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Zeroing the main telescope and the finder

It is not critical whether this is done before or after focusing. It does assume the normal state of affairs - i.e. that neither the CCD nor the autoguider is seriously out of focus: look back through the log to other observers' efforts.

1. Zeroing the telescope (alias SNAFU)

   (N.B. This process is greatly simplified if the rotator is at a cardinal point). At the beginning of the night, slew the telescope to a star of accurately known position. The star should always be visible on the chip, even immediately after a top-end change, because the field of the CCD is large. If the star is not there, either the end change has gone catastrophically wrong or you have done something stupid. If you have problems, check that the telescope control software has been reconfigured for the Prime Focus, not Cass,(see the INT Manual). If all is well, select SNAFU mode on the user interface console and move the telescope so that the star is at the apparent rotation centre (which is normally set up to be the centre of the CCD chip (see Chapter 2 for details of how to do a GLANCE and to determine the position of the star in pixel coordinates using the LIST option of PHOTOM). The centre of the RCA chip is at 160, 256 (192, 289 for the GEC chip) and the pixels are 0.74 and 0.54 arcsec across, respectively. If the rotator is at a cardinal point, then the Lexidata display has North and West marked, so that it is easy to see which way to move the telescope (pixel coordinates have 1,1 at the readout corner). Convert the displacement from the centre to arcsec by multiplying by the pixel size and move the telescope by increments. To exit from SNAFU mode, press the ACKNOWLEDGE button on the control desk.

   The centre of the chip is approximately 10 arcmin away from the optical axis, but the telescope pointing is adjusted to keep the object at the centre of the chip for any rotator angle.

2. Zeroing the finder

   The finder flexes with respect to the main telescope by 110 arcsec in RA and 75 arcsec in Dec. The former effect is very reproducible; the latter has a maximum uncertainty of 45 arcsec in the mean relation.

   This effect must be calibrated out if guide stars are to be located using the finder display. The zero point of the relation must be reset at the beginning of the night; thereafter adjustment is automatic. Proceed as follows:

   Track any position within about 2 deg of the zenith.

   Move the guide probe to its park position: press the guide probe CENTRE (really means PARK) button on the A&G panel or type

   Adam:> GDXYPOS 550 900

   Adam:> ROTPOS 180

   on the ADAM terminal.

   Enable cursor 1, select SET FINDER ZERO-POINT on the Grinnell menu, and immediately press ENTER. This puts up a cursor at the approximate reference point. Fine-tune (if you know better) and press ENTER again.

   Select DIRECT to unfreeze the display and move a bright star to the cursor position using the handset.

   Turn on the autoguider high voltage (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. Do a FIELD integration (section 1.6.2). The star should be visible on the autoguider display. If it is not, do a spiral search around the position, moving the telescope (NOT THE PROBE!) by 30 arcsec increments until you find it.

   When you have located the star, type AC/TEL on the user interface terminal and use the autoguider to ACQUIRE the star (section 1.6.2).

   Select SET FINDER ZERO-POINT on the Grinnell again, but this time move the cursor to the star's position on the Finder and press ENTER.

   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.