Las Campanas Observatory

THE WFCCD IN MULTISLIT GRISM MODE

Draft Version (incomplete in places!)

Ray Weymann, Bill Kunkel, Andy McWilliam, Mark Phillips
September 1999



Table of Contents


1. Introduction

This manual describes the use of the Wide-Field CCD (WFCCD) camera for obtaining multislit grism spectra on the 100-inch du Pont telescope. The WFCCD reimages a 25 arcmin diameter field onto the TEK#5 CCD camera, with a scale of about 0.774 arcsec/pixel. It produces good images over the full field, and has good transmission from 3750-9000 Å. Field distortion is minimal so that the camera can be used for drift scanning in strips close to the equator. It also produces a collimated beam with a 70 mm pupil in which filters, grisms, or a Fabry Perot etalon can be placed.

This manual provides a description of the advanced preparations, setup, and operation of the WFCCD for multislit spectroscopy.

2. Slit Masks

There are 6 slit mask holders, together with the screws and "holddown bars". These 6 holders are kept in a wooden box labeled "WFCCD SLIT MASK HOLDERS" which is in the WFCCD cabinet in the small room behind the mezzanine. Please keep all the hardware in this box and return the box to the cabinet. Please do not leave bits and pieces lying around in the library or the filter room. One of the holders currently contains a long slit. There is also a special mask for astrometry and a calibration mask for mapping the slit and wavelength distortion.

Preparation of the metal masks is covered in a separate document, which is still in preparation. In the meantime, users should contact Andy McWilliam (andy@ociw.edu) for information at least 2 months before their observing run.

The metal masks are fabricated on Las Campanas, and will be handed to you upon arrival. It is suggested that before mounting any mask, it be inspected at low power with the binocular microscope in the library. With a used engraving tool (kept in the aperture box), small remaining threads of brass or other junk can be easily removed manually. Place the blanks in envelopes until you are ready to mount them.

To mount a mask, orient the mask frame holder so that the curved piece is away from you and the 8 small screws holding the 4 holddown bars are up. There is then a single pin on the right border of the frame and two pins on the lower border. (These pins define the position of the metal masks precisely). Using a 3/32" Allen wrench, remove the 4 holddown bars. The metal slit mask is should be oriented so the single notch bears against the pin on the right side of the holder, and the two lower notches bear against the two pins along the bottom side of the holder. Place the 4 holddown bars over the mask and screw them in. If the 8 holes in the slit mask interfere with the screws, you may need to slightly enlarge them with a small rattail file.
 

3. Mounting the Mask in the Aperture Wheel

Up to 3 mask frames can be mounted into the aperture wheel; the 4th position should be left open. The wheel is accessed by loosening the two red thumb screws and sliding off the cover. You will be holding the mask with the curved slot facing down, and the observer looking up toward the dome during this operation. Push the frame gently into the wheel and the wheel will "grab" the frame.
 

4. Hooking up the PC

The PC which controls the aperture and filter/grism wheels should be in place and the cables connecting the PC to the controller, and controller to the WFCCD itself should be hooked up. The cable connecting the data acquisition computer, Canopus,  to the PC should also be hooked up. Turn on the power for the PC and controller if the day crew has not already done so.

The PC which controls the aperture and grism wheels runs under Windows 3.1. When the PC is brought up it will display a panel with "Monitor & Control" as one button, and "Configuration" as the other. When clicking on "Monitor and Control", the application window will overfill the screen. Click and hold the left mouse button on the upper blue border and drag to the left so you can see all 4 buttons for both the aperture wheel and the filter wheel. Clicking on any of the 4 aperture wheel or filter wheel buttons will move the wheel. The wheels can also be controlled from the CCD data taking window (see section 7).
 

5. Grisms

There are presently two low-resolution grisms, both of which give a FWHM resolution of about 375 km/sec. The undeviated wavelength of the blue grism from a central slit is about 4700 Å and for the red grism is about 6000 Å. The blue grism gives about 3 Å/pix, and the red about 4 Å/pix. Slits ±6 arcmin from the center of the field will enable coverage of roughly 3800-7600 Å for both grisms.

A higher-resolution grism sometimes referred to as the "HK grism" is also available. This grism has a resolving power of R~1,400 (roughly 1.3 Å/pix), corresponding to a FWHM of about 220 km/s. The undeviated wavelength is near 3700 Å. Spectral coverage depends on the location of the slit; however, wavelengths larger than ~5300 Å are inaccessible.

