Subject: 3.5m Collimation Monitoring Program

From: Ed Turner

Submitted: Mon, 23 Sep 2002 22:32:49 -0400 (EDT)

Message number: 613 (previous: 612, next: 614 up: Index)

Beginning in the near future and not later than October 1, a program of
regular Shack-Hartmann observations will be instituted on the 3.5-meter.
The purpose of this project is to monitor the telescope collimation.  These
observations will be carried out by the Observing Specialists and will take
approximately 15-30 minutes on each night they are carried out.  They will
require rotation of the tertiary and must be done after the telescope and
enclosure have reached thermal equilibrium, which can take up to several
hours depending on when it is possible to start the ventilation of the enclosure
and how long it takes to reach a stable ambient night time temperature.

The Observing Specialists will attempt to acquire Shack-Hartmann
collimation data on the next suitable night if it has been >=3 nights since the
previous successful such measurement.  During a middle of the night
instrument change or at the end of the night (including morning twilight)
will probably be the most convenient times to interupt science operations
for this purpose.  In any case, the Observing Specialists will discuss the
timing of the collimation measurement with the affected observer(s) on
each night in order to try to minimize the impact on science observations.
However, it is important to understand that this critical engineering
program is NOT optional; that is, the affected observer(s) may NOT elect
to forego the Shack-Hartmann observations for that night due to their
impact on the scheduled science program.  If an observer or PI believes
that his/her program should be exempt from interuption for collimation
measurements for some compelling scientific reason, he/she should email
me a request for such an exemption along with its justification well in
advance of the program's scheduled time.  I anticipate granting very few
such exemptions.

As stated above, the initial cadence of Shack-Hartmann collimation
monitoring will be once per 3 nights to the extent weather permits.  The
results of these measurements will be monitored continuously and it may
prove necessary to increase their frequency or possible to reduce it, or
even to discontinue the project, depending on what the data show.  The
time required to obtain the Shack-Hartmann data will also evolve and may
be longer than estimated above until a smooth procedure is established
and practised.

The background of this new engineering effort is as follows:  During the
conclusion of the summer shutdown, Shack-Hartmann measurements
indicated that the telescope optics were capable of delivering 0.3 arcsec
FWHM images!  This encouraging result is to the credit of the long standing
and ongoing image quality campaign and to all of the work that has been
done on the support and control of the telescope optics.  However, over
a period longer than one day and less than three days, the telescope
collimation drifted or jumped, it is not clear which, to the extent that
its limiting image quality was worse than 0.5 arcsec FWHM.  The most likely
culprit is believed to be motion (tilt) of the secondary, but that
explanaton is not definitely established, and even if it is correct, the
cause is not clear.  The goal of program described above is to allow a
better understanding of the telescope decollimation process.  If it is
not possible to fix the problem until a new top end is put into service,
which seems quite possible, a program of frequent and regular recollimation
may be required.  The Shack-Hartmann monitoring program will also allow
a determination of how this might best be done.  No recollimation during
the night will be attempted initially however.

I appreciate your cooperation with the Observing Specialists in carrying
out this important engineering project with minimum impact on science
observations and trust that we can all agree on the importance of
achieving the best possible imaging preformance.

Ed Turner

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