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Echelle Spectrograph Users Manual old

 

 


Bill Kunkel, Steve Shectman, Mark Phillips, Ian Thompson
December 2001


Table of Contents


1. Introduction

This document describes the operating procedures for the 100" du Pont telescope Echelle spectrograph. With the principal optical components held by a short optical bench attached at right angles to the telescope optical axis mounted directly on the lower face of the IMB for freedom from flexure, the instrument was originally designed for use with a two-dimensional "2D-Frutti" photon-counting detector. In early 2000 this was replaced by the TEK#5 CCD camera with a 2k by 2k format and 24 micron pixels, providing an order separation of not less than 11 pixels in the red, and more in the blue. Simultaneous wavelength coverage extends from ~3700 to 7000 Å at a fwhm resolution of about 45,000 or 7 km/s with a one arc-second slit. The longest non-overlapping slit projects to six arc-seconds on the sky.
 

2. Optical Layout

The optics employ a cemented triplet lens of 500 mm focal length operating as both collimator and camera in a Littrow arrangement. Immediately behind movable entrance apertures there are two filter slot positions, one a wheel, now fixed open, the second permitting the insertion of an external filter, normally a diffuser to permit obtaining "milky flat" frames from daytime sky exposures. Behind the shutter blade the beam is deflected 90 degrees by a small right-angle prism. A negative lens on the exit face of the prism converts the telescope's f/7 beam to f/5. Light then travels to the objective lens, through the cross disperser to the echelle grating, returning through the cross disperser and the lens a second time, passing immediately below the prism to a plano-concave field-flattener fixed to the exit surface of the echelle case body where the CCD camera is mounted. Comparison light from a Th-Ar hollow cathode lamp (mounted on the back face of the spectrometer body) is introduced via a movable prism above the aperture plate. Visual inspection of the dewar on the instrument shows a variation of spacing between the dewar face and the spectrometer body. This reflects a tilt in the dewar to compensate for chromatic variation of focus with the orders, and is the normal appearance.
 

3. Data Acquisition

The TEK#5 CCD camera should be mounted and powered up, and the data acquisition PC should be booted and loaded with the ccddriver (as opposed to the irdriver). If this is not the case, see the CCD manual, Appendix A for instructions on how to change the driver, or ask one of the day crew for assistance.

3.1 Starting the CCD Command Window

In the console window of the data acquistion computer, Canopus, do a cd to the directory/subdirectory where the data are to be stored. For example,
    cd /export/data1/obs100/ccd_data/n1
Next, in this same window, type ccd [return]. A dialogue box will appear. In the top panel where the "shutter option" exists, hold down the right mouse button which displays a total of 3 options; select Uniblitz. Next, in the "CCD option" pull-down menu, select TEK5. At this point you can change the number of overscan columns if you so desire. Finally, in the "Telescope" menu, select LCO-100. Clicking on OK will then bring up the ccd command window.

The present manual will refer to certain specific functions in this program -- see the CCD manual for a complete description how to take data with the ccd command window.

3.2 Setting the CCD Gain

ccd command window allows for the selection of 3 possible CCD gain settings. For echelle spectroscopy, it is suggested that gain 1 be used, which gives approximately 1.0 electrons/ADU and a read out noise of ~5.6 electrons. Gains of 2 and 3 give approximately 2.0 and 3.0 electrons/ADU, with a read out noise of ~6.6 and ~7.0 electrons, respectively. Note that the window is brought up with the gain set automatically at 1.

3.3 Starting IRAF

IRAF is used to display the data and to make quick extractions of the orders. To start IRAF, first open an IRAF xgterm window using the right button on the mouse (while on the screen background) and selecting "IRAF", and then "IRAF-Xgterm". Then in the IRAF window, go to the data directory from which the CCD command window was started.

