COSMIC Notes





COSMIC is unique in many respects for Palomar. It is the only narrow line imager. It is also the only multiobject spectrograph. Some general notes:

1) Making the Slit Masks


2) Mounting and Installing the Masks

You will probably need to start mounting and installing the slits around noon, so that you will have time to take all the needed calibration data before dark. The reason is that you will need to take calibration data for every slit mask you intend to use that night, and this can take a really long time, given the COSMIC readout time.

The stuff you will need to mount the slits. There should be two bags full of machined slit mask holders waiting for you (the day crew will get them). There will also be tape in the bags. Bring some liquid paper with you. The lightbox is probably in the office off the control room. You'll also need to borrow some screwdrivers from the office as well.

Trim the slits so that that they have a border like this. They must lie flat in the holder, if you have to pinch them to fit them in your slit alignment will be off. You will probably see some tiny pinholes in the mask. These will show up as out-of-focus doughnuts in your spectrum. Using a toothpick and some liquid paper, cover these up. There is a type of marker you can get at a professional photography place to do the same job.

This is a slit mask holder. There are two types in the bag. You need the ones that look like this (they notches and such are machined a little different). Note that it is laid out as if you were about to put a slit mask on it, with the notch closest to the mask at left. Place the mask such that the writing is backwards and in the upper right corner, when the slit holder is positioned as it is here. Try and line the box marks up as closely as you can with the slit holder. After you've attached the cover, use thin slices of tape to cover up the box marks along each side, as well as the writing on the mask. When you are done, the only light shining through should be from the slits.

Looking down from the prime focus elevator into the prime focus cage. You will unzip the black shroud over COSMIC in order to place the slit holders into the slit wheel. You do not need to remove the shroud. The blue box is the COSMIC controller. You will use the inc and dec buttons to spin the filter wheel. Note - tell the day crew you are heading up to prime focus. Also, the interlocks on the elevator are pretty finicky - be sure to solidly close all the gates, or they will lock out the telescope.

You place the slits into the slit wheel, using the blue box to advance or reverse the wheel. Write down which holder is in which wheel position. The holders are inserted and twisted slightly, they will snap into place. Some of the holders are a bit sloppy in the wheel. Note the tape used to cover the four edges of the slit mask to blot out the writing and alignment box. Also note the little bit of tape used to secure the holder. There's very little clearance for the wheel, though. You will need to leave at least one position open, and you will probably want to leave another with a long-slit in, for standard stars and such.


3) Basic Theory of Slit Alignment

This part was incredibly unclear to me from the instructions, so I will try to explain it here.

The basic problem with using a spectrograph like COSMIC is that it is impossible to assemble all the components (like the slit masks) with such precision that they simply work when pointed at the sky. Instead, the masks must be actively aligned after they are inserted. This is critical, since if the targets aren't right down the slit you will lose some if not all of your sensitivity. The basic steps are these:

  1. After you have inserted all of the slit masks into COSMIC, you will turn on the cal lights, withdraw the dispersion grating, and take images of each slit mask. Each FITS image will just be an image of the mask. Note that this has to be done for each slit mask each time it is inserted into the wheel. If you change masks around during the night, or remove and put one back in, you must repeat this step. This happens during the day.
  2. Then, you will run a piece of software that will step through the slits. For each slit you will mark the center and each end. This will generate a text file to be used later. You will need that printout of the slits with numbers on them for this step.
  3. After you've opened and done the usual stuff (focus, pointing check), you will slew to your target. Withdraw the dispersion grating, and rotate the slit wheel to the open position (remember?). Now you can take an image of the sky.
  4. Now you'll need that finder chart, labeled with the numbers of the slits. Using the provided IRAF scripts, you will identify a few of those bright objects you included on the slits. It will generate centroids, compare them to the known slit locations (from the second step), and generate offsets and rotations.
  5. The offsets get sent to the telescope. The rotation is sent to COSMIC - it has a rotator that will rotate the detector relative to the sky. You will probably need to repeat this whole process again in order to fine-tune the positioning. While this all sounds complicated, once you have done it once or twice it is really easy. You'll waste much more time waiting for the ccd to read out.