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

Attendees: Darren DePoy, Paul Byard, Jen Marshall, Chris Morgan, Dan Papallardo, Pat Osmer, Tom O'Brien, Mark Derwent, Bruce Atwood, and Rick Pogge.

Mechanical

Tom and Mark visited Indian Creek, a steel fabricator near Dayton, to review issues about how to build fixtures for the upper structure of MODS. Indian Creek has already completed work for us on the Grating and Camera handling carts. The discussions went well, and will help us reduce the risks in the top structure fabrication. We are currently preparing a fabrication package for the upper structure. In our discussions with them they told us that they could get us a more accurate quote on the upper structure with a whole package than by components, and we now know better how to dimension things for fabrication. We are currently discussing delivery times as well.

Mark Derwent has also been working on the design of the MODS Instrument Cart (note: one of the fabrication fixtures to be built by Indian Creek will become one of the main pieces of this cart). Only one structural element of this cart is critical for flexure testing, namely the top support element that replicates the Gregorian docking fixtures and which has about the right stiffness to be a reasonable structural surrogate for the Gregorian ring for in-the-lab flexure testing. The handling cart has no requirement to handle zenith pointing at the telescope: all handling of MODS at the telescope will be done in the horizon-pointing orientation at a fixed rotation angle. The final design is still progressing but should be done soon.

In the lab so far, Tom has assembled and been testing the first of the camera primary focus mechanisms. The MODS cameras are focussed by translating the primary mirror of the camera. The total focus travel is +/-2.5mm, with a +/-25-micron tolerance. Because the camera is decentered, the mechanism must move the camera in such a way that it maintains proper optical centering. So far it has come together nicely. The preloading is good, and the tensioning of the drive belt is as we want it. Remaining to be tested is drive motors under power, and the lateral stiffness (which parameter maintains the centering of the mirror).

The first complete camera assembly is coming together (mounts, struts, filter wheel, etc). Still to be integrated are the dummy optics, the focus mechanism (described above), and the shutter. The handling carts are proving to be a great help, allowing us to work on the cameras standing up, and they allow a lot of flexibility of motion to put everything together.

Other experiments on-going are working on epoxy release procedures for the grating mountings. This should allow us to be able to dismount and remount the gratings from their cells. Experiments on the current procedures are in progress with Tom and Jerry Mason. Mark is working to fine tune the last details of the grating tilt mechanism.

The last set of parts for the collimator truss assemblies are being fabricated by the ASU shops. They've done great work for us so far (thanks!), but they will be on a short haitus while they relocate to new shop facilities.

In design work, Tom is about to start detailing the multislit mask mechanism (storage cassette and insert/retract mechanism). The first cut at design of the calibration system has gone very well, the parts will fit and integrate nicely. Tom has put this aside for now to focus on other problems. The Front-side Acquisition/Guide stages are done up to the point where we need to have details of the optics and camera designs to proceed further.

Optics

The first two MODS gratings have been delivered by Richardson Gratings (a division of Spectra-Physics, the grating maker formerly known as Thermo/RGL and Bauch & Lomb in earlier incarnations). These are the low-resolution Red and Blue gratings (see the MODS Gratings page for details). They look bang on spec. The bluegrating has a slight bit of power on it (0.27 waves), but initial calculations show this does not present any issues. There is also a glitch in the interferogram that runs parallel to the rulings, about the location of a barely visible line seen from behind (we have not yet unpacked the gratings from their carriers and inspected the frontsides, though we can look through the zerodur substrates at the the back of the rulings). Bob Martin at Richardson reports that this is a 1/10 wave glitch in the master itself, but it is very minor and so presents no problems. Richardson has also provided us with electronic versions of the measured blaze curves, which Rick is using to update the instrumental performance estimates (see the MODS Estimated Performance page for details).

Steve Miller at SOML has sent us the interferograms and structure function measurements for the first of the four camera primary mirrors they are figuring for MODS. So far they have exceeded our structure function specification, and so we can declare this mirror essentially "finished" except for remaining trimming and drilling work. They are now moving the blue camera corrector parent to the polishing machine to begin work on the blue camera corrector lenses.

