 'Configuration Window'
Fig. 9 - LDSS3 GUI
Note that the data files will by default be written into the user's home directory. The data path can be chaged in the LDSS3 GUI using 'DataPath' which is found in the 'Options' pull-down menu. The user should make sure the data path has been properly set before taking any exposures.
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.
Loops allows a series of exposures with identical exposure times to be taken. This can be a convenient way to take bias frames, flat fields or make synoptic observations.
ExpType specifies the type of exposure (e.g. object, bias, flat, etc). This information is written to the image header and can be useful for data reduction purposes. A special type of observation type is "nod & shuffle". If this type is selected a second window is launched (Fig. 10a) that allow the nod and shuffle parameters to be specified.
Fig. 10a - Nod & Shuffle Dialog Box
The software also provides for a very convenient way to set up subrasters using the menu to the right of "ExpType". When "Full" is selected, the entire CCD is read out. If "Subraster" is selected, a second window is launched (Fig. 10b) that allows the appropriate pixel values to be entered [Note, pixel values for the subraster must be the values associated with the Quick Look Tool and not from a stitched image]. As many as 8 subrasters can be selected; the user fills in a table of X0 and Y0 coordinates and the size of each sub-array. The cursor on the Quick-Look 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 CCD is 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 SaveMode = minimal or embedded in a full frame, with zeros filling the non-subrastered areas, by choosing SaveMode = full in the Subraster dialog box. The SaveMode = full is the default.
Fig. 10b - Subraster Defitition Dialog Box
For LDSS-3, the part of the chip that is exposed to light is smaller than the available pixels. Thus, you can shorten the readout time considerably by using subrastering.
Be careful if you are reading the subraster values from a file that you input the correct file name. If the filename is misspelled, the code will accept it, but not really apply a subraster.
Aperture,Filter and Grism are used to move the wheels in LDSS-3. To move one of the wheels, press the left button of the mouse on the box with the current position name. This will pull up a menu with the various wheel positions. When a selection is made, the box will remain yellow until the move is complete. If you hit the "Edit", a second window will be generated. This window allows the instrument specialist to put in names for each of the aperture, filter and grism slots. This feature is password-protected and may only be used by the day crew during initial set-up.
Focus is used to change the focus value of the instrument.
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 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.
Exposures can be stopped or aborted. At any time a user can stop the exposure and a 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 time be taken to insure a clean chip, before any science exposure is begun.
File# sets the file number.
Speed sets the readout speed. The CCD camera has three readout speeds. The "fast" speed readout is the default, with a readout time of 43 seconds (note: there is a moderate pattern noise when using the fast readout). This may still be appropriate for some spectroscopy, and for imaging where background levels are higher and contribute more to noise. The "slow" speed achieves the lowest noise at the cost of a ~72 sec read time. The 'turbo' readout is intended only for setup frames and tests.
Disk shows how much disk space is available in the current directory.
Although the aperture, filter and grism wheels in LDSS-3 are fairly robust, they do occasionally fail. If a wheel fails to reach the desired position, the following steps should be taken:
1. First, try moving the wheel again by selecting another position. If this works, try a few more positions just to make sure that everything is working properly.
2. If selecting other positions does not work, the failure is probably related to a communication problem between the electronics and the control software. Try closing (menu File-Exit) the LDSS3 GUI and starting it up again. Be sure to reset the path to the data directory before taking more data.
3. If that problem still persists, exit the LDSS3 GUI and have the night assistant power down the LDSS-3 electronics. Once it's been powered back up, re-launch the LDSS3 GUI and try to move the wheel again. Note that after the power has been cycled, the next image will always be saturated. Therefore, observers should first take a snap exposure immediately following a power cycle.
4. If the solutions above fail, it is probably a mechanical issue with the wheel and an instrument specialist should be notified. If the aperture wheel has failed, the observer should check to see if the wheel is in an undefined position. In this case, the wheel can be rotated manually until a detent position is reached. Restart the LDSS3 GUI and you should be ready to go. If the failure persists or is instead with the filter or grism wheel, you will need to call an instrument specialist. These wheels are unfortunately very difficult to reach and should only be worked on by someone very familiar with the instrument.
