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,

MODS PDR

The MODS PDR was held on Monday, June 11, at OSU. The discussions were "frank and productive", and many good suggestions were received from the committee. We are grateful for their time and effort, and believe that MODS will be a better instrument as a result.

Tom Herbst, on behalf of the SAC, gave us a verbal report from the committee, and the SAC voted to accept the report and declare that we "passed". A written summary of the PDR committee's recommendations, with amplifications by the SAC, will be forthcoming from Tom.

Optics

Most of the optics discussion centered on revised bids received from vendors. This is a closed confidential discussion that may not be reported here in detail, but suffice it to say we are making progress, if slower than we would like.

Interestingly, regarding the issue reported in the May 22 meeting of whether we can deploy the phase 1 MODS with the blue or red channel first, the PDR committee suggested we build the blue channel first. This recommendation is largely on scientific grounds, rather than our technical issue regarding the availability of red vs. blue glasses for the correctors! Great minds think alike, although not always for the same reasons.

We will therefore plan to deploy the phase 1 MODS1 with the blue channel only at around Mirror 1 first light, with the second channel coming online after the initial commissioning and engineering phases. Given the amount of work required (and additional mechanisms), this will probably be 1 year after the Phase 1 deployment.

Detectors

While there is no urgency on final selection of the CCDs, work is moving ahead to define the CCD detectors. Among those we will speak to are Mike Lesser's CCD Lab at Steward (since they are LBT partners), but also will consider current developments at MIT/Lincoln Labs, LBNL, and Marconi regarding our options.

Some technical and scientific issues to be resolved are:

• The blue sensitivity needed to meet our performance goals

• The possibility of getting extended red sensitivity, either now or in the future, e.g., by using deep depletion or other "red boosting" devices. Because we are probably deploying the red channel after the blue, this buys us time to resolve these issues, esp. vis-a-vis the rapid developments of late in red-sensitive CCD technologies.

• Readout noise requirements. In particular, do we especially need to pursue more advanced (but correspondingly more expensive and risky) low-noise technologies like skipper amplifiers (and related non-destructive readout tricks) and frame-transfer devices, or with regular delivery of almost "commodity" CCDs with 2-3 electron readout noise performance, can we use these more "standard" components? The issues for spectroscopy can be stated as these questions:
  1. At what spectral resolving power does readout noise begin to affect things like sky subtraction.
  2. In our "dual spectrometer" mode with both MODS, the idea is to have minimal readout noise penalty for co-addition of spectra of the same objects from MODS1 and MODS2 to mimic the "dual" 11.3m aperture collecting area. When is the readout noise good enough for this?

The next MODS team meeting will be July 3, combined with the general ISL monthly meeting, in the astronomy conference room.

R. Pogge, 2001 June 20