To start up an "ximtool" window for displaying your data, use the right mouse button to select "IRAF" and then "ximtool". Then within the IRAF window once again, set the ximtool window to the correct size by typing:

    set stdimage=imt2048

3.4 Taking an Exposure

To take an exposure, first make sure that the "DISK FILE" number in the ccd control window is set to a number in the range 0-9999.  (This number is incremented by one at the end of a regular exposure, and is NOT incremented for a "SNAP", nor when the "SAVE" flag is set to "test".) The disk file number can be set by the user by clicking the left mouse button on the panel field and entering the new value.  Note that if a frame with the same number as the current exposure already exists on the disk, it will be overwritten.

Next, the exposure time in seconds should be entered in the "EXTIME" field.  The object name may be entered in the "OBJECT" field (note that this value, as well as the "COMMENT" field may be modified during an exposure, prior to read-out). The "IMTYPE" button can be used to set the image type that is written to the header.  Clicking on the "START" button will then begin the exposure.

Note: The ccd control system displays a small window which says "abort readout" during the readout of the chip. Do not be alarmed! This simply gives you the opportunity to abort the readout by clicking on this button during the readout. During readout, the CCD window is "locked" and no operations in it or changes in it can be made. For gain 1 and the default overscan of 16 lines, the total readout and storage time is 124 seconds for the full chip. For gains 2 and 3, the readout time decreases to 84 and 68 seconds, respectively. The orders do not fill the entire CCD. To save readout time, it is possible to read out a subraster of the full image. To do this, click on the "Subraster On" button. This will generate a window where the subraster dimensions can be entered. For normal echelle observations a subraster of x pixel from 1 to 2048 and ypixel from 1 to 1450 works well. An image subrastered in this way takes about 90 seconds to read out for a CCD gain of 1.

 

4. Echelle Controls

Control of the instrument has evolved with the years. Spectrometer focus remains an entirely manual adjustment. The projection optics for the Th Ar comparison light remain as in the original design and positioning of the aperture plate is controlled with a bidirectional contact switch without encoder readout. These latter two functions are activated by the left two of the three toggle switches on the Echelle Control Box. The filter wheel multi-position switch is currently disabled, and the voltmeter switch serves only for engineering diagnostic purposes.

The operations are described below in the order in which they would normally be performed during an instrument setup operation.

4.1 Aperture Positioning

There is no encoder reading the aperture plate position. The activating switch should NOT be moved until the plate position can be verified visually in the TV guider screen. Two cursors, xy1 and xy2, should be positioned at roughly mid-screen in y, and separated by some thirty pixels in x. When the row of apertures is seen on the screen, the two cursors xy1 and xy2 should be placed on a straight line connecting the aperture centers, the left cursor at screen center, and the right cursor on a continuation of the same line, one aperture separation to the right. Currently the values are approximately (x1, y1) = (178, 153) and (X2, y2) = (203, 158). See the TV guider manual for details of this operation. With the cursors in this position, place a suitable aperture midway between the two cursors by briefly pressing the APERTURE PLATE toggle switch either upward or downward. This motion should be done in short steps. The guider TV has considerable time lag inherent in its "leaky memory" averaging, and the tendency is to overshoot. It is convenient to use the narrowest 6 arc-second slit (just to the right of the 4 arc-second "square').

With the comparison mirror positioned in the beam and the lamp current set to 15 ma, a 90 second exposure read with gain 3 will produce a usable spectrum that should be displayed to select a strong fairly isolated emission feature near the center of the frame. Its profile may then be examined with the IRAF task "implot". Put the cursor in the plot and type :l yyy [return], where yyy is the mean frame ordinate of the feature to be plotted. The light measured is that seen through both the slit and an isolating window whose edges mask off all other slits. The edges of the hidden isolating window can be found by shifting the aperture mask a fraction of the spacing between the cursors and comparing the new profile plot with the previous one. Normally the profile will look the same as the previous frame, only shifted in x. However, at the edge of the hidden isolating window two things happen: the x-locus of the profile jumps in a direction opposite of the aperture motion, and the profile shape will alter. If the 0.5" x 6.0" slit was used, the new "jumped" profile will be of greater area, because light admitted through the 4.0" x 4.0" or 0.75" x 6.0" slits admit more light. Locate a cursor at the TV guider position of the 0.5" x 6" slit, and reverse the direction of the aperture motion, displacing the slit by half the spacing between slits. Now expose and search for the other hidden window edge, with smaller aperture displacements. Locate the other cursor at the 0.5" x 6" slit when the profile discontinuity is encountered. Half-way between these two cursor positions is the point where the observing slit should be centered.

