The Ohio State University College of Mathematical & Physical Sciences Department of Astronomy

The first part of this meeting was a "closed door" discussion of vendor quotes for optics materials and fabrications.

Mechanical Design

Tom O'Brien gave a report on mechanical design progress to date. The highlights are as follows:

Mirror Support

This generically includes support for the large reflective optics, namely the gratings, camera primary, and collimatory mirrors. The gratings will be solid zerodur, while the mirrors will be Hextex honeycomb mirrors. A simple 3-point support will work. The Hextex mirrors offer terrific stiffness/weight performance compared to solid mirrors. In our discussions with Hextex, we have designed mounting bosses in an optimum triangle, not at the edges. We can live with what are at first-sight poor figures resulting from this support scheme because the footprints for the different field locations are small comared to the sizes of the mirrors. Thus each field location is only subject to small "wedge" distortions, even though the overall figure of the optic looks poor. The total distortion at the field corners are very tiny when the gravity load is shifted from zenith through horizon pointing - much smaller by at least an order of magnitude than our best flexures specification.

We still don't have a good hard-point design, but this is in progress. Overall the mirror support design is pretty far along.

Camera Design

The center section of the instrument is the structural cornerstone. The camera mounting ties into the primary steel structure. The basic design (figure online soon) is the camera primary and corrector cells joined by invar struts. this makes it pretty well athermalized for focus between the primary and detectors. Still needs to be some work done on the point where the camera structure ties into the detector module.

Filter Wheel

The filter wheel mounts in front of the detector/shutter system. The design is almost done, and is fairly simple. The filter positioning tolerance is less than before because there is no power on the filters (recall that an earlier design had field-flattners bonded to the filteres proper).

Detector Dewar Vacuum Window

The vacuum window for the detector dewar is a field-flattener. Pressure loading and edge thickness look OK.

Grating Tilt Mechanism The guys in the shop are busy making the parts for the grating tilt mechanism and other test systems. A prototype grating tilt system is working in the lab, but Tom hasn't had much time to work with it. First impressions are that it is looking very good.

Ohio State will host the LBT Science Advisory Committee (SAC), to be held in the Astronomy Conference room in McPherson Lab on Thursday, 2000 November 30 to Friday December 1. While a formal agenda has not yet been sent by SAC chair Tom Herbst (Heidelberg), we know that Darren will be presenting an update on MODS, there will be some discussion of the upcoming MODS PDR, and we will be giving SAC members a tour of our shop facilities.

Interested members of the MODS team and the Astronomy Department are welcome to attend open sessions.