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

Attendees: Darren DePoy, Dan Papallardo, Jerry Mason, Tom O'Brien, Paul Byard, Jen Marshall, Mark Derwent, Amy Stutz, and Rick Pogge.

Gratings

Two post-baseline modes for MODS in consideration are a very-low resolution (R=few hundred) "survey" mode and a high-resolution (R=15000) mode, the latter possibly with cross-dispersion. We are considering these now to assess their demands on space inside the instrument as the structural design is getting closer to being finalized.

An earlier idea to use direct-vision prisms to implement the very-low-res survey mode for MODS did not produce good results: the resolution was too low and changed rapidly with wavelength. Synthetic spectra created by Pat and Jen were yielding results that were not better than what could be accomplished with multi-filter medium band imaging techniques.

Paul Byard has instead been exploring grating options for implementing survey mode resolutions. A good design, iterated with Thermo RGL, is a 60 g/mm grating with a 1.26-degree blaze. This produces ~70% peak efficiency and a resolution of ~600. It would require a new ruling, with a cost of about $100K/ruling plus replica costs. At present the ruling engine at RGL is in high demand, so lead times are long (~2 years). Both gratings look straightforward to make.

The red survey grating design would peak at about 800nm with 70% efficiency, dropping to 40% at 550nm, and only droppong to 60% at 1-micron.

A blue design peaks at 70% between 300 and 350nm, dropping to 40% at 550nm. We have asked Paul to explore alternative designs that shift the peak to be centered at 350nm, which should boost the red performance.

An interesting possibility with these gratings is that the orders will not fill the full detector, so you could operate in a side-by-side mode using order-blocking filters and double the number of slitlets. One limiting factor is that the 0th order image is also on the detector, so a slit-mask design algorithm that mitigates overlap is an issue. With a full 6-arcminute science field, we don't fill the full 4Kx8K detector, and with a 4-arcminute field we can fit all of the spectra onto a single 4Kx4K device (w/o double-stacking slits).

High-resolution (R=15000) grating designs are proving problematic. There is one 800g/mm, 37-degree blaze design that would meet our specifications but it may not be possible to actually make the ruling in our size. Further, the properties of the orders are not as amenable to cross-dispersion. In all, the high-res mode of MODS is looking less likely, and it is also becoming clear that there is more scientific interest in the Survey mode low-resolution modes. We will continue to explore these options for future upgrades.

Mechanical

The camera primary mirror focus mechanism design is done and Tom is very happy with it. It is very low risk and robust. A work package of drawings is being prepared to be sent to the ASU shops for fabrication.

Overall the cameras are in very good shape, with >80% of the work done.

We are studying the first round of bids received for the structural fabrication, and investigating other vendor leads. So far, however, the picture is encouraging, as the structure could well cost less to fabricate that we had originally thought.

Now that the basic design is converging, Tom, Bruce, and Dan have been tasked with preparing a "What is a Mechanism" specification document that describes what kind of power supplies are allowed (switching or not), motion controllers, sensors, components, etc. This will become our standard reference document for MODS mechanisms.

Optics Fabrication

Word from REOSC has been very good, and we could be seeing the first of the collimator mirrors during 2003 Q1.

Darren will be in Tucson next week and will be meeting with Steve Miller of SOML, so we can expect an update on progress on the MODS camera optics upon his return.

R. Pogge, 2002 September 17