Problem(s) Encountered:

I cannot find the guider star using JSkyCalc, so have to use some fainter stars in the guide camera. Do I need to manually set an offset even if I'm using the field center?

Solution:

I assume you're taking direct images or using the center slit, so the 'field center' box is checked in the guide star selector.  That's key, since the coordinates sent to the telescope by "Coords->TCS nextobject" are offset otherwise and you won't find your objects.

Sometimes when you get an object centered where you want it the guide star you've dialed in from JSkyCalc will be a little outside the guide camera field. In those cases you need to move the MIS X-Y guide stage a little bit to bring it back into the field.  I think the last time I was consistently adding 200 to the MIS X coordinate before looking for the guide star, or else it was right at the bottom of the guide field.

Here are some things you can try.

First, if you haven't already, on xmis pull down the preset positions menu and go to "origin".  This will back the probe up against the limit switches and re-zero the coordinates. If you haven't done this it's possible that all your troubles were due to a mis-calibrated guide probe.

If that doesn't do it, the next thing to try is to center a bright star in the OSMOS field and then look for a known guide star and find the offsets needed.  Here's a way to do that:

1) On xmis, set guide probe to "center".

2) On JSkyCalc,  dial up "nearest bright star" (I think it's called).  This will pull up a star with mag < 6 nearby. "Read guide stars" to get the guide stars, and then slew to the star by setting "Coords -> TCS next object" and slewing.

3) Be sure Maxim DL is reading out continuously.  Use the telescope paddle to find and center the star in the guide camera field. It will be extremely bright so you should find it by just paddling around.

4) Be sure OSMOS is set up to take a direct image. In prospero, call roi1k to save readout time.

5) In JSkyCalc, set up a bright guide star for this target, and move the probe to it.  The bright star will disappear from the camera.  WRITE DOWN the coordinates that JSKyCalc sends the guide stage to.

6) Without moving the telescope -- which should track open-loop pretty well -- hop the guide probe around to find the guide star. Assume you know nowthing about the direction, try them all.

7) Once you've found the guide star, note how far, and in what direction, you had to move the guide probe to pick up the star. If you apply that 'dx' and 'dy' with each new target, you should find the guide stars without too much trouble, if the telescope is pointing well.  If it isn't pointing great, you can set up the dx and dy and then paddle around a bit to find the guide star..

8) If you want to be a little fussier, take some OSMOS images and move the telescope to center the star on the detector where you want it.  Note where the guide star is on the guide camera, and if you had to hop the probe over to get it into the guide field, note the new offsets.

This shouldn't take as long as it took me to write it down, so it may be worth your time. [Thorstensen]