Locating guide stars at Prime Focus

The standard method of finding guide stars for the Prime focus CCD Camera is to locate them on the wide-field Finder display. Having reassured yourself in advance that a suitable star exists (see above) and having zeroed the finder model at the beginning of the night (section ), proceed as follows:

1. Move the rotator to your required position angle with the ADAM command ROTPOS. The default angle on startup is at 180.
2. Point the telescope to your object to be observed; take a GLANCE to check its position.
3. Select FINDER and DIRECT on the Grinnell panel to route the raw finder image to the monitor.
4. Offset the telescope so that the guide probe field becomes visible on the finder. This done by pressing the BEAMSWITCH A button on the Control Desk. The offset is made in a coordinate system fixed to the Prime Focus Unit; its value on the sky depends therefore on the rotator Position Angle.
5. As a result of pressing A, stars should appear to move along the major axis of the chip as the beamswitch takes effect and this will be visible on the finder image. If they move otherwise, or if nothing happens, perhaps the default aperture offset values have been overwritten. The correct offset values are set up on startup of the control software; to check the values, type at the User Interface console:

   AD(VISE)/T(ELESCOPE)

   Aperture A should be set to 0,-590 arcsec. If you have accidentally overwritten these values, type:

   AP(ERTURE)/A 0 -590

   and press A again. The aperture in use is shown on the Information Display with the offsets converted to RA and Dec.

6. Select the Grinnell function GET GUIDE STAR. A display of the field accessible to the probe will appear on the screen. Move the cursor to your chosen guide star in the permitted area and press ENTER on the Grinnell joystick control box. The coordinates of the star will be transmitted to the instrumentation computer which will then move the guide probe to the correct position.

   A star will be bright enough for autoguiding if it is visible on the finder with 4 frames of integration at full TV gain. Avoid very bright (6) stars.

7. Finally press the BEAMSWITCH NOM button on the Control Desk to return the telescope to your object and press DIRECT on the Grinnell panel to unfreeze the TV display.

The telescope is now pointing to your object, which should be positioned close to the centre of the CCD detector, while the guide probe is positioned such that the guide star is at the centre of the acquisition area. The guide star must now be acquired with the guide probe, and the guide loop enabled as described in section below.

Note: if the acquisition of a guide star fails, the most likely cause is that the mapping of the autoguider acquisition field onto the finder has gone wrong. This will occur if the TV scales are more than about 5 percent off the correct values, and these should be checked/redefined with the ADAM command TV\?SETSCALE. Nominal values at Prime are 3.38 arcsec/pixel in x and 2.75 arcsec/pixel in y (see section 3.1.8 of the INT Prime Focus User Guide).