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 problem. 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 APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 383 in the apo35-general archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-general/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO