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

Attendees: Darren DePoy, Pat Osmer, Bruce Atwood, Paul Byard, Tom O'Brien, Mark Derwent, Jen Marshall & Rick Pogge,

Optics

Ohara has sent information about the BK7 blank it has produced for the red camera corrector lenses. Paul plugged the index of refraction homogeneity numbers into Code-V and the piece is acceptable. We will approve the numbers and then Ohara will ship us the piece. This is about 3-6 months ahead of the original schedule.

We also received a quote on a piece of NZK7 for the field flattener, to help us assess the cost impact if we have to use a zero-potassium glass rather than BK7 to avoid radiation effects from the latter on the CCD (the field flattener lens is also the dewar window and will sit only 2 millimeters away from the CCD). The radioactivity of BK7 and its impact was discussed in the 2001 Nov 21 meeting.

The collimator and camera primary mirror blanks made by Hextek are done and the vendor would like to ship them. Darren will we out in 2 weeks for observing and will inspect the pieces and decide on acceptance then.

We are awaiting the arrival of the pyrex imaging flats we have purchased from JML. These have the same mounting fixtures as the eventual zerodur grating substrates, and will be used with prototype grating cells to test our grating support design before we commit to purchasing the gratings (they are expensive, long lead-time items, and we want to get it right).

Mechanical

The prototype grating cells are being fabricated now, and should be ready when the flats arrive from JML for integration and testing. We anticipate assembling the first test system in early-to-mid March.

The first two filter wheel assemblies are basically done, but only one has been assembled for testing. Mark Derwent will be doing these tests in the next couple of weeks and report on the results at a later meeting. Overall, the machining of these complex parts and their assembly went better than expected. There were virtually no mistakes in the machining, and are considered among the hardest-to-make parts that will be in MODS. [Note: a filter wheel does not seem that hard, but these have a lot of fine machining work in them, like the small bits for the radially-mounted, user-insertable filter cell fixtures].

Tom has tested the camera support trusses, measuring in particular the lateral stiffness. The test was done by building a test support truss with the prototype invar truss members and two of the corrector lens cell parts to form the top and botton structures (left picture below). The bottom ring of the stucture was bolted to the milling stage of the Haas CNC mill to provide a very stable horizontal base (it is our best rigid table). The second picture below shows Tom pressing on the top ring with a force gauge. A micrometer on the opposite side of the top ring is used to measure the amount of lateral motion in the top ring before and after putting it under stress.

The results are very close to predictions. Going into the tests he was most concerned about verifying the behavior of the ball joints and adjustment nuts (like turnbuckles). Everything performed extremely well. There is less than 0.5% hysteresis in the structure, and the entire ensemble worked as elastic elements without ticks or sticks. This result confirms the mechanical soundness of our basic adjustable support truss design, and we can feel confident to proceed with making the rest of the parts for all of the cameras. Tom is very pleased with the results of these tests.

A prototype collimator mounting system has is being erected in the mechanical lab to provide early indications of the expected performance of the collimator mirror tip/tilt/focus motions. We need these performance numbers to inform the final design of the flexure compensation system. After the rig is assembled, it will undergo testing for repeatability of actuator motions over the next couple of weeks.

The design of the primary structure for MODS is proceeding well. Tom is now doing all the detail work. The big job right now is working on the area around the focal plane of the instrument, where we have to fit in the slit mask exchange/storage mechanism, guide cameras, calibration system, etc. This is a crowded place with lots of constraints.

In exporations to determine how we will get the structure fabricated, Tom and company have been visiting various local steel fabricators to see what our options are. It would help us regarding labor and schedule if we could get a big steel fabrication shop to make it for us, but at what cost to the project? We could conceivably do it in-house, but we would need to outsource a lot of the larger parts because we are not equiped to machine such large pieces.

With an LBT Board meeting in Tucson and Darren's trip to visit Hextek to inspect the mirror blanks, we will probably not meet again until either late January or early February.

R. Pogge, 2002 January 18