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,

This report covers this meeting and a brief meeting on Friday, Apr 27 after Pat Osmer's return from Tucson.

Note:

There are no reports for most of April because those meetings have discussed various "closed door" issues having to do with bids and procurements.

New MODS Team Member: Mark Derwent

With this meeting we welcome Mark Derwent who has just joined the Astronomy Department as a Mechanical Engineer. Mark will be working with us on MODS and other projects. Welcome, Mark!

Mechanical Systems

Tom O'Brien reported on progress on various MODS mechanism efforts:

• The first grating turret frame has been fabricated, and has turned out extremely well. This was a crucial test of how well we can make and assemble large Aluminum weldments. The structure was very stable and easy to weld, with virtually no post-welding machining required. The part (JPEG image) consists of 16 welded pieces and 2 bolted units (the top pulley and one internal part). After welding and assembly the shape of the frame was well within tolerances.

• The top pulley wheel for the grating turret was machined in our Haas CNC mill, and is one of the largest parts (850mm) we can make on this machine (JPEG image). It came out very well. All other parts we are contemplating are smaller than the pulleys. This suggests we will be able to make other such parts with good efficiency and results.

• The first camera cell for the red camera corrector lens is being fabricated.

• Next up is to make parts for the fold truss and differential screws for testing the camera assembly concept.

Low-Resolution Survey Modes

Paul Byard presented designs for direct vision prisms (2-element composite prisms that give dispersion w/o deviating the beam) that might be used in a low-resolution, faint-object survey mode. The low-res disperser would be located in front of the red camera at the location reserved for the cross-disperser mechanism. The goal is to provide low (R=few hundred) resolution over two octaves from 400-1000nm over the full MODS FOV. An attraction of direct vision prisms over grisms is cost: for a grism we would need a custom ruling on a roughly 260mm square grating that would cost >$120K, whereas two direct vision prisms might do the job for $20-30K, depending on materials. Dispersions are determined by the indexes of the two glasses used and the base thickness of the prisms. The prisms are limited in thickness by the amount of space available ahead of the red camera (146mm).

Paul evaluated two designs for direct vision prisms that could provide a throughput in the 95% range.

1. BK7+SF6 prisms: R=200 @ 500nm to R=30 @ 1000nm range, with reduced transmission shortward of 400nm (50%).

2. FK51+SF57 prisms: R=450 @ 500nm to R=80 @ 1000nm, but with much reduced transmission below 500nm.

Contact Paul Byard if you want all the details.

Jen Marshall and Pat Osmer have been tasked with taking these direct prism performance specifications and folding in spectra of various objects (QSOs, Galaxies, AGNs, Stars) to see if the "observed" spectra will be at all useful for faint-object survey work.

Atmospheric Dispersion Corrector

There was considerable discussion of what an Atmospheric Dispersion Corrector (ADC) should do, what the effects of atmospheric dispersion are and how astronomers usually try to compensate in the absence of an ADC, and ultimately, what size of an ADC can the current design accomodate before it ceases to be an upgrade and becomes a design driver.

To summarize the discussion, it was decided that an ADC capable of covering up to the central 4x4-arcminute FOV can be accomodated by the current design without becoming a driving feature. This FOV is for an ADC located ~900mm ahead of the MODS focal plane and ~200mm in diameter. Currently an ADC is not part of the baseline instrument plan and is considered a potential upgrade. What we want to decide now is not the ADC design details but rather if we are doing anything to preclude adding the ADC later, and what is limitations will be.

Given the image quality in the extended 6-arcmin FOV, it is clear that an ADC makes most sense in the inner 2-arcmin region where we get the best (0.3-arcsec) image quality, and arguably might be useful the 4-arcmin main FOV. It was decided we could live with the 4-arcmin limitation, as going for the full 6-arcmin FOV would start driving the design in undesirable (and potentially problematic) ways.

A critical issue is that we must guide through the ADC for it to be effective. Two approaches were discussed. One was that you build a second XY guider system in front of the slit (in addition to the current behind-the-slit guider system we are designing). The second was that you still use the BTS guider, but make a special slit mask with sizeable holes (1x2 arcmin or longer) inside the "science field", and then devise a baffling scheme that will adequately block out unwanted light from entering the instrument. It comes down to answering the question "how much baffling is adequate"? Darren has been tasked with coming up with a "light tightness" specification.

Getting back to atmospheric dispersion in general, Rick will work up a series of calculations and report on the impact of using MODS in various modes without an ADC (which, by the way, is the operating mode for nearly every other MOS on 6-10m class telescopes). A full working out of these issues will be required to develop baseline observing strategies, especially for the MOS mode.

The next MODS meeting will be Tuesday, May 8 at 3pm in the Astronomy Conference Room.

R. Pogge, 2001 May 2