4.2 Filter Wheel

The eight position filter wheel is currently disabled to avoid positioning error. With the TEK#5 CCD, the filter wheel should normally be left in position 1 (open).

4.3 Comparison Mirror and Shutter

The comparison mirror is controlled with the leftmost toggle switch on the Control Box. The motor takes about two seconds to move the comparison mirror into place, and a red LED comes on. The comparison mirror sometimes sticks, and when that happens the LED will come on for only a short time -- about half a second. The recovery is to try again. The shutter is controlled by the CCD acquisition program, and the shutter control toggle switch on the Echelle Control Box is disabled.

4.4 Aperture Mask

The thirty apertures machined into the plate are listed below in order from left to right on the slit viewing TV. Note that the tiny holes (numbers 2 through 5) are hard to locate by eye on the TV monitor because of their size. Their spacing along a straight line connecting all aperture centers is uniform however, and positioning them with the help of a third cursor xy3 on the TV guider may prove useful. When slits longer than four arc-second are used the closest orders toward the red end of the spectrum will begin to overlap.

Apertures
Position Apertures Remarks

1 0.06 x 0.2 in (full open)
2 0.75" x 0.75" (arc-sec) centered
3 two 0.75" x 0.75" spaced 4.0" apart
4 0.75" x 0.75" hole, 5" below midline
5 0.75" x 0.75" hole, 5" above midline
6-11 0.5", 0.75", 1", 1.5", 2", 4" 4" slit
12-16 0.5", 0.75", 1", 1.5", 2" 6" slit
17-24 0.5", 0.75", 1", 1.5", 2", 3", 4", 8"   8" slit
25-30 0.5", 0.75", 1", 1.5", 2", 4" 45" slit

 

5. Comparison Source

A Th-Ar hollow-cathode lamp serves as the comparison source for the Echelle spectrograph. The power supply should be turned on only when the lamp is being used. The current should be set between 10 and 20 ma. DO NOT EXCEED THE 20 ma LIMIT. We may not have a spare Th-Ar lamp. Typical exposure times run to 90 seconds.
 

6. Focusing the Spectrograph

Initial focusing of the spectrograph is normally done by LCO staff. Nevertheless, changes of temperature during an observing run of greater than 3 degrees Celsius are cause for redetermining the focus setting, and so the observer should become familiar with the procedure. The basic steps of this procedure are covered in this section.

Focussing consists of taking a series of ThAr exposures, moving the lens for each exposure, and analysing the frames with the IRAF script fechelle. The lens position is controlled with a micrometer screw, and motion steps of 0.20-mm or 0.25-mm give the best results. Micrometer adjustment should always be from larger to smaller values to ensure smooth motion of the lens.

Focus observations should be made with one of the 0.75" x 0.75" holes (#3 or #4 from the apertures list), this images onto the chip as a point source. A satisfactory criterion for focus quality is a fwhm of about 2.0 - 2.2 pixels. The sharpest focus in the X-direction differs from the Y-direction by about 0.3mm in the micrometer setting. An "optimal" best setting half-way in between can be achieved with a degradation in fwhm in both axes no larger than 0.05 pixels. The observer has to decide on the compromise that will meet the anticipated goals. Caution must be exercised in deciding on a compromise with focus along dispersion (the X-axis) and along the slit (the Y-axis): the two differ just enough to require some thought.