LDSS-3 can be focussed using an internal stage which moves the location of the camera along the optical axis. Filters sit in the collimated beam, and the system is almost achromatic; the focus offset of each filter has been measured, and is typically very small. Typically blue filters focus at slightly smaller focus encoder values. Under normal operations the focus compensation with filter changes is automatic and the user should not need to change the focus manually when changing filters. This will be true as long as you are using one of the standard filters. Observers are encouraged to verify that the focus does in fact change when the filter is changed the first afternoon of their observing run. Note that some user filters do not have filter focus values measured and thus observers may need to change the focus manually for these filters.
The focus of LDSS-3 is temperature dependent. A temperature adjustment is built into the automated system.
Also note that the focus system moves to the nearest commanded encoder position with a coarseness of 5 units (i.e., 510,515,520 etc.). Note that a one-pixel image seperation (i.e., 0.189 arcsec) in a Hartmann-Mask test of LDSS-3 corresponds to 182.4 encoder units; a coarseness of 5 units is more than adequate.
The calibration lamps are run from a GUI similar to the one used for IMACS on the Baade telescope. The row of green boxes represent individual arc or quartz lamps mounted near the flat field screen. Below the row of lamps are the controls for the flatfield screen. Generally, observers should only be concerned with the "Go In" and "Go Out" boxes. These move the screen in and out of the light path.
When you are ready to take some calibration exposures, click on the "Go In" box to move the screen into the light path. While the screen is moving, the word "Wait" flashes in the black box in the gui. To turn on the arc lamps, click on the boxes for the lamps you want. The boxes of the lamps that are on will turn red. You are now ready to take some exposures.
For the spectral resolutions of LDSS-3, the helium (He), argon (Ar) and neon (Ne) lamps are useful. The integration times needed for arcs will depend on the grism you are using and the width of your slits. Some example arcs will soon be available here.
Quartz lamps for dome flats are also on controled from the Calibration Lamp GUI. There are two lamps available. We recommend that you do NOT use the lamp marked Qv. This lamp allows you to vary the intensity of the light, but experience suggests that it is not very stable. We recommend instead that observers use the fixed Quartz lamp. For lower intensities better suited to broadband imaging, users are advised to use the independent quartz lamp power supply installed in the dome beneath the NASW platform (settings of 1-3 volts should be sufficient for most filters - and the power supply should never be set higher than 5 volts).
Once your masks have been mounted in the aperture wheel, you are ready to take some calibration and preparation images. The first thing to do is to take images of your slit masks in the afternoon. Typically a 10 second integration will be sufficient (there's no need to turn on any lights since there should be plenty of light leaking in from the dome). These images should be used to verify that the masks have been mounted properly in the holders. When displayed with IRAF, the slits should run along the columns (unless you have used tilted slits). Compare the image with the postscript output from the mask generating code.
You should also take some bias frames. Set the exposure time to 0 and start a loop or 10 or 15. You need to take bias frames for all the gain settings you plan to use during the night. Thus, if you are doing both spectroscopy and imaging, you should take a sequence of bias frames at multiple gain values (i.e. at the same CCD readout speed settings as you intend to use for your science frames).
Observers are also encouraged to become familiar with the mask aligning software (see cookbooks) their first afternoon at the telescope.
Several IRAF scripts have been written to assist with the alignment of multi-aperture masks. To run these, start up iraf on the control computer and type "ldss3". This will load the appropriate scripts. The relevant scripts are described in detail in the spectroscopy cookbooks.
Finally, observers should produce an observing catalog for their run. More information on the format of these files can be found here. Note that the relevant information for multi-slit masks (such as the appropriate rotator offset angle) can be found directly in the *.SMF output file created by Ken Clardy's mask making software.
