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4-m Telescope to be Shut Down for Four Weeks...(1Jun94)
in August
(from CTIO, NOAO Newsletter No. 38, 1 June 1994)

The CTIO 4-m telescope will be shut down for an unusually long
realuminization and engineering run starting in early August.  We are
presently planning to take 29 days, probably 8 August - 6 September.
The realuminization is routine (and would take over a week by itself),
but there are a number of other things that we need to get done while
the mirror is out of the telescope.

These include:

                        Installation of Active
                    Primary Mirror Support System

The 33 air bags that provide the axial support for the mirror will be
"re-plumbed" with air lines leading to 33 individual air pressure
controllers, which will be under computer control.  We will also be
able to continue using our old air-pressure controllers, so the plan is
to carry out just the basic installation in August and then use
engineering nights over the next several months to debug the new active
system.  We believe that this is a low-risk way to make this changeover.

                      Inspection and Alignment of
                    Primary Mirror Radial Supports

As is noted in an accompanying article, the telescope's astigmatism at
large zenith distances has recently increased.  The new active axial
support system should be able to remove any reproducible components of
this astigmatism.  The problem most likely comes from the radial
supports and we obviously need to try to correct the problem at its
source.

                     Improvements to Air Ducts for
                     Primary Mirror Cooling System

We will install some missing baffles and make other modifications to
this system, which has been in routine daytime use for the past several
months.

		    Inspection and Possible Repairs to
		           Declination Drive

We had a close call a few months ago when one of the declination drive
motors suddenly stopped working.  The drive uses two torque motors
working in opposition to each other.  Fortunately, it turned out that
after we used the surviving motor to move the telescope a small
distance, the other one came back to life again.  Since it takes three
days of down time to remove and inspect these motors, we will do it
during the August shutdown when we can schedule it, rather than risk
having the telescope die for three days during somebody's observing run
(as was almost the case).

                    Removal of Unnecessary Counterweight
                               from Chimney

The chimney (light baffle) sticking up from the center of the primary
mirror has been carrying around an extra ton of weight for the last 20
years.  This was installed instead of the flip-in tertiary mirror for
the (never-implemented) coud focus.  This extra weight causes
undesirable amounts of flexure in the primary mirror cell, so we feel
it is high time to lighten the ship.

The 29-day schedule for the shutdown includes 6 test nights at the end,
as well as 5 contingency days on top of 18 days of planned work.  We
hope that this will let us deliver the telescope back to the scheduled
astronomical observers on time and in good shape.

Jack Baldwin

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