The program gs24 (read "GS two-four") finds guide stars for the 2.4 meter MIS offset guider. It should make it possible to select guide stars in advance and "dial them in", so you don't have to waste time hunting for guide stars in sparse fields.

Usage Summary

The program is launched from a terminal window on Agung by typing

gs24 [target list] [rotator angle]

• target list -- an MDM-style pointing list.
• rotator angle -- the instrument rotator setting, which defaults to zero if not given.

Description:

The program should be fairly self-explanatory. It looks up stars in the USNO A2.0, selects the brighter ones (with a bright cutoff around 5 or something), and figures out the guider XY coordinates for the stars. It then plots them on a map of the focal plane, with a vignetting circle and an approximate location of the forbidden zone where the guide probe will shadow the detector (see the Cautions and Gotchas just below). If you select a star with the cursor its coordinates will be typed out; you have to write 'em down to record them.

There are two Cautions and Gotchas!!:

• Note carefully that the guide probe can obscure the CCD in certain positions. The suggested limits given by the program may not be conservative enough to clear your field of view. They do seem to be fairly good for the Echelle chip with a 2-inch filter wheel, but that's a little smaller than the field of view with the four-inch filter wheel, so caveat emptor. A good cloudy-night exercise might be to take domeflats with the mirror moved closer and closer to the center in each coordinate, and see where the shadow begins to appear when the test frames are divided by a guaranteed unobscured picture. If you do this please share your wisdom with the community.

• Also note carefully that if the guide probe is commanded to go past its limits, it will silently lose track of where it is, in which case you won't be able to set it to a star correctly. If this happens Bob assures me that sending the probe back to "Origin" is sufficient to clear it, but to be safe you may wish to hit "Initialize" in xmis, which will surely restore the coordinates.

Pointing lists

If you give a pointing list, the program cycles through all of them. Low-latitude fields will have so many stars that you'll see right away if you can just take "pot luck" when you get to the field; some high-latitude fields may be pretty empty, and in those cases you can save precious telescope time by finding a guide star in advance and writing it down so you can "dial it in" when you get to your field.

One-time use

If you don't give a pointing list on the command line, the program assumes you want to do a single object and prompts you for the coordinates, which you give as sexigesimal triplets (e.g. 18 22 22.3 for 18 hours, 22 minutes, 22.3 seconds). It also prompts for the rotator angle (zero for most observers).

Limitations of the USNO A2.0 ...

The USNO A2.0 catalog from which the stars are selected does have some limitations. Most importantly, in any location where a sky survey plate would saturate, it knows nothing. This will present a problem in the burned-out areas around bright stars. The catalog is also empty in a few super-dense regions near the Galactic center, and in the centers of bright galaxies and globulars, but those cases aren't of any practical importance, since you'll surely be able to find guide stars at random in those regions.

The USNO magnitudes are to be taken with a large grain of salt (they often use negative magnitudes to flag some kind of problem, for example), but they're better than nothing.

The USNO's strength is in its astrometry, which is excellent. However, the A2.0 is based on 1950's-era Palomar Sky Survey plates, so an occasional star will have moved significantly by the present time. This well seldom be a problem, especially since we just need to be fairly close.