Newsletter - 24 August 1993

This is another update concerning the current status of instruments
at CFHT.  Comments and suggestions about the newsletter may be
emailed to newsletter@cfht.hawaii.edu or newsletter@uhcfht.bitnet
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SIS ACTIVE GUIDING UPDATE
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The SIS active guiding was having troubles since December 1992. After
extensive investigation, the following problem(s) were identified and 
corrected:

- Intermittently failing quadrant on the SIS photomultiplier unit. PMT 
  unit was replaced.
- Optical fiber feed of the guide star to the guider slipped in its
  holder, feeding a defocussed image to the PMTs. The optical fiber was 
  refocussed and secured in its holder.
- One of the piezo actuators on the active mirror was not responding to
  the guide signals. It was replaced with a spare.

These three failures account for all problems encountered during
previous runs.

After these fixes, we were able to put the SIS active guiding back into
operation during one engineering night (August 19, 1993). SIS and active
guiding are at this moment being successfully used by observers, with images
recorded with FWHM better than 0.5 arcsec, and spectroscopy conducted 
under stabilized imaging conditions. The SIS user interface to initiate
active guiding has also been much improved.

As a consequence of the fine tuning of the guiding system, the limiting
magnitude for active guiding is now V=15.5 for 20 ms integration for 
the active guiding loop, and a S/N=3 per photomultiplier channel.

We are still planning to replace the current SIS guiding optics and PMTs
by a more efficient optical train and avalanche photo-diodes to increase the
limiting magnitude for active guiding to V=18.5. This upgrade will 
probably not be operational until the end on the 1st semester 94.

Olivier Le Fevre (lefevre@cfht.hawaii.edu)

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