SNAFU at Prime Focus

  The SNAFU at Prime Focus is done completely differently, because there is no TV camera for field viewing, and involves the taking of a test GLANCE exposure on the prime focus CCD, under ADAM control. One approach would be to simply follow the procedures for Cassegrain focus (sections and ), and move the star iteratively to the SNAFU reference point on the CCD by means of repeated GLANCE exposures on which the star's position is determined by using the LIST option of PHOTOM. The reference point is obviously the centre of the chip. However, the following procedure is more convenient, since it requires only one GLANCE to be taken, and makes use of standard ADAM functions:

1. Move the rotator to position angle 180 with the ADAM command ROTPOS 180.
2. Select the finder on the TV monitor at the Control Desk.
3. Slew the telescope to the SNAFU star, as for Cassegrain mode.
4. Startup ADAM (See the Prime Focus User Guide).
5. Take a test GLANCE exposure of the field (1-5 seconds; use filters if the star is very bright). The CCD image will appear on the LEXIDATA and the star should be obviously visible near the centre of the chip. If it is not, either the top-end change over has gone catastrophically wrong (is the telescope control software configured for Prime?), or you have done something silly, such as mis-typing the SNAFU star coordinates.
6. Use the MOVESTAR command, and locate the star with the cursor. ADAM wil now calculate a position correction and send those via the inter-processor link to the Telescope Control Computer. The telescope will move to the corrected position: the star will move on the Finder TV image.
7. Mark the position of the star now on the Finder TV screen with a felt pen or similar marker. This position corresponds to the centre of the CCD detector.
8. Now send the telescope again to the original position by pushing the SRCE button.
9. Type at the User Interface console:

   SNAFU

   and move the telescope with the handset such that the star image on the finder will coincide with the position marker put on the screen with action 7.

10. Press the ACK button on the Control Desk.

The reason behind the last three steps is that although ADAM sends the corrections to the Telescope Control Software, this does not update the pointing model zeropoint. That is done in the same way as for Cassegrain mode during the last three steps.