Subject: Status of APO 3.5-meter Telescope

From: elt@astro.Princeton.EDU

Submitted: Wed, 27 Oct 1999 14:11:29 -0400 (EDT)

Message number: 390 (previous: 389, next: 391 up: Index)

As described in the attached copy of a posting to sdss-general which
was issued on the evening of 21 October, the Sloan 2.5-meter's secondary
has been seriously damaged.  The cracks were first discovered on
19 October, and the state of the mirror is still being investigated
carefully.  So far as I know, the cause of the damage has not yet been
definitely established.

These events are the cause of the further extension of the 3.5-meter
shutdown which was also announced on 21 October (see apo35-general
message #389) and for the delay of return to science operations
significantly beyond 25 October.  The mounting and motion mechanisms for
the new 3.5-meter secondary which had just been installed in the 
telescope are very similar in design to those of the Sloan secondary.
Moreover, Jon Davis has identified a failure mode of this mechanism
which *might* produce damage to the secondary similar to that which
has befallen the 2.5-meter M2.  (To be clear, it is not necessarily
thought that this failure mode is the actual cause of the Sloan problem.)

Given these facts, it would have been irresponsible, of course, to put
our new secondary into operation in its new, "Sloan-like" mounting
cell, etc. and before clear diagnosis of the cause of the damage to
the 2.5-meter secondary.

Two steps are now being taken to allow the 3.5-meter to return to
science operations as quickly as practical after implementing 
the necessary safety systems.  First, the new secondary will be mounted
and moved using the cell and mechanisms previously used with our old
secondary (i.e., not the new "Sloan-like" system) after a small amount
of necessary retrofitting.  Second, a long-planned (conceptually, at
least) set of safety mechanisms (software, electronic and hard mechanical
limits) will be implemented in the secondary and tertiary motion
mechanisms.  Both projects are being pursued as rapidly as possible
on a "crash priority" basis.  These steps should make it possible to
operate the telescope without undue risk to M2 and M3.

It is estimated that this new work will require roughly three weeks
beyond the previously planned return-to-operations date of 25 October,
but this figure cannot be regarded as very firm.  A more definite date
for resumption of science operations will be announced as soon as it is
possible to do so.

Ed Turner

>Dear Colleagues:
>
>As many of you may have heard by now, the secondary mirror of the Sloan
>2.5-meter telescope has suffered some damage to the central part of its
>faceplate.  We have spent much of the last twenty-four hours assessing
>the extent of the damage.  It was decided last night to relieve some of
>the gravitational loading from the mirror and this morning to very
>carefully remove the mirror from the telescope.  This seemingly rash
>move was prompted by the conviction of everyone involved that the
>loading of the mirror on its central support posed a real danger of
>further, possibly catastrophic, damage in its weakened condition. 
>
>The preparation and removal went very smoothly and the mirror is
>now safely in its shipping crate in the plate-loading building.
>The central support structure has been successfully removed and
>all speculative theories as to what caused the damage have apparently
>been refuted; we will spend the next few days attempting to ascertain
>what happened and how future repeats can be avoided.
>
>We have been in contact with the Mirror Lab and with Hextek; the mirror
>lab folks are guardedly optimistic that once the damaged area is removed
>the mirror will be OK; previous experience with lightweight hex-cell
>mirrors indicates that the figure damage due to the relief of stresses
>at breaks does not propagate very far, and we do not use the damaged
>portion of the mirror at all optically because of the large central
>obstruction in this design.  How long this cleanup, subsequent testing,
>and any necessary support rework will take is not clear at this point,
>but it IS clear that there will be no November run.  We will keep you
>informed as the situation progresses. 
>
>Needless to say, all involved are severely disappointed, but there
>is no apparent reason why essentially all of the intensive development
>work facing us in the next few months should not go ahead on an
>accelerated schedule, and we can hope that the survey start will not
>be excessively delayed.
>
>John Peoples
>Jim Gunn
>Bill Boroski
>
>
>
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