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

Attendees: Mark Derwent, Dan Pappalardo, Ross Zhelem, Jerry Mason, Ray Gonzalez, Bruce Atwood, Jason Eastman, Tom O'Brien, & Rick Pogge

MODS Detector Systems

Bruce started with a show-and-tell of the black dewar with the detector box and 4K detector mount with thermal test piece installed. So far the work is going well, the usual routine start-up issues with sealing and such as expected. The boil-off rate with the full rig is about 7l/day, about as expected. So far the system is cooling very slowly, a more precise quantification of "slowly" will require more data this week.

An issue that has to be addressed during this testing is how we will support the detector box when the system is off the camera. This should be straightforward.

Bruce reports that we should be receiving a mechanical sample of the 3x8K CCDs from e2v soon. He is still trying to parse the latest progress report, but early indications are that the initial CCD wafer runs has "low yield" of blue devices and "very low yield" of red devices. Realistically, this all probably means we won't be seeing blue science-grade 3x8K devices until the later part of this year, and red devices sometime in early 2008.

Mike Lesser is sending us a new blue 4K CCD, it should arrive later this week. The flats he sent show some blocked columns, but we can manage with this for initial testing. Blue QE below 400nm is less than with the "red" device, but it'll do for initial work.

MODS Optics

Ross reported on the Blue Corrector Lens acceptance tests. Using better analysis tools, Jason measured the on-axis spot FWHM of 1.3 pixels, which corresponds to a camera PSF with a 80mas FWHM. This is well within the specification (110mas FWHM max). D80 is 2 pixels, again well within the design on-axis image quality specs.

Initial off-axis images show larger FWHM (3pixel and 5pixel, bottom and top, respectively, same left-to-right) on the off-axis corners. These are best-focus FWHM. The main difference is due to tilt in the focal plane that has not been removed by re-alignment of the optics. If this tilt is factored in, the numbers for the image quality are consistent with the off-axis design specification. This verifies that Blue Corrector #1 meets our specifications. We will install Blue Corrector #2 and subject it to the same tests. If it passes, both will be packaged and sent out for coating.

Full fine alignment of the camera optics will not proceed until we have the final coated correctors in hand, and will use the Aluminized camera primary for the final blue camera. The current round of acceptance testing has fully validated the measurement and alignment approach that Ross developed.

Jason and Ross have tested a sample of a visible-blocking filter for the IMCS laser channel (a 1.55-micron laser). The filter keeps unfiltered visible light from leaking into the CCD camera. Each camera filter wheel position has a corresponding IMCS filter position above it (just off axis) that needs one of these 1-inch diameter filters. Tests show that with a slight field tilt the filters provide ~70% transmission in the IMCS IR laser band, and <10^-4 transmission in the CCD bands, so they meet or spec. Ross is exploring possible vendors as we need 32 filters minimum (4 filter wheels at 8 filters/wheel), plus spares. One vendor quoted us $70/filter, but they're just resellers so we are trying to find a direct source.

MODS Enclosure

Tom reported initial results on his structural analysis of the MODS enclosure design.

The panels in the hexagonal top section will require diagonal bracing to work correctly. His design is a cross-brace using rods and turnbuckles in the corners. Aluminum rods work better than cables in this application because they will not stretch like cables, and their CTE matches that of the aluminum framework. There is only one interference with an ingress point, at the blue camera filter wheel, but it should not be a proble to omit the braces on that panel since they'll be present on the others.

Wind loading analysis using manufacturer's data on OmegaLite panels shows that they can handle a 2400-lb maximum stagnant wind load. The maximum computed wind load is 1000-lb, so they have plenty of margin. The panels are quite strong, and can survive a constant wind load of 80mph with no problems only supported on the perimeter. Adding the cross-braces to provide a center support improves this performance over the bare-panel loads.

While doing this analysis, Tom updated the MODS mass and center-of-gravity (CoG) spreadsheet, now including real data on the enclosure, the external electronics boxes, and the detector dewars. The current total mass is about 2300kg, will probably be 2500kg in round numbers in the end. The CoG is now 1.1-meters behind the mounting balls, a little further than before, but still well within margins.

Using this Tom is now looking at ways to lift the instrument, working out a rigging plan. There was some discussion of the concepts (with or without a load spreader, etc.), and Tom will come up with a detailed plan that can work inside our shop, the mountain Instrument Lab, and for lifting it to mount/dismount on the LBT.

Mark reports that the miniMODS prototype enclosure is about ready for light-tightness testing in the lab. They've validated a scheme for 2-panel removal for instrument servicing that works, but involves some complicated instructions for removing bolts. We may need to color code the relevant bolts - he and Dave Steinbrecher will think about this some. Currently they are running down light-leaks along various seams and gaskets.

Overall the prototype is proving its worth. The final work will be to make sure with all the appendages and modifications that it all fits within the LBT direct Gregorian envelope. Once the final enclosure design is done, drawings will be sent to LBTO for verification that we don't violate our envelope before proceeding.

The last steps are to start compiling the total parts list (which will be long), and then make up the order. Mark expects about 1 month needed from start of making the list to receipt of the kits from item. We can expect a May/June timeframe for assembly of the enclosures on West Campus. The process will need some extra hands (visualize a high-tech barn raising and you get the idea), but it should be fun.

MODS Insturument Electronics

Dan has taken delivery of the ethernet switch and has completed the in-box ethernet service fan-out for IEB#1. He's had a few problems working with the WAGO serial interface, mostly getting used to the new software applet from WAGO, but Ray has it going with the ethernet interface w/o problems.

Ray is ready to receive IEB#1 and start working with it in the lab.

MODS Schedule

Rick gave a brief presentation at the end of work on the assembly and integration plan. The example given was the integration and testing of the MODS Focal Plane Suite, the set of mechanisms that are common to the blue and red channels that reside mostly above the focal plane. These are the dark hatch, calibration system, AGW stage, mask storage and selection system, and the dichroic select, plus the assoicated optics (calibration optics, field lens, red folding flat, and dichroic). Technically the red folding flat and dichroic select are part of the red channel optics, and their integration can be deferred to red channel integration.

This part of the instrument is the most complex, with the greatest density of mechanisms, and with the only intersecting mechanisms that require hardware and software interlocks to prevent collisions (namely, the AGW stage and Calibration Tower intersect, and the mask storage and insertion mechanisms work together). It also has the greatest overlap among the team disciplines (electronics, mechanics, software and optics), and completing this integration is a major milestone. An integration flow chart has been developed and Mark is turning it into a detailed Gantt chart with resources and time estimates. After review, the notion is to begin focal plane suite integration around May 1 or so.