ANDERSON MESA - 72" News From Larry Wasserman, 31 August 2006
If you use the guider at the 72", I would highly recommend that you read this...

This memo is prompted by problems that Wes and Tom were having finding guide stars using the new spectrograph. I went out last night and found that the problem was that there is a large offset between the instrument pointing and the guider zero point. Unless you account for this offset, you will never find any guide stars.

But, once a large offset is set, it remains until reset. And, this will screw up the guider initialization for the next instrument/user unless they are aware of this problem.

I am therefore suggesting the following startup procedure for all observers at the 72" which will ensure that the guider works for everyone. Note that this procedure will only be needed on the first night of a run.

1) Focus the telescope

2) Put a bright star into the instrument, center it up and update coordinates on that star.

3) Zero the guider offsets by issuing a GZO command. This is critical if you happen to be the first observer following a spectrograph run, but is a good idea to do in general.

4) Send the guider home (GHO)

5a) For the Spectrograph – Use the GSO (Set Offsets) command (see below for details) to move the guider head until the bright star which is now on the slit is at the center of the guide screen. This may require a bit of fishing around since the offset is relatively large. Last night, with the spectrograph, I was able to center the star when the offsets on the screen were 3900/13000 microns. I suspect that you could just move the stages until you get those values – it's likely that this offset will not change much from one run to another. Once the star is centered, be sure to exit the GSO properly with F2 which will remember the offset.

5b) For other instruments – Do the same command (GSO) as for the spectrograph, but the star will not be very far away so moving it into the center of the field is much simpler.

6) At this point, you could move the telescope to a PP star to do a GOS (Orientation and Scale), but with the new camera, even the faintest PP star is a bit too bright for the GOS. I would suggest the following instead:

Do a GGS and note a star which is around 10.5-11.5. Write down its coordinates and then to exit GGS. Then do a CO command to that position. This procedure will put that star in the guide screen (remember that the probe is still at home). Then focus (GFO) the guider on this star and use it to do the GOS.

I expect I will eventually add an option to automatically find a GGS star into the GOS command, but for now it is necessary to do this roundabout procedure.

IMPORTANT: The GOS gives you the option of zeroing the offsets. Do NOT do that at this point – you just set them in step 5.

7) After doing the GOS, check to see if it works by doing a GGS and moving the stages to a few (at least two) of the stars to see if they actually appear. Note that if they are all offset from the center of the field you can do another GSO (Set Offset) to center them up (again, see notes below).

8) I note that you can do the extended GOS (GEG) at this point, but if you are observing stars or slow moving objects, it's not really necessary.

Notes on GSO:

The GSO works exactly like the GXY command – the keypad keys move the stages such that if there is a star in the field, the 8 key moves it UP, the 2 key moves it DOWN, the 6 key move it RIGHT, the 9 key moves it UP and RIGHT, etc). The size of the step is adjusted with the keypad + and - keys. The difference between the two commands is that, on exit with F2, the GSO takes the net offset between the start and end positions and adds that offset to the existing offset. This means that, (a) no matter where you wander using the GSO, only the start and endpoint positions matter and (b) once you make the large adjustment in (5), you can "tweak" the adjustment as in (7) because the adjustments are cumulative. Finally, I note that in step (5), if you do not find the star, be sure to abort out of the GSO with rather than exit normally with F2 so that you do NOT save the offset.