Field Acquisition

Acquiring the field is relatively simple.

• GOCAT to field on TCS (remember the rotator to AF2 offset- Section 4.2.)
• Check telescope is at the correct sky PA for the field (and correct RA and Dec - check equinox!)
• View a guide star on a fibre near the centre of the field with afvobj n (n is the pivot number).
• Centre up the star on the cross of the mobile probe using X,Y offsets.
• View a second guide star at the field edge (afvobj m) and centre up as best as possible using telescope rotation. Check a few other stars at the positions of guide fibres using afvobj (you should have at least 4 bright 13;SPMlt;B;SPMlt;15 stars on guide fibres in the field).
• Park the robot (afpark) and you should see the stars coming down the guide fibres on the TV. Note that the fibre-to-fibre spacing on the guide bundles is approximately 1 arcsec. The TO can tweak them up to come down all the fibres, or you can run the af2_acquire program to calculate the shift and rotation. N.B. Due to a slight offset between mobile probe co-ords and AF2 co-ords (1.7 arcsec in X, -1.5 arcsec in Y - measured during May 1996 run), the X,Y shift reported by af2_acquire may not be quite correct - but the rotation (which is the most difficult thing to get right) will be good. **check this** Once the rotation is accurately determined, it should be a straightforward task to get the guide stars down the fibre with X,Y offsets.
• Make sure you take at least one arc and offset sky (typically 200-300 sec) during the observation of each configuration. The offset skys are essential to ensure the correct normalisation of the fibre transmissions in the program frame (which WILL change from configuration to configuration due to vignetting). It is always a good idea to take at least 3 offset skys (in different directions) and take a median, in order to get rid of any faint stars that will inevitably turn up in at least one of two fibres in one or more of the offset skys. The ability to define the fibre transmissions is usually the limiting factor in doing sky-subtraction with fibres (2% is good). (A `flat' twilight sky is also very good for sky normalisation - it contains lot of counts and won't be affected by faint stars). At least one arc is also advisable for each configuration in order to ensure that an arc spectrum is going down every non-parked fibre in that configuration.
• So that the data reduction routine wyf_red can recognise which data frames are arcs, skys or program frames each should be taken with different commands (it should be obvious which is which):
  • arc wyffos 10 "Arc at program field"
  • sky wyffos 200 "Offset sky at program field"
  • run wyffos 1800 "Program Field"
• After sufficient integration has been achieved on each field, the TO should move the telescope to the next field, and the observer should re-setup there, after the telescope has stopped. You should not do a new fibre afsetup until the CCD has read-out, since the CCD shutter is not lightproof against the fibre backillumination used during a setup. It is also a good idea to wait until read-out is finished before moving the telescope. You might save a few minutes but lose an hour's worth of data! Finally, the telescope should not be moved while fibres are being moved.