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You are here: Home Telescopes Irenee du Pont Instruments Website WFCCD The WFCCD in Multi-Grism Mode THE WFCCD IN MULTISLIT AND LONG SLIT GRISM MODE1

THE WFCCD IN MULTISLIT AND LONG SLIT GRISM MODE1

Ray Weymann, Bill Kunkel, Andy McWilliam, Mark Phillips

September 1999R

evised Miguel Roth, Mark Phillips, Nidia Morrell

December 2003


Table of Contents


1. Introduction

This manual describes the use of the Wide-Field CCD (WFCCD) camera for obtaining multislit grism spectra direct images and long-slit 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 with a modest radial degradation, and has good transmission from 3750-9000 .. 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 and long-slit spectroscopy.

 

Remember that you must send in your set-up form prior to your run.


2. Slit Masks


There are several slit mask holders, together with the screws and holddown bars 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! Several long slits permanently mounted in their holders are available. In particular there are 2 useful long slits. These are:

 
      1. A 1.65 arc-sec slit (150 microns), and

      2. A 8.65 arc-sec slit (750 microns) used for spectrophotometric standards.

 

These two long slits are off centered in such a way as to avoid a bad column in the detector and, more importantly, to get a full wavelength coverage on the detector.

 

Preparation of the metal masks is covered in a separate document on this same web page . Make sure to send in you mask generation files as far in advance as possible. Masks submitted later than 6 weeks prior to the run can not be guaranteed. Avoid painful experiences...

 

The metal masks are fabricated on Las Campanas, and will be handed to you upon arrival.

 

To mount a mask, orient the mask frame holder so that the curved piece is away from you and the small screws holding the hold-down 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 hold-down bars (only two of the hold-down bars are currently used for the metal masks). 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 hold-down bars over the mask and screw them in. If the 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. Someone on the technical crew will assist you with this and other set-up procedures.

 

4. 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 . and for the red grism is about 6000 .. The blue grism gives about 3 ./pix, and the red about 4 ./pix. Slits .6 arcmin from the center of the field will enable coverage of roughly 3800-7600 . for both grisms.

 

A higher-resolution grism sometimes referred to as the "H & K grism" is also available. This grism has a dispersion of 1.3 ./pix, corresponding to a FWHM of about 220 km/s. The undeviated wavelength is near 3700 .. Spectral coverage depends on the location of the slit; however, wavelengths longer than ~6300 are inaccessible.

 

It is expected that an echellette will be available in late-1999.

 
See Appendix A 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.

 

Normally you will mount one grism and 3 filter in the grism/filter wheel.


 

5. Taking Data with the WFCCD

 

The TEK#5 CCD should be mounted and powered up. Your account on the data acquisition computer (clarity) will be initialized. You will decide, with the technical person present to assist you where you want to store your data (i.e. /data/yyyymmdd).

 

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:

    B
    V
    I
    red_grism

And aperture.names:

    Myfield_1
    Yourfield_2
    Ourfield_3
    OPEN

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, you must check the screen of the PC wheel controller program. A fault will be indicated by the particular button in question remaining "red". If this should occur, go to another position of the wheel until you get all blue. Then send the wheel to the desired position again. Coming in from another side often facilitates the correct positioning. With some patience, it is almost always possible to successfully seat the wheel in the desired position. This is an irritating "feature" because failing to notice this problem will result in one of the wheel motors burning out and time being lost.


5.1 Setting the CCD Gain

 

The ccd command window allows for the selection of 3 possible CCD gain settings. For direct imaging, it is suggested that gain 3 be used, which gives approximately 3.0 electrons/ADU and a read out noise of ~7 electrons. For faint object spectroscopy, a gain of 1 corresponding to approximately 1.0 electrons/ADU and a read out noise of 5.6 electrons is probably more advisable. Note that the window is brought up with the gain set at 1, not 3.


