• This document applies to the Direct Imagers, CCDS, and TIFKAM.
• It does not apply to Modspec, MarkIII, or the 8k imager.

If you are using one of the applicable instruments, it is possible to center on targets quickly and accurately using offset guide stars. This can be a substantial time-saver even if you don't have a lot of targets. The procedure has been made straightforward and convenient, as described here.

• The USNO A2.0 catalog is a list of 500 million stars derived from scans of the POSS-I (1950's era sky survey). We have an abridged version of this catalog on the disks of Agung and Hill. The USNO A2.0 has high astrometric accuracy (typically 0.3 arcsec); some stars have moved since the 1950s, but most have scarcely budged.
• The MIS guider unit views an off-axis guide star using a movable mirror. It is easy to move the mirror to a precise location. The geometry of the focal plane is well-known.
• It is therefore possible to "dial in" a guide star ahead of time, instead of searching for one.
• What's more, it turns out that in practice this is so accurate that if you position the guide probe ahead of time, and you know where on the TV screen to put the guide star, you can center on your object to within a few arcseconds -- without ever seeing it!

The description here is verbose for newbies, but it is extremely quick once you've done it a few times. First, to set up:

(0)You can do everything from Hiltner's (or McGraw's) terminal, but you need to run the program on Agung (or Hill).

(1) Pop up a terminal window using the icon at the lower left of the KDE screen on Hiltner (or McGraw). In this terminal window, type

   ssh agung         (at the 1.3m,   ssh hill)

(2) If you have an object list that you have put on Agung somewhere, change directory to where the list resides:

   cd mydirectory

(3) Launch the Python GUI skycalc with

   skycalcdisp.py

(4) Near the bottom of the gui skycalc window you'll see a button labeled Guide Star Config. Press this to get a tiny window that sets the telescope and the rotator angle. Be sure the telescope rotator is set to a precisely known angle, probably zero. Once you have the mirror cover open (!!!!) you may want to tweak the rotator to put it on exactly zero. The procedure described here depends on having a precisely known rotator angle.

(5) If you have a pointing list, load it into skycalcdisp.py with the Get object list button. This is not strictly necessary but it makes the operation vastly more convenient. The format of the list is identical to the MDM pointing list, viz.

   name_no_blanks  hh mm ss  dd mm ss  equinox

   grb090909  7 50 18.23  -0 12 13  2000

OK, now to use it:

(6) Enter an object's coords into the program. You can do this by typing them into the boxes in the upper left, or by selecting the object from the list. It also works to type your object's (exact) name into the "objname" box, and hit return.

(7) Press the Get Guide Stars button (bottom row). A temporary bright blue xterm pops up, and a big black xwindow appears. The red diagram shows the MIS field of regard on it, with the stars plotted on it. Click on your favorite star, and its guider XY coordinates appear in the blue window (this all takes up a fair amount of screen real estate so you may need to push windows around to see everything).

(8) Advice: select a guide star that is not near the very bottom of the screen -- those are actually vignetted. Avoid guide stars brighter than about 11 (but use them in a pinch). Avoid guide stars that will be ambiguous - if there's a cute little pattern to help you ID it, that's great. Also, avoid guide stars at Y greater than 10500 or so, since these make the guider lose track of where it is!!

(9) Go over to the xmis window. Select the "x:" field, and use the delete key (not backspace) to erase what's in it; replace with your X value, and hit "Enter". This moves the guide probe. Do the same for Y. The blue field above the entry boxes shows the actual location of the probe.

(10) Set on your target by putting the guide star near the middle of the screen. Center up your target accurately - e.g., with a test exposure, or by viewing the target on the CCDS slit jaws -- and once you're centered, move the guide box to the star (using the numeric keypad on the guider computer keyboard). This will be the fiducial position where you put all guide stars.

(11) Take your science exposure. When it gets near time to move to the next target, repeat steps 6-11. Carefully check your centering on the first few targets to be sure you haven't blundered, and pretty soon you'll get a feel for just dropping the guide star into the box to get a very accurate centering. It won't be accurate enough to put a star down a slit, but it'll be plenty accurate enough even for rather exacting direct programs; for CCDS work it will give a valuable check on your target ID, and make it much quicker to set up.

(12) Note -- once you're set up on target, typing q in the black Xwindow makes it go away (together with the blue Xterm). They take up a lot of room so you'll only want one up at a time.

Cautions:

• If the guide probe is driven past its limits, it silently loses track of where it is. If you suspect this, simply go to the pink "Preset" menu in xmis to select "Origin"; this sends the guide probe up against its limit switches and resets the coordinates. It only takes a minute or so and can save you big trouble.
• As noted earlier, the rotator angle has to be accurately known for this scheme to work.
• Don't assume this is working without checking your images, or spectra, or whatever. If you're using a CCDCOM-based direct imager, there is a "centermdm.py" program that automatically matches the stars in the image to the USNO A2.0 and tells you exactly where you are. The MDM mountaintop web server hosts a manual for this program.
• Not all objects in USNO A2.0 are real, and some are galaxies that are misclassified as stars. If you cannot see your guide star, it doesn't necessarily mean your pointing is wrong -- just dial in another. If you can't see any guide stars, then there is something wrong (maybe you're not looking out the dome, or it's cloudy, or your pointing is off).
• The USNO A2.0 has some blind spots because it was compiled from photographic plates. Anywhere a plate would burn out, it will be useless -- right near bright stars, in the Galactic center, and so on. You're on your own there.
• Occasionally a USNO A2.0 star will have moved significantly in the 50+ years since the POSS-I was taken. This is unusual.

Skycalcdisp.py is useful in its own right -- you can learn it by playing with it, and it has an online help text and reference manual built in. There's also an html manual (with screenshots and the like) on the MDM mountaintop web server. The java version JSkyCalc is similar to skycalcdisp.py, but it does not have any MDM-specific goodies. Java code is extremely portable; JSkyCalc it will run on Macs and Windows machines as well as Linux boxes, provided only that you have Java Runtime Environment 1.5 or higher.