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CTIO Instrumentation News (1Dec94) (from CTIO, NOAO Newsletter No. 40, 1 December 1994) A major activity during the past three months was the 4-m shutdown, which took the telescope out of service for four weeks starting 8 August. During this time we installed the active primary mirror support system (see accompanying article), realuminized the primary mirror, made improvements to the ducting for the primary-mirror cooling system, installed insulating material on the interior of the telescope structure above the primary mirror, and worked on the radial support system for the primary mirror. All of this went very successfully except for the last item. The primary mirror is supported in the radial (sideways) direction by 24 counterlevers that are attached to the side of the mirror by epoxy. We found that the epoxy joint had broken on one of these supports on the SE side of the mirror. We reglued the broken joint and put the telescope back together. For reasons which we are still trying to understand in full, during the test period after reassembling the telescope, the reglued joint plus the epoxy joint on a different radial support detached themselves. The net result was two broken supports instead of one. This seemed unacceptable, so we removed the primary mirror from the telescope again, reglued the broken supports, and reassembled the telescope. This was a grueling five-day job for Jorge Briones and his crew of telescope mechanics, which they undertook with great cheerfulness and competence. Then when we started testing the telescope again, the second radial support (the one that had been reglued just once) detached itself again. At that point we decided to leave the telescope in operation while we tried to figure out what was going wrong. Subsequent laboratory tests have shown that our gluing procedure sometimes produces very weak joints, and we believe that this explains why the same radial supports are failing twice in a row. It is unclear why the glue joints failed in the first place, although this has been a continuing problem throughout the life of the telescope. We plan to take the telescope down for another week in May 1995, and at minimum reglue the broken joint using improved procedures. The 4-m shutdown was a large effort that depended on having a considerable number of people put in their maximum effort for a long period. We are especially grateful to Eduardo Aguirre, Gale Brehmer, Jorge Briones, Mario Fernandez, Eduardo Huanchicay and Oscar Saa from the Telescope Operations division on Cerro Tololo, and to Fabian Collao, Rodolfo Diaz, Eduardo Mondaca, Juan Orrego, Victor Pasten, Gabriel Perez, Fernando Saleh, German Schumacher and Pedro Vergara from the La Serena Engineering and Technical Services division. On a different front, mechanical engineering design work has now started on the f/14 tip-tilt secondary for the 4-m. The mechanical, electronics and software parts of this project are being done at CTIO. At the same time, the optical work is now underway in Tucson, under the direction of Gary Poczulp. The mirror will be reduced in weight by boring a number of holes in its back surface, and then ground and polished. The target date for delivery of the polished mirror is the third quarter of 1995. CTIO is aiming to have a mirror cell ready to receive it at that time, with a computer- controlled collimation system suitable for active optics applications and the basic piezo-electric tip-tilt actuators installed. The actual tip-tilt implementation will then follow during the next several months. This will require the addition of a tip-tilt detector system with software for calculating the guide star centroid and sending the appropriate commands to the piezos. The tip-tilt detector will be on a remotely controlled x-y stage mounted at the straight-through position on an instrument cube installed at the Cassegrain focus. Dichroic mirrors will permit the tip-tilt detector to view a guide star while light is sent to an instrument at one of the side ports. The initial instruments will be IR and optical imagers. Eventually we hope to also offer an IR spectrograph. Work on the Arcon CCD controllers and characterization of CCDs during this period is described in a separate article. The Arcons have reached new low levels of readout noise and continue to be enthusiastically reviewed by our users. The major effort on the user interface part of the software is now winding down; the very considerable efforts of Pedro Gigoux and Steve Heathcote have led to a very popular user-friendly interface that continues to get even better during this mop-up phase of their work. Pedro will now gradually be switching over to work on the 1.5-m telescope control system. Dan Smith and Gary Webb are continuing to work full-time on other aspects of the Arcon Software. In addition to the projects mentioned above, the mechanical group has been designing a new filter wheel and shutter assembly for the CCD system on the Curtis Schmidt telescope. The unit will then be fabricated by the University of Michigan, in a spirit of shared investment in improving the telescope. J. Baldwin
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