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

Attendees: Pat Osmer, Paul Byard, Tom O'Brien, Mark Derwent, Darren DePoy, Jen Marshall, & Rick Pogge,

Optics

We received information from Thermo/RGL (formerly Richardson Grating Lab, formerly Bausch & Lomb...) regarding gratings for MODS.

The largest monolithic astronomical gratings in the Thermo/RGL catalog are built on a 270x400mm substrate.

The issue with the MODS gratings as quoted to Thermo/RGL is that we want a thinner substrate than usual, 50mm versus the usual 74mm. This means a potential for surface figure errors of <0.75 waves (peak-to-valley) with the thin (50mm) substrates compared to 0.25 waves p-v with their typical thick (74mm) substrate. The cause of the greater surface error is that the epoxy used to bond the replica to the substrate strains the substrate, making it more of a sphere.

Paul analyzed these numbers with Code-V and finds that even with the pessimistic estimates from Thermo, they are still well within our tolerances for image quality, and can be compensated for by a slight refocus.

From the point of view of delivery, for the R=2000 red and blue gratings, for both of which a suitable master ruling exists in the Thermo/RGL catalog, the delivery times are 16-18 weeks from receipt of the order. For the cost (not quoted here as confidential), we get a measured grating efficiency curve (littrow mode, unpolarized) and an interferogram of the grating.

We have decide to order the pyrex substrates for the imaging flats right away, but will defer the order of zerodur substrates for the gratings. This will allow us to test the grating mounting system rigorously with the flats before committing to that design (since it is such a critical component). We will probably not begin scheduling integration and testing of a prototype grating cell and flat until the end of Q1 of 2002.

The remainder of the discussion, as often before, was a confidential discussion of details of the optics bids that cannot be reported publically here.

The next MODS meeting will be in two weeks, either August 28 or 29, in the astronomy conference room, again depending on how may poeple in town.

R. Pogge, 2001 August 16