Calibrations

Updated on June 15, 2021, 7:33 am

During the night, at every slew position and every configuration: (these are typical numbers) echelle: etalon, 180s; quartz, 120s rc mode: penray 10s, quartz 10s for KPGL3

Note that "penray" is actually a rack of a number of lamps, including neon, helium, argon, and xenon. It is supposed to emulate a more traditional (but much dimmer) he-ne-ar lamp. The line ratios are very different from those in the old lamps. The henear lamp is available but is dim and not very dense in lines.

During the day: (do all these with the fibers at the large circle position:
select "File->Run Script->largecircle (or configlargecircle)->Open" from the
menu bar in the Hydra GUI, or type "execfile largecircle" in the command window)

25 biases
echelle:
3 daytime skies @ 45s + etalon (with dome closed!)
3 th-ar, 600s
3 penray,henear, 300s
darks (I ususally leave darks running at the end of the night when I
go to bed)

rc mode:
10 dome flats (make sure that you tell ObsSup that you want the color
balance filters removed to do these. We have some blue color
balance filters on the flat-field lights on the ring of the telescope
for direct CCD work. These must be removed)
3 penray, 1s
3 twilight skies (right after sunset)

During the run once:
milky flats


The ObsSup will be more than happy at the chance to install the milky flat diffusing screen in the spectrograph. To do the milky flats, you have to bring in the fibers to the large cicle. Do this by selecting the "largecircle" script from the File->Run Scripts menu. Do the milk flats.

You may want to take some short exposures of the milks to get data for the mask image. The division of the short milk and the long milk is a good way of revealing the traps and bad pixels.

I STRONGLY suggest you worry about the dark frames. You should verify that the dark count is low. The CCD should have only a few e-/hr/pixel behind a dark slide. The spectrograph is wide open, and although we try as hard as possible to keep stray light out, sometimes doors aren't closed and light gets in. The dark count should be no more than 10e-/pix/hour. The Mosaic CCD chip should have less than 3e- read noise, so any dark more than ~10e-/pixel in your exposure is going to reduce the S/N of a faint object.

 

USING THE COMPARISON LAMPS:

In the Arcon parameter file "instrpars", set the "lampsys" parameter to "new" and select the comparison lamp you want to use in "complamp", using "pen" for the HeNeArXe penray. When you take a comparison lamp exposure (through "comp" or "observe"), the Arcon will automatically move the system into place and turn on the lamps. You can watch all this happening from the "Lamps" menu in the Hydra GUI. Note that Arcon will turn off the lamp but leave all the mirrors, etc. in place after the exposure is finished. If you go to a field and can't see any stars through the gripper camera, you've likely forgotten to park the comparison lamp system. Do this from the Hydra GUI by clicking the "Park" button.

MAKE SURE THAT THE GRIPPER IS OUT. IF THE SOME OF THE SPECTRA ARE MISSING, IT IS PROBABLY THE GRIPPER IN THE WAY.

FOCUSING:

It is not too difficult to focus the telescope using the FOPS. You can ask the night assistant to focus for you and he (she) will use the following procedure. Remember that the FOPS are fiber bundles of 7 1 arcsec fibers. Choose a single FOPS with a star on it near the field center. The idea is to get most of the light into the central fiber by changing the focus.

The object will move during focus and the FOPS will lose centration. I found to focus, you must do the following:

  1. Move the focus about 10,000 units above nominal focus in IN direction.
    Check centration.
  2. Move 2500 units in OUT direction. Check centration.
    ...etc.
  3. Go through about 10 focus steps and pick one with best intensity and smallest xy movement through image.
  4. When the focus is set, note the temperture of the "Serier Truss" by typing "settemp" on the IRAF command line. If you set "setfocus" to "auto" in "instrpars", then Arcon will look at the difference between "temperature" (which is updated every time you type settemp) and "reftemp" and adjusted the focus for change in temperature. Just make sure that you have entered the temperature at which you focussed the telescope in the "reftemp" parameter and the correct focus in the "basefocus" parameter. If you'd rather enter the focus value yourself for every exposure, set "setfocus" to "yes". Setting it to "no" will cause Arcon just to use the focus value in the "telfocus" parameter.
  5. To modify the focus as the temperture changes by hand, note that the focus changes as df/dT = -766.2mu/deg C.

Typical values:
12.8 degrees 156200

TELLURIC OR SPECTROPHOTOMETRIC FLUX STANDARD:

If you are using echelle more or observing longward of 6000A and need high S/N spectra, you probably need a telluric standard. It is best to do it through a few fibers. You can do it one fiber and exposure at a time. Alternatively you can bring in a number of fibers sequentially. Put the fibers at the large circle or park position. Bring one fiber into the center. Start a long exposure on the CCD (3600s). Expose for say 100s and then "pause". Then:

unwarp
gripin
park 000
move 001 0 0 <== some other fiber here.
warp
gripout

and expose for 100s starting with "resume" on CCD window. Etc. (Check to see if light is leaking in from gripper light.) You can do this on about 4-6 fibers with the telescope in open loop tracking.

Alternatively, you can put a FOPS down on one guide star (and have the object in the center). After moving the a new fiber to the center and warping the plate, recenter the guide star on the FOPS. You can turn on the FOPS guider to guide, but I recommend just guiding by hand on these short exposures.

You can get the *.hydra fibers of the tertiary spectrophotometric standards of Baldwin and Stone (1984, MNRAS, 206, 241; 1983, MNRAS, 204, 347) as calibrated at CTIO by Hamuy et al. (1992, PASP, 104, 533; 1994, PASP, 106, 566). These are 11-14th mag stars. I also have *.hydra fields for the secondary spectrophotometric standards (also called Hayes standards) as give in the Hamuy reference. These have fluxes every 16A. The stars are 4-6th mag.

The coords are taken from USNO-A2.0 and have the standard in the center with FOPS stars and skies scattered around in the field. See the directory:

/uw52/nick/hydra/text/specphot/*.coo

A selected list of bright, hot stars culled from the Yale Bright Star Catalog can be found in:

/uw52/nick/hydra/text/bstar.dat