Zeroing the finder at Prime

Zeroing the finder at prime is best done by acquiring the test star with the autoguider probe. It is best to use the SNAFU star following the SNAFU. Proceed as follows:

1. Move the guide probe to its park position: press the guide probe PARK button on the A&G GUIDE PROBE panel, don't confuse it with the TV PARK , or type at the ADAM terminal:

   GDXY 550 900

2. Move the rotator to position angle 180with:

   ROTPOS 180

3. Track a test star within about 2of the zenith. This will usually be the SNAFU star.
4. Select the Finder TV on the Grinnell display.
5. Enable cursor 1, select SET FINDER zeropoint on the Grinnell MENU, and press the ENTER button twice. This puts up a cursor at the reference point used before. (If you have an ink mark from the previous night move the cursor to that position and press ENTER again; but this isn't essential.)
6. Select DIRECT mode to unfreeze the display. Press the BEAMSWITCH B button and the star will begin to move to the left of the screen. Wait until the star is apparently stationary and the telescope is tracking.
7. Turn on the autoguider high voltage (with the red-white button below the mini-monitor at the top right-hand corner of the control desk) and select the autoguider display (A&G) on one of the TV channels.
8. Do a FIELD integration (section ). The test star should now be visible on the autoguider display. If it is not, do a spiral search around the position, moving the telescope (NOT THE PROBE) by 60 arcsec increments until you find it.
9. When you have located the star, type AC/TEL on the User Interface terminal and use the autoguider to ACQUIRE the star (again, see section ). You are moving the telescope so that the star is at the centre of the autoguider probe when at its PARK position.
10. Select SET FINDER ZEROPOINT on the Grinnell again, but this time move the cursor to the star's position on the Finder and press the ENTER button.
11. Put the autoguider into WAIT state and type AC/PR on the User Interface console for later use.

The purpose of this elaborate procedure is to mark the position corresponding to a reference point fixed with respect to the main telescope (the guide probe centre) on the finder field at a known position on the sky (the zenith). From now on, the software will recompute the position of the CCD chip and guide probe field overlays on source change and will always show them in their correct place on the finder display.