6.1 Moving the Lens

The lens is held in dove-tailed ways clamped via two dark-anodized hand wheels some two inches in diameter. These wheels should be turned half a turn counter-clockwise, and then be tapped sharply with the heel of the palm to snap them free of "stickage." Once freed, a clockwise motion of the micrometer screw to lower values may be applied. If the value to be set is higher, always back off the micrometer by a full turn (0.5mm per turn) beyond the destination setting, and approach the final position from greater values. When the desired setting is attained the hand wheels must always be clamped. Leaving the clamps loose leaves the lens in an indeterminate location.

6.2 Running the Focus Script "fechelle"

To run the echelle focus script "fechelle" you must first load the "c100echelle" package by typing c100echelle at the IRAF prompt. Now type fechelle. The script will ask for a frame name. Since the comparison lines will retain their relative positions on each focus frame, the table of line positions need be determined only for the first frame. The script asks for the line list name. Enter a new value or accept the default. The script will then ask whether to use the existing table, or measure a new values, and offers a default (the last table used). If a new table is to be generated, the observer positions the cursor at each feature, and adds the line to the table by hitting the space-bar. As many as a hundred features may be selected. The selected lines should be as widely distributed across the frame as practical, since the true focal surface does not coincide completely with the chip surface. When the last feature has been added, the loop may be exited by striking the "q" key. If during the loop one wants to exit because an error was committed, the control-C sequence WILL HANG IRAF, requiring IRAF and the xgterm window to be reloaded! Instead, use the "I" key to produce a controlled abort. Alternatively, one can restart the task of marking the features by using the "r" key. On proper exit of the loop, the script will measure the profile of each line, and then plot a map of the measured fwhm values, with the symbol size proportional to the fwhm in each axis for each measured line, as well as the mean fwhm values in the x and y directions. A paper copy of the map may be obtained by typing "=gcur" at the IRAF prompt, which puts a curser in the plot. With the cursor in the plot strike the "=" key, and when the print - done message appears, strike the "q" key to exit. The plot will print to the Laser-Jet printer in the observing room.

 

7. Flat Field ("Milky Flat")

Fringing in the TEK5 CCD is just visible at the longer wavelengths (longward of H-alpha), so that any flat field illumination must reflect the wavelength illuminating the local neighborhood of each pixel. A sufficiently bright and smooth illumination is obtained from the day-time sky, which is very much "hotter" than any lamp (equivalent to about 10,000K), and so produces useful flat field illumination even near the H and K lines of CaII. The solar absorption spectrum may then be smoothed over optically by placing a diffusing opal glass in a filter slot on the side of the spectrograph. The opal glass is located far enough behind the entrance aperture to spread each wavelength over a bell-shaped distribution some fifty pixels in diameter. Since the order separation between adjacent orders is never less than 11 pixels, each pixel will be illuminated by light not more than 400A different from its working wavelength. The opal spreading is sufficiently smooth to remove all traces of solar daylight absorption features. The opal glass diffuser is mounted in a bracket that slides into a slot in the side of the spectrograph located to the to the far left of the focus hand-wheels on the spectrograph body, just above the face mounting the TEK5 dewar. The slide is stored in the filter loading room off of the dome floor. The slot must be kept covered with black tape when the slide is not in place.

A member of the day crew should be asked to open the du Pont dome sometime around 5pm, when the sun is far enough from the meridian to permit positioning the dome so that no direct sunlight reaches the interior of the dome or falls on the upper end of the telescope. Doing this much later puts telluric molecular absorption bands across the spectrum, and flats should be done before the last hour of sunlight. Good exposures are obtained, depending on the sky, the season of the year, and cloud cover (yes, this observing can be done in overcast conditions!) in times between 180 seconds or longer. To process the "Milky Flat" make a median-averaged copy of the bias-subtracted and bad-column-corrected frame, running a "box-car" some fifteen pixels on a side on the copy, keeping the frame dimension the same. The box-car may perhaps be made shorter in the y-direction, and longer in the x-direction. The "Milky Flat" is then divided by the box-car smoothed frame, yielding a new "flat" whose value is nearly unity over the useable portion of the frame. The edges of the final normalized flat are noisy, but, of course, these parts of the frame are not used anyway.


8. Observing


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