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

Attendees: Darren DePoy, Paul Byard, Jen Marshall, Dan Papallardo, Chris Morgan, Jerry Mason, and Rick Pogge.

Overview

This is the first MODS meeting of Spring Quarter, and given better coordination of teaching and travel, we will hold weekly meetings this quarter insofar as possible.

AGW System

Darren was in Tuscon a couple of weeks ago and spoke with LBTPO personnel about the AGW system. LBTPO is offering to purchase four of the Steward Cameras for use in MODS so that we can integrate them into the TCS system as seamlessly as possible. We would prefer not to have glycol flowing into the MODS focal plane area, so Mike Lesser is going to provide us with information about their dark current rates when operated at -20C without the liquid recirculation system. The Steward cameras are 4-inches diameter and about 3-inches deep. This is a fine size for our above-the-slit guider and wavefront sensor package, but not for the below-the-slit viewer (more on that below). We are hoping to get one of these units in hand later this year to start working with.

From an operational point of view, using the same cameras and software as the Potsdam AGW greatly simplifies MODS operation on the LBT. The Guiding and WFS systems are run via the TCS software. We have been in contact with Tim Axelrod to keep abreast of development of that system so we can interface our XY guider stage into the TCS software. From the looks of things it should be relatively straightforward to provide the interface.

Chris Morgan has been researching behind-the-slit viewer cameras, looking at small commercial units. The issue (reported in previous reports) is that we have a confined space behind the slit, and so our ideal camera has a 2x2x2.5-inch (WxHxD) form factor, and dissipates only 5W of heat. There are a couple of decent candidates that we have been able to identify from multiple vendors, both frame-transfer and interline-transfer devices that would feed into a video frame grabber. Since these are not an immediately critical item, we will continue to explore options and possibly decide on a camera to purchase and test later this year.

CCD Detectors

Darren met with Mike Lesser during his recent visit in Tuscon, and Mike is making progress on the 4Kx4K CCDs. For first-light with MODS, we will probably want to use a single 4Kx4K detector (it is all we can illuminate with the low-res gratings available at that time in the blue channel), and then plan on deployment of the 4Kx8K mosaic with the full red+blue channel Phase 2 system later. If TSIP money is forthcoming, we will probably explore creation of a more ideal 3Kx8K monolithic detector that will avoid the problems of the large gap between the two 4Kx4K detectors (they are not meant to butt as closely as, say the 2Kx4K 3-edge buttable detectors common in large-format mosaics). The current gap is about 135 pixels, which would be in the middle of the detector plane. We can only fully illuminate 8000 pixels in dispersion with the medium-resolution (R=8000) gratings that will come on line later (we currently only have money for the two low-res R=2000 gratings).

Optics

Paul reports that our first red and blue low-resolution gratings will soon be delivered from Richardson Gratings (formerly Thermo/RGL). These will come with full efficiency measurements so we can update our throughput estimates. Both are aluminum coated. In principle, we could deply the red low-res grating with the first-light blue-channel Phase 1 MODS, but at some loss of efficiency longward of 700nm. Eventually this grating becomes our work-horse low-res grating on the red channel, which will be deployed in Phase 2.

We have received email from Sagem/REOSC regarding final checks on the mounting holes for the collimator mirrors. All four collimators have had their spherical surfaces generated, and, after the mount borings are complete, will be figured into their final parabolas and polished. Delivery is now expected to be in September this year.

Steve Miller from SOML has been in contact with us regarding the first of the camera mirrors, verifying the structure function specs with Paul. Indications are that the first of the four camera primary mirrors is close to done and on spec. The null lens is also nearly completed. The spherical surfaces have been generated on both sides of the blue camera corrector's parent blank, and work will proceed on generation of the surface asphere.

The NZK7 saga continues. Paul decided to purchase suitable NZK7 blanks from Schott after iterating with Steve Miller and deciding that the roughly equivalent Ohara material was too fragile and presented potential fabrication and handling risks, especially since it is to be used as a dewar window. Schott replied that they cannot deliver NZK7 to a homogeneity spec as high as we indicated, but Paul thinks that what they can deliver will work well. The lens is close to the CCD and so has much looser tolerances than other optics. We will, however, have to wait and see what Schott actually delivers.

Flexure Compensation System

en and Darren have ordered an IR camera (a phosphor-coated CCD) for use in the lab with the IR laser system. This will let us evaluate an FCS array sensor option in addition to the quad cell that was used for the initial prototyping.