 

5.2 Starting IRAF

 

IRAF is used to display the data and to measure alignment errors for the aperture masks. IRAF and DS9 are started by means of an strong-box icon on the lower bar on the screen of clarity. Make sure that the image is displayed correctly:

 

set stdimage=imt2048

 

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 DS9 it is simple to switch polarity of the display. 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:

    wfccd


panew

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


6. Daytime Mask Exposures

 

Take a short un-dispersed image using the dome flat lamp or the light leaked into the dome and check the slits. You may want to check that a dispersed image of a flat field or residual dome light produces spectra which don't overlap. If they do, then the grism will need to be re-seated-- 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 available arc lamps, a HeAr and and a Neon lamp to get a good solution. If you want to take flat fields of for your slits, ask the person assisting you to set the telescope for flat-fields.



7. Pointing and Focusing the Telescope

 



7.1 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. On the white board in the console room, the latest value of the xy coordinates (in CCD pixels) of the zero point should be listed. Have the the operator point the telescope to a bright setup star. Take a short (~1 sec) frame and display it in the DS9. Now compute the appropriate telescope offset to place the star at the zero point coordinates. (Alternatively, you can use the IRAF task "cobject" in the WFCCD package to calculate the correct offset. Type "help cobject" in IRAF for details on how to use this task.) Once the star has been centered, ask the telescope operator to update the zero point of the telescope pointing.

 

Note that if the ring angle is substantially changed during the night, this procedure should be repeated.

 


7.2 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).

 

8. Alignment of the Field and Slit Mask



Use the following "cookbook":

 

http://vizcacha.lco.cl/~nmorrell/Manual/wfccd_guide.html

 

 

9. Alignment for long-slit operation

 

As mentioned before, the more commonly used long slits are off-centered from the center of the chip (called the Zero Point above). You can either use a pre-determined Zero Point (normally written on the white board) or determine the center of rotation by rotating the telescope Cass ring through 180 degrees and determining the center. You should also determine the position of the slit and the point on the slit where you want your spectrum.

Since you will probably be using parallactic angle, alignment of an object is not so trivial. Here is a short cook-book type recipe:

 

Use an IRAF routine called pangle. Enter the requested values for hour angle (estimate the hour angle you will be at when starting the exposure), declination and expected exposure time. The output will be the parallactic angle, the Cass position angle and the "pa" on the TV guider. The operator will slew the telescope and set the Cass angle and other values.

Since your object is going to be close to the center of rotation and you will be at an arbitrary Cass angle determined by pangle, use toslit in order to get the object close to the center of the slit. You will need to enter the coordinates of the Zero Point, those of the center of the slit and the Cass angle. Give the output generated by toslit to the telescope operator and ask him to offset the telescope by those values.

 

Next use cobject to get the object into the slit. cobject is interactive, works in every way like align and is pretty self explanatory. Iterate if needed.

Finish by taking an undispersed image through the slit and check that your object is located where you want it to be. You will have to use ad-hoc z1 and z2 parameters for the display command.

 


 
 

Table 1. Relative Efficiencies of Grisms

Wavelength

red low-res / blue low-res

blue hi-res / blue low-res

4000 .

0.59

 


4500 .

0.78

 


5000 .

0.95

 


5500 .

1.16

 


6000 .

1.25

 


6500 .

1.33

 


7000 .

1.38

 


7500 .

1.45

 


8000 .

1.48

 


8500 .

1.51

 


9000 .

1.51

 


Appendix B: Observing Hints

 


THINGS TO LOOK OUT FOR:

 
  • Try to avoid different raster or sub-raster schemes during one night.

  • Watch out for wrong GAIN setting. Notice that the default gain is 1. If you are well aware of what's going on, you can risk doing alignments with Gain 3 and therefore cutting down your cutting overhead. DONT FORGET TO CHANGE BACK TO GAIN 1 WHEN EXPOSING A SPECTRUM. Taking a long exposure and reading out with the wrong gain can be infuriating.

  • It has happened to many of us: After a superb alignment, don't forget to put in the grism if you want to see spectra...

  • When aligning, keep in mind that the positions of the masks may vary slightly with time and zenith angle. Never use slit mask images taken in the afternoon. Take you slit mask image and field images in consecutive order.

  • Check the alignement periodically if you are taking several spectra of the same field.

  • Comparison spectra (and dome flats) are taken in situ. Ask the operator to close the dome when the CCD starts reading out the last exposure. Closin the dome takes about the same time as reading out (although the light that indicates that the dome is closed takes significantly longer to turn green). You can start the arc exposure as soon as the CCD has read out.

 

 

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