Paul is working on getting digital versions (data tables) of the grating and primary mirror phase maps from Richardson and SOML, respectively. These can then be converted into a form that we can use with Code-V and LightTools software to re-analyze the performance of the whole-system optics using actual surface figures.

Front-Side AGW Cameras

Darren reported on an offer by LBTPO to purchase 4 Steward-built thermo-cooled CCD cameras (see Gary Schmidt's Steward CCD Guider Camera webpage) for MODS that would be used as our front-side Acqusition/Guide and Wavefront cameras. These are the same cameras being used in the Potsdam AGW units. They use 512x1024 frame-transfer CCDs in a TEC head that attaches to a PC-operated Leach GenII controller. Mike Lesser says that the Lincoln Labs CCDs he is currently using in these cameras deliver 3e- readout noise in 100kpixel readouts. Binned 2x2 we should be able to operate them at ~1Hz full-frame, or ~10Hz subframe readouts.

The attraction to us and the LBT Project is that these are essentially "facility" cameras, so MODS would in principle be able to integrate more easily into the TCS system for guiding and wavefront sensing. MODS, in effect, is providing the mounting fixtures for the cameras (the X-Y stages above the MODS slit), and the optics to deliver guiding and wavefront images. We would need to provide the software interface to provide the LBT TCS software access to the X-Y stage controls (this should be straightforward).

Chris Morgan has been tasked with getting answers to some questions we have about these cameras. In particular, (1) What are the camera dimensions and how are they mounted (current mechanical drawings), and (2) Can they be operated w/o the liquid glycol cooling system?. We will generate a list of questions and have Chris report on them at the next meeting.

In general, we think this will be a good thing for us to do, we are awaiting answers to our questions so we can make a full evaluation.

CCD Detectors

The design of MODS is for the science detectors to be a 2x1 mosaic of 4Kx4K CCDs (15-micron pixels) giving us an 8Kx4K detector plane, 8K along the dispersion direction. 4K overfills the MODS slit which only projects to 2880 pixels of 15-microns each. This is still our design objective.

MODS1 and MODS2 will be deployed in three phases, starting with the blue channel only of MODS1 for Gregorian first light and commissioning activities. The red channel will be added to finish the MODS1 deployment, and (money permitting), the final phase will be deployment of a full 2-channel MODS2 on the other Gregorian focus. Because MODS is highly modular, this phased deployment can be done with nearly zero downtime for the instrument. Single-channel initial operation allows us to address the essential telescope and system integration issues with the full physical structure and the optical channel deemed "most important" by the PDR committee (who recommended to us that we deploy the blue channel first, or at least give it priority). Since we only have the red and blue low-resolution gratings at present, and this grating only illuminates 4K pixels (1000 resolution elements), we can work with a single 4Kx4K detector, rather than a mosaic, at first light and have the full scientific capabilities of this mode available to us. This allows us time for Mike Lesser et al. to solve the problems of integrating the 2x1 mosaic, whereas Mike can deliver a single detector to us in a few months from now, guaranteeing we have a proper detector in time for first light.

Given all the many things that must happen between now and MODS1 first light, this option makes the most sense to us. Our plan, therefore, is to deploy a single 4Kx4K detector in MODS1 Blue channel for first light commissioning activities, and then deploy the 2x1 mosaic detector cameras with Phase 2 when we install the red channel in MODS1. Since the CCD dewars are interchangeable modules mechanically (the only difference between a red and blue dewar is the window/field-flattner: NZK7 on the red and Fused Silica on the blue), upgrading the Phase 1 blue channel system in MODS1 to a 2x1 mosaic requires only a dewar swap, and therefore entails nearly zero downtime. MODS2 will be deployed in Phase 3 with 2x1 detector mosaics from the start.

Long term, however, the 2x1 mosaic with its irreducible 135 pixel gap smack in the middle of the imaging and spectral fields is not the optimum detector solution for MODS (recall that our nominal slit projects to 4 pixels wide). This gap presents a number of operational problems that would be obviated by a monolithic detector with 8K uninterrupted pixels in the dispersion direction. If the TSIP grant we have applied for comes through, there will be funds available to begin this exercise, but we emphasize that this is for a post-baseline detector upgrade, not a change in our plans.

Darren is preparing a detailed report on our detector options and the overall deployment plan that will be posted on this website.