An echellette grism is now available. It provides a resolving power of R~3,000 (FWHM~100 Km/s), with spectral coverage from 3700 to 9000 Å, in three orders. An inter-order gap is present in the interval 6200-6700 Å. As with the HK grism spectral coverage depends upon location in the field. Presently, there is no cross-dispersion available, for the echellette so orders, or parts of orders, must be isolated with filters. To fit the echellette grism into the available space it was necessary to vingnette the beam; as a consequence its transmission is low.

See Appendix B for the further information on the performance of these grisms.
 

Let the LCO staff know well in advance of your run (via the WFCCD instrument setup form) what items you wish mounted in the filter/grism wheel. The grisms have been carefully mounted in their cells; alignment of the grisms in their cells and the cells in the filter wheel is described in a separate technical document. It should only be done by one of the LCO staff who know how do to this.

If you are not planning to use more than one grism during your run, and have need for at most one of the 3" circular filters, it is suggested that you use the "standard filter wheel" complement. If you elect to do this, you can use the cookbook focus procedure for the spectrograph, focus using the two Hartmann masks. This complement has the filters arranged as follows:


 

6. Taking Data with the WFCCD

The TEK#5 CCD 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, or ask one of the day crew to do this.

6.1 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 is to be stored. For example, In this directory, create two files called, respectively, filter.names and aperture.names. In each file, enter 4 names with no blanks in them. For example, filter.names might look like: In the console window in the directory where the data is to be stored, 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 wfccd. This will automatically select the correct dewar and other appropriate defaults, including the number of overscan lines. 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.

In the ccd command window are menus for the Aperture wheel and Filter wheel. Selecting one of the 4 positions in each will move the wheel(s) to the correct position.

CAUTION: There may still be occasional problems with either the aperture or filter/grism wheels not properly seating in their detents. Whenever the position of either of these wheels is changed, it is a good practice to check the screen of the PC wheel controller program. A fault will be indicated by the particular button in question remaining "red". See section 4 for instructions on how to correct this problem.

6.2 Setting the CCD Gain

The ccd command window allows for the selection of 3 possible CCD gain settings. It is suggested that gain 3 be used, which gives approximately 3.0 electrons/ADU and a read out noise of ~7 electrons. Note that the window is brought up with the gain set at 1, not 3.

6.3 Starting IRAF

IRAF is used to display the data and to measure alignment errors for the aperture masks. To start IRAF, first open and 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:

The orientation of the CCD image (and the TV guider) will be a function of the Cass ring rotation angle as illustrated in Figure 1. However, with ximtool it is simple to switch polarity of the display. Inverting the left/right (i.e., columns) display will yield "sky polarity". Column numbers on the chip are not affected, but on the display, columns increase toward the left (rows still increase to the top). With the Cass ring angle set to 270 degrees, east will be towards the bottom of the screen, and north to the left. With the Cass ring angle set to 180 degrees, then north is to the top, and east to the left.

Finally, to load the IRAF tasks that will be used at night for aligning the field and slitlet masks, type:


 

Orientations of CCD and TV Guider

Figure 1. Orientations of CCD and TV Guider for the WFCCD

 

6.4 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, and the "imtype" button can be used to set the image type.  Clicking on the "start" button will then begin the exposure.

Note: There is a new feature of the ccd control system which displays a small green window which says "abort" , 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 3 and the default overscan of 16 lines, the total readout and storage time is ~70 seconds for the full chip.

6.5 Defining Subwindows

This feature allows one to read out up to 8 small rectangles on the chip, shortening the readout time. This can be useful during the procedure for aligning the field and slit masks (see Section 10). As outlined in this section, the IRAF mksubr task in the WFCCD package should first be used to generate a file containing the subraster coordinates for each of the masks to be used during the night. The subrastering is then activated as follows:
 

7. Focusing the Spectrograph

The spectrograph will normally be focused by the day crew at the start of your run. Should this not be the case, or should you feel that a refocus is required, refer to section 2 of the Aligning and Focusing the WFCCD Optics manual.
 

8. Dewar Alignment

Alignment of the dewar rotation should also be carried out by the day crew, but is also covered in the Aligning and Focusing the WFCCD Optics manual.
 

9. Daytime Mask Exposures

Take a short dispersed image using the dome flat lamp. For a slit near the center of the field, the direction of dispersion should be very near 90 degrees near the central wavelength. None of the spectra should "cross". If they do, then the grism will need to be rotated -- but this should only be performed by knowledgeable members of the LCO staff.

