Locating guide stars at the Cassegrain focus

The best way to find guide stars is by using the Finder TV probe. The Grinnell menu function GET GUIDE STAR draws an overlay on the display of the Finder field which shows the locations of the spectrograph slit and the area in which guide stars may be selected (Figure ). It is very important that the zeropoint of the finder be determined beforehand, in order to enable the TCS to map the guide probe acquisition area accurately onto the finder field.

 

1. Slew the telescope to the object to be observed and move it onto the spectrograph slit using the Cassegrain TV camera.
2. Press the FINDER button followed by the DIRECT button on the Grinnell Panel.
3. Select the Grinnell menu function GET GUIDE STAR: a horseshoe-shaped acquisition field will appear on the Grinnell TV monitor (figure ). Any stars which appear within the permitted area of sky can be reached by the guide probe without obscuring the spectrograph slit. Choose a star which is visible without integration (otherwise it will be too faint for the autoguider).
4. Move the grinnell cursor to the desired guide star position and press the ACCEPT button on the joystick. The probe will now move to this star.
5. Select probe mode by typing at the User Interface:

   AC/PR

You are now ready to set up the autoguider and acquire the guide star as described in section furtheron.