Subject: 3.5m M1 status/science operations suspended

From: elt@astro.Princeton.EDU

Submitted: Thu, 23 Sep 1999 09:37:21 -0400 (EDT)

Message number: 383 (previous: 382, next: 384 up: Index)

Science operations at the APO 3.5m are suspended until further
notice in order to allow critical engineering work on the primary
mirror and its support system.  This work is necessary to insure
the short term safety of the primary and to initiate a longer
term project to remediate the problems which have been recently
discovered.  [Although this amounts to an extension of the current
shutdown for installation of the new secondary and other planned
tasks, the added down time is needed for unrelated and unanticipated
work.]  See apo35-general message #379 for a first report of this

I am happy to report that the primary is not in any apparent
danger of failure or of being (further) damaged and that there is
now a reasonably clear path forward which is expected, though
not guaranteed, to bring the telescope back into safe operation
within a period of perhaps 10 days to a few weeks.  Moreover,
it does not seem likely that the existing damage to the primary
has compromised its figure or performance.

An "executive summary" of the current situation is given below:

- Small but significant "clam shell" cracks have been found in the
  walls of two of the mirror's internal hex-cells at the positions
  where the feet of a transverse spreader metallic support are
  epoxied to the unfinished glass surface.  In addition, a very small
  crack has been located in a third hex-cell.

- The two hex-cells with the larger cracks are located on the
  rim of the mirror at positions 180 degrees apart and which are
  the highest and lowest cells on the mirror when the telescope
  is parked in the horizontal position.

- The cracks in at least one of the two more damaged hex-cells,
  and perhaps all of them, were already present three to five
  years ago, as can be seen in some old photographs of the
  primary's back plate.

- Experts from Steward Observatory Mirror Lab., where the mirror
  was fabricated and polished, have inspected the mirror in
  detail and are giving us a high level of support in dealing with
  this problem.  Additional experts on glass and fractures will
  probably be consulted.

- The cause of the cracks has not yet been determined with any degree
  of certainty although several possibilities are being actively
  explored, and there is evidence that more than one possible mechanism
  may have played a significant role.  However, the positions of
  the cracked hex-cells and the positions of the damage within each
  cell strongly suggest that the installation or performance of the
  primary support system is at fault.

- Deficiencies of the current primary support system have been
  uncovered during the investigation of the cracks; these may or
  may not have caused or contributed to the cracking, but it
  will probably be necessary to remediate them eventually even
  if they did not.  The magnitude of the effort that will be
  required is not yet known.

The 3.5m User's Committee will hold a one day meeting at APO next
week on September 29.  Those attending will be fully briefed on
the situation at that time; thus you may wish to consult your
institutional representative on the Committee for further details
and news after the meeting.

The team working on the telescope and mirror at APO, notably
Jon Davis, Mark Klaene and Jeff Morgan, have displayed an
extraordinary degree of professional expertise and dedication,
as well as working long and hard throughout the ongoing shutdown.
In addition, the Mirror Lab staff, notably Steve Miller and
Randy Lutz who traveled to APO on very short notice, have
provided us with excellent and generous support.  I am personally
grateful to all concerned, as indeed we all should be.

Ed Turner

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