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

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

Note: there were no "public" MODS meetings in October, the meeting times taken up with confidential discussions of optics bids, some other vendor bids, and ISL business that took precedence (e.g., LBT aluminization).

Optics

An interesting issue that has come up has to do with using BK7 as the dewar window on the red channel. Recall that our optics design has the field-flattener acting as the dewar window in both cameras (in the blue camera the glass is fused silica).

BK7 has 7% potassium, which includes radioactive ⁴⁰K. ⁴⁰K beta-decays by emitting few keV electrons and 1.5MeV gammas. About 10% of the decays produce positrons. The decay rate for this concentration is 30 decays/sec/gram. The gammas would pass right on through the CCD for the most part, but the electrons impinging on the CCD would result in a substantially higher "cosmic ray" rate, that could be especially undesirable if we have thick red-sensitive CCDs. Bruce reported anecdotal discussions at a recent CCD meeting that suggested this may be a problem in other cameras elsewhere.

An alternative is to use NZK7, a Schott glas that contains no potassium, (ZK7 contain As, which would not be so good for various reasons). Paul is exploring costs and properties of NZK7 (it may not be cheap). Also to be investigated is verifying that this is a problem using a lab dewar with a CCD, and putting reference samples of BK7, NZK7, FuSi, and others in proximity (1-2mm) from a CCD in vacuum and comparing the effective "cosmic ray" rates.

Mechanical

Tom reported on recent piecework done by the Chemistry Department shop. They turned out good work at a very good price, much cheaper than out on the street. Their equipment and skills is a nice complement to ours, and they don't have much work lately, so we can get some help with MODS from them.

The structural design and analysis study for MODS is nearly complete. Flexure of the collimator assembly at the ends of its long support tubes, as predicted, dominates the flexure-induced image motion budget. Flexure of the gratings is very small, and flexure of the cameras contributes even less. There are very few bolted subassemblies, so the flexure is relatively deterministic, although it would be most difficult to build and maintain a look-up table for all but coarse-grained correction of flexure. This verifies the results of earlier, less sophisticated studies that we should be working with the collimator for flexure-induced image motion compensation for the spectrometer.

One option Tom explored was to make the structure out of stainless steel. Again, the dominant term in the flexure budget is the self-mass of the tubes, so keeping the weight down drives us to tubes with the appropriate diameter to provide the necessary strength-to-weight performance. A nice feature of stainless steel tubes is their ready availability, ease of manufacture and handling, and we can find pre-made all kinds of tube and coupling shapes. This would reduce the amount of special machining, design, and special material handling required. But, we do pay a penalty in weight. For the collimator assembly support members this is an issue, but certainly for the top structure (where MODS bolts to the telescope and holds the gratings and cameras) stainless steel is the way to go (weight is not the same issue there as on the long reach down to the collimator assembly).

An alternative we are explring is carbon fiber tubes for the collimator members. The can offer superior strength to weight (actually only about twice that of steel because while the carbon fibers themselves are much stronger, about 30-50% of the mass of the tube is the binding epoxy which has lousy properties). They might be affordable, perhaps adding $10K per member to cost of MODS. Being aware, of course, that small $10K increments add up real fast, we should do a full cost/benefit analysis before deciding. Also, carbon fiber tubes carry much more complex handling and "working" problems, esp. we have to take care that carbon fiber dust doesn't get into and gum stuff up while machining, etc. It is not a no-brainer of a decision.

The prototype filter wheel has come together nicely in the shop. The wheel itself was a complex part to machine for the computer-controled mill, but it cranked out two of them that matched perfectly and had no errors. We're quite pleased with the results. The filter wheel assembly came together so well, we stuck it into an anodizing run, which will help protect this part from corrosion while it sits on the shelf. It jumped from prototype to being the first "production" component of MODS because it worked out so well (why waste it). It will be used for testing of the encoding and actuator system for the filter wheels after the start of the new year.

Next up in the shop is to build a prototype grating cell. This will be used to rigorously test the mounting design and where the mount points for the grating should be. This test needs to be completed before we can finalize the specifications for the gratings and seek an order from Thermo/RGL for these long-lead time items. The delay may increase our price, we need to ask Thermo/RGL what the deal is with that.

Work is also proceeding on setting up a prototype collimator frame and tip/tilt/focus actuator system with a real collimator cell.

Schedule issues: We are currently working on a revised schedule, based on spot checks of actual performance in the last 6months against Jerry Mason's original Project2000 timeline. We are doing very well on most things so far as predicted times are concerned. Mark's work has been bang on for design and testing of components, and that is proceeding quite well. Some items, like the prototype collimator mechanism parts cam in much faster than expected, and the structural design exercise is way ahead of schedule.

On the flip side, however, we also missed a few things, including time to design and fabricate a handling cart for the MODS gratings. Fabrication times for some items are also 50-100% longer than expected, so we will be revising the work schedule accordingly. The schedule is currently under review.

We can anticipate two major tweaks to the schedule in the future:

1. We will be pausing to work on the LBT aluminization system when it arrives in Columbus. Last word is that won't happen anytime before March 2002, could be into summer depending on Ansaldo's disassembly and packing schedule, and the shippers' schedules.

2. A still indeterminate schedule pause will be waiting for the optics to arrive. That depends on negotiations on work schedules with the vendors for the optics currently in progress. It is clear, however, that delivery of the finished optics by the vendors is on the critical path for all phases of MODS deployment.

Rick and Bruce will be traveling to Tucson for the next LBT SAC meeting in early December, and Darren is going to be in Chile until mid-January. The next MODS team meeting will be in December after Autumn Quarter classes are over (probably the 17th or 18th), but before the break for the holidays.

R. Pogge, 2001 November 23