When you arrive at the telescope after dinner, the night assistant will have already opened the dome and the telescope mirror. Next, the telescope will be focused. This process can take a little time (10 or 15 minutes) and generally does not require the astronomer to do anything.
If this is the first night that LDSS-3 has been on the telescope, the observer should verify the orientation of the CCD on the instrument has not changed. Typically, for the West Nasmyth port of the Clay telescope, a rotator offset mode and angle of (EQU -62.0) will align the standard long slits (and slitview mode) along N-S. Targets in the South will have N up and E to the right on the array. Targets in the North will have N down and E to the left on the array. To align the slits along the paralactic angle, use (HRZ -62). If you are using multislit masks, the latest version of the mask generation software now contains a program named "obscat" that will generate the appropriate catalog file entry with input of the .SMF file. The fiducial values for the rotation are known to shift by several degrees. The latest values are usually documented in a figure that should be posted in the control room. Multi-slit users should note any offset to the angle for the first mask they align and update their observing catalog appropriately.
A Cookbook for Multi-Slit Spectroscopy with LDSS3 can be found.
Although LDSS-3 was designed primarily for multi-object spectroscopy, it also serves as a good wide-field imager. On Magellan, LDSS-3 reimages approximately a 8.3 arc-minute diameter field onto the CCD camera, with a scale of 0.189 arcsec/pixel. The quality of the images are good across most of the field (they degrade somewhat at the very edge of the field).
Up to 7 filters can be mounted in the filter wheel at once. The filter wheel is not very easy to access, therefore a standard set of filters has been adopted and users must request any special filters well before their run. The standard filters include a Sloan set (g',r',i' and z'), a Harris B filter and an OG590 order blocking filter.
A Cookbook for Imaging with LDSS3 can be found .
Fig. 12 - Color image (g',r',i') of NGC 2217 (courtesy Dan Kelson)
LDSS-3 can be used for longslit observations of one or more objects. A standard set ofalready exists, but users can also customize their own longslits using the multi-object mask making software. Unlike traditional longslit spectrographs, LDSS-3 does not permit slit-viewing. Instead, observers must take images of the field of interest and the longslit mask and calculate the appropriate shift to put the object(s) on the slit. Software exists to calculate the appropriate shift. Overall, this process is fairly efficient and the total process should take less than 5 minutes:
A Cookbook for Long-slit Spectroscopy with LDSS3 can be found.
- In order to minimize the effects of atmospheric differential refraction (see Filippenko 1982, PASP, 94, 715), it is recommended that the slit be aligned with the parallactic angle. (Unfortunately, this means that the CCD image will not necessarily be in an easily recognizable orientation.)
Nod-and-shuffle spectroscopy is a technique that uses very short slits (typically a few arcseconds) and chopping the spectra between opposite ends of their slits, while at the same time shuffling the charge back and forth on the CCD and nodding the telescope in the opposing direction. The advantage of nod & shuffle spectroscopy is that it allows excellent sky subtraction. However, the technique is considerably more complicated than standard multi-slit observations and should only be attempted by experienced observers. For more on the technique, see Glazebrook & Bland-Hawthorn 2001, PASP, 113, 197.
Nod & shuffle spectroscopy is now possible with LDSS-3. "Microshuffle" mode, i.e. nods of a few arcsseconds and shuffles of up to about 20 pixels, has been successfully tested on LDSS-3 and appears to work very well. "Macroshuffle" mode, i.e. shuffles of a significant fraction of the CCD width, has also been tested. While this mode can in principle be very powerful, early tests suggest it is not an optimal mode for LDSS-3. This mode is therefore not recommended and is not supported.
Note that nodding with LDSS-3 is done by moving the guide box on the guide CCD and not by moving the guide head. This effectively limits nods to some fraction of the width of the guide probe field of view or approximately 0.5'.
LDSS-3 data can be reduced in a fairly straightforward fashion using the reduction package.
- John Mulchaey
- David Osip
dosip at lco.cl
- Last updated: 11 September, 2011 by D. Osip