Rotator Centre

After a twilight sky, you must determine the rotator centre, to enable the TO to do a calibrate.

• Point at a bright calibrator star (the TO will find a suitable one, say around 10th-12th mag).
• From TCS, tv on and in autoguider console, fm 3. You should be running version 4.4.3 alpha of the autoguider software (as of May 1996).
• Find the X,Y rotator center position (in AF2 coords) from the last run in the AF2 folder, or **here**. In ICL, type afvsky X Y, where X,Y are the values you just found-this will move the mobile probe to that last position.
• You should be able to see the star in the mobile probe autoguider window (the left one). If not, you will have to try moving the telescope to find it. You could also try a brighter calibrate star, and make sure it's not cloudy! As a guide, the field-of-view of the mobile probe is approimately 30 arcsec by 20 arcsec and the scale is 0.15 arcsec/pixel.
• Once you see the star, go into eng mode on the autoguider, and type ccdcur in the autoguider. You should then see a cursor on the mobile probe autoguider window, which you can center on the star with the arrow keys. Once the cursor is centered on the star, hit return on autoguider console, and the location of the cursor (star) will be given (in CCD chip coords).
• Change the sky PA by 180 deg, and you will see the star move in an arc (about the rotator center) on the autoguider window. Hopefully the star will stay in the window.
• Center the cursor on the star again, and note the cursor location again, exactly as before. Alternatively you can guide open loop on the star as the sky PA changes, closing the loop at the beginning and end of the rotation will give the mobile probe X,Y co-ords of the star on the TCS info display at the two different Sky PAs.
• The rotator center position is given by the midpoint of the two locations. For instance, if you measured the star to be at 122, 67 for PA=0 and and 179, 47 for PA=180, the rotator center (still in autoguider CCD xy coords) is at 150, 57.
• On the ICL type agcuro x y (e.g. agcuro 150 57). This will move the autoguider cursor to the rotator position you've just determined.
• Use the TCS to center the star on this cursor position. The star should now be on rotator center. Try moving the Sky PA back by 180 degrees to make sure star stays on rotator center.
• Until now, the rotator center has been determined only in autoguider CCD xy coords. To determine the rotator center in AF2 XY coordinates, first center the star in the mobile probe window by moving the robot probe (af2_offsetrobot dx dy, where dx and dy are the desired shift in x and y). When the star is centered, read and record the probe position from the ICL mimic-this is the new rotator center position in AF2 coordinates.
• The TO can now focus the telescope using this star. After that, the TO can do the CALIBRATE. For each CALIBRATE star, type afvsky Rotx Roty (where Rotx and Roty are the rotator center coordinates you determined in the previous step), to center the star in the mobile probe window-this will improve the CALIBRATE precision.