For both grisms, wavelengths increase with decreasing column numbers. The wavelength solution is not well approximated by a linear fit. Use both HeAr and Neon lamps to get a good solution.
 

10. Pointing and Focusing the Telescope

10.1 Initial Pointing of the Telescope

The mask design program prints out the RA and Dec of the position at the nominal mask center. The pixel on which this position of the mask lands does not coincide with the instrument rotator axis and therefore will depend upon the Cass Ring angle. Have the the operator point the telescope to a bright setup star. Take a short (~1 sec) frame and display it in the ximtool. Now run the IRAF task "zpoint" in the WFCCD package. This program prompts for the Cass ring angle and has hidden parameters corresponding to the rotation center on the CCD. The ximtool cursor will appear, and you will be asked to hit "space bar" with the cursor placed on the star, and then hit "space bar" again with the cursor at the approximate location of the mask center. The task outputs the required E/W and N/S offsets to place the setup star at the mask center. Ask the telescope operator to update the zero point of the telescope pointing.

Not that if the ring angle is substantially changed during the night, this procedure should be repeated.

10.2 Focusing the Telescope

The telescope should be focused in the normal way by using the "focus" function in the ccd command window. Enter some number (e.g., 7 or 9) in the loop field, set the exposure time, and then use the focus button in the ccd command window (see the CCD manual for more details).
 

11. Alignment of the Field and Slit Mask

For each field, the following procedure should be followed for insuring that your objects are aligned with the slit mask:
 

Appendix A: Observing Hints


 

Appendix B: WFCCD Performance


 

Appendix C: Suggestions and Aids for Data Reduction

Appendix D: Slit Mask Fabrication

Slit masks are fabricated from brass, or aluminium, plates with a computer-controlled milling machine at Las Campanas. The milling machine code is generated from sky positions by use of program "maskgen.e", which is available on the Carnegie Observatories ftp site.

Input to "maskgen" includes id number, X (arc sec), Y (arc sec), priority, magnitude, Position angle. Input is in free format; data beyond the sixth column do not affect operation of the program. Note that positions are relative to the field center, with positive X to the East and positive Y to the North. The slit PA, measured in the usual sense clockwise from North, is only used if the user requests tilted slits in the interactive session. An example input file is shown below.

Sculptor 0, mask 5 
   ID   X(arsc)  Y(arsc)   Priority   Mag.    P.A.
  9028  -487.03   175.15    -1.00    13.42     0.00
  9055  -544.44  -309.28    -1.00    14.68     0.00
  9031   382.77   137.40    -1.00    13.61     0.00
  9025   275.20  -270.04    -1.00    13.30     0.00
 10001     0.0    100.0      0.0     13.0      0.00
 10002   260.0    100.0      0.0     13.0      0.00
 10003   520.0    100.0      0.0     13.0      0.00
    44  -650.83    11.46     16.90   16.90     0.00
    43   607.59   -10.60     16.95   16.95     0.00
    56   519.86  -152.24     17.11   17.11     0.00
    64   462.51  -384.99     17.14   17.14     0.00
   100   -94.16   159.28     17.26   17.26     0.00
    98   378.38   262.68     17.31   17.31     0.00
   111    37.34  -625.07     17.32   17.32     0.00
    79  -537.72  -238.84     17.33   17.33     0.00
    96    89.10   292.48     17.35   17.35     0.00
   134  -279.86  -190.27     17.37   17.37     0.00
    84    49.52    44.83     17.37   17.37     0.00
   144   277.17   -67.92     17.39   17.39     0.00
    82  -609.85    38.28     17.39   17.39     0.00
   110  -282.72   360.20     17.40   17.40     0.00
   129  -277.62   412.20     17.43   17.43     0.00
   135    97.93  -506.50     17.45   17.45     0.00
   161    22.00   184.44     17.46   17.46     0.00
   127   341.55   -40.79     17.50   17.50     0.00
   119    83.25   -83.31     17.51   17.51     0.00

An arbitrary number of comment lines can be placed at the top of this file; the user is queried for the number. Notice the negative priority of the first four objects, which indicates alignment stars; they take priority over all porogram objects.

The next three objects with priority 0.0 are optional, but recommended if holes are to be used rather than slits; these objects are part of a grid of sky holes.

Finally the list of program objects. Notice that the priority and magnitude are the same. Since smaller numbers have higher priority the program will choose the brighter object when clashes occur.
 



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Last modified: Sep 29, 1999