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

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

Focal Plane

Tom reported on continuing work to finalize the design of the focal plane area of MODS. He has come up with a design for an X-Y stage for the above-the-slit viewer that looks like it will fit and be able to cover the entire 6x6-arcmin science field plus the 2x6-arcmin offset guiding field.

Paul Byard continues to work on the design for the calibration system. The current design uses a Fabry relay lens that needs to be inserted into the focal plane aread roughly ~50-100mm above the slit plane. This is the lens that forms an image of the pseudo-pupil on the grating, with light from the off-axis integrating sphere feeding a retractable 45-degree folding flat mirror. This calibration mirror will be integrated with the instrument dark slide so that when the calibration mirror is inserted into the beam the instrument dark slide will be closed. This has a couple of nice advantages: when we are taking night-time calibrations we can flood our instrument chamber with light but not have any leak out into the telescope dome, and during the day when the instrument cover is closed, we can shut out stray light from the enclosure while taking basic calibration data with MODS.

Acquisition & Guiding System

Tom also reported on continuing work on the MODS AGW system. We are looking at two systems, above and below the slit. The above-the-slit (AOS) system would include acquisition, guide, and wavefront sensing, the latter provisionally using a 12x12 Shack-Hartmann sensor, currently conceived as a copy of the Potsdam system, with some modifications as noted below. The below-the-slit (BOS) system would only be a single-camera acquisition and guide unit with now WFS capability.

The AOS viewing system seems to be converging nicely (see above). The BOS viewer is next to be designed and presents some challenges. The real estate behind the slit is fairly constrained by the location of the field lens that lives behind the slit. Paul Byard needs to make a more compact BOS viewer optics design than we currently have.

Another question is what guide star can we use for guiding and 12x12 Shack-Hartmann testing? Darren computed that we should be able to do 12x12 S-H tetings on an 18th magnitude star at about 1Hz, depending on the design. Similarly, we should be able to guide on a 21st magnitude star at about 1 Hz. This means we should never lack for either a guide star, or a star to do relatively quick Shack-Hartmann wavefront sensing for verifying the telescope setup (note: this is not S-H sensing for adaptive optics).

The proposed Potsdam design splits the beam with a dichroic into red and blue beams; red light going into the wavefront sensor, and blue light into the guide camera. For MODS this presents us with a problem, since because of differential refraction we want to guide at about the same wavelength as we are taking spectra. Guiding on a blue signal could cause us problems, depending of course on just how blue it is. An alternative is to use a gray beam splitter instead of a dichroic. Typical metal-film gray beam splitters have an efficiency of 30%/30% into both beams (maybe up to 40%/40% at best). The loss in light is pretty minimal in impact sice we will still not lack for guide stars even dropping the limits down a magnitude. We will explore this option more carefully. It does mean we want to put a small filter wheel in front of the guide camera so we can satisfy our "guide at roughly the same wavelength" requirement.

The final issue we have identified is whether we want to permit glycol cooled cameras into the instrument focal plane. The concern is that a leak could be devastating, involving a very expensive cleanup job, especially if the gratings are involved.

What cameras can we (should we) use?

One option is to simply adopt the Steward camera design that is being used in the Potsdam AGW concept. Our current design work is using these camera as placeholders since this is a likely option. These cameras use the E2V (ex-Marconi) 512x1024 device in frame-transfer mode to give a 512x512 pixel guiding field. The advantages of using the same units as other focal stations is clear. However, one disadvantage is that these units are liquid glycol cooled, which makes us nervous about leaks.

The other question is whether these cameras are the right solution. Another alternative might be to investigate the new E2V backside illuminated "zero noise" devices. E2V claims its can sell a "commodity" camera for about US$15K that incorporates this technology (though current units use the front-side illuminated devices, they are talking about incorporating the higher QE bsi devices soon). We might want to acquire one of these cameras to use on our CCDS at MDM to replace the aging ISIT camera as a test in the short term.

Mechanical Fabrication

Mark Derwent visited 3 shops today in town taking around a set of drawings for a filter wheel we want to build for a 4Kx4K CCD cameara for MDM. The goal is to get the 4Kx4K filter wheel built, and to qualify shops for future MODS work. Mark will report on his findings later (though since these involve discussing vendors, those notes will not be reported here for obvious legal reasons).