Blind Offsets at Cassegrain

  It frequently happens that an observer working with the FOS wants to take a spectrum of an object too faint to be seen. Such objects should be measured astrometrically before leaving for La Palma relative to a comparatively bright reference star close to the object. An accuracy substantially better than an arcsecond is recommended. Blind offsets are made with the autoguider, because it is more accurate than simple offsetting the telescope especially in Declination.

At the telescope, proceed as follows:

1. Place the reference star on the slit of the spectrograph.
2. Acquire an autoguide star in the ordinary way with the guide probe.
3. Switch over to ACQUIRE/TELESCOPE mode.
4. Enter the offset values at the User Interface console with:

   AU/B RA-offset,Dec-offset

   where both offsets are in arcseconds. The telescope will be offset by the appropriate amount to put the invisible object (approximately) on the slit, while the guide probe is moved to the precise position where the autoguide star (step 2) should be after the telescope offset.

The autoguider will now lock onto the guide star at the offset position to centre it on the probe. When the measured star position is centred in the circle, the invisible object will be on the slit. On big offsets the autoguider may lose the guide star because the telescope moves faster than the probe. However, you can always re-acquire the guide star (with the probe still fixed).

One important drawback to blind offsetting is that when the autoguider probe is offset it may actually be carried into an area where it either vignettes the spectrograph slit or is itself vignetted by the ironmongery of the A&G box. For this reason one should try to find a reference star as close to the invisible object as possible. If no suitable reference star is found, one has to fall back on the simple telescope offset/beamswitch method referred to in section , which is inherently less accurate.