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

Note: this report is longer than usual as it includes progress made since before the Christmas & New Years holidays, and this was our first formal MODS team meeting for 2000. [rwp]

One of the still unresolved issues in the current design is the fact that the grating mechanisms still project above the instrument focal plane (IFP) into the space between the IFP and the back of the primary support. We have plenty of room, but it impacts on the volume that must be shared with the Guide/Acquire system. It is clear we need to interact more closely with the design group in Potsdam working on the G/A module.

Drawings for the optics are converging to the point that we are ready to start compiling the optics fabrication bid package. The designs for the Off-Axis Paraboloid collimator mirrors are such that we will need to have four (4) indentical pieces fabricated for the full two-instrument two-channel MODS. The Spherical Primary for the Cameras will also be identical for the 4 cameras in the full MODS.

The Camera Correctors are still being worked out. In the current working concept, the cameras are vacuum cameras, and the corrector plates will act as the dewar windows. If we continue with this concept, the optimal shape will be round. At present they are 480mm in diameter.

Paul presented some more refined optical designs, with a view to understanding the image quality along the slit. Questions about how to "measure" this for typical observing modes (small field, 4-arcmin field, and extended 6-arcmin field) that will be studied and addressed at the next meeting. In looking at materials for the correctors and field flatteners, it looks like a good combination is to use Fused Silica in the blue-channel lenses, and BK7 in the red.

Design questions remaining:

Gratings:

Is a 400mm "ruled width" the maximum-sized grating MODS will ever need?

The answer to this appears to be "yes". Above this size, the vendors tell us we need to use grating mosaics. The only reasons would be to push to very high resolutions (20000 and greater), at which point you are really designing a high-res echelle spectrometer, and are outside the MODS basic design mandate.

Must the largest grating be able to reach zeroth order (e.g., for alignment purposes)?

The answer again appears to be "yes", and after some discussion we decided that we are willing to accept a small amount of vignetting in this mode since it is used primarly for alignment. This has no impact on imaging with a flat mirror, since the vignetting is mostly due to anamorphism in the grating which would be absent in an imaging flat.

Is three gratings plus one imaging flat mirror per beam adequate?

Pat Osmer and Rick Pogge have been tasked with assessing this from the perspective of science capabilities for the next team meeting.

What is the range of tilt angles for every grating?

Tom and Paul will iterate on this.

How many cross-dispersers are required per beam?

One. In the baseline proposal, this was only on one channel.

What are their dimensions?

Good question. Paul will examine the issues.

Are we still satisfied with the integrated filter/flattener approach? Is eight filters per beam adequate?

There was some discussion of this. For "fixed" filters used for order separation, there are clear advantages to this approach. It was not thought to be a good option, however, if there will be a lot of call for a large suite of user filters for MODS for direct imaging or special projects (e.g., the medium-band filter set concept presented by Rogier Windhorst at the MODS meeting in Columbus last year).

At a slight loss of throughput (extra surfaces), one concept discussed would be to have 2 filter wheels. The present configuration has a single 8-position lower "fixed" filter wheel located immediately in front of the CCD detector in each beam. This filter wheel carries a clear FF with no filter and a set of integrated filter/field-flatteners (e.g., order separation filters). A second "user" filter wheel would be located above the lower filter wheel, with one position reserved as empty to permit the beam to pass through to the primary fixed filter/FF set. The remaining positions of the user filter wheel would be available to be populated with regular filters without integrated field flatteners (e.g., UBVRI filters, medium and narrow band filters, etc.). These user filters would be used in combination with the clear FF in the lower filter wheel to ensure good images on the CCD. If we were to simply replicate the lower 8-position filter wheel, this would provide a total of 7 additional user filters/beam (red and blue), for a total of 14 filters per beam. This is just one idea, other user filter wheel options will be explored. Input on this from the MODS working group would be appreciated.

What is the optimal camera offset?

This is a crucial issue, as it requires a careful trade-off between vignetting in the 6-arcminute "extended" FOV of the instrument and image quality in both the extended and "design" unobstructed FOV of 4-arcminutes. If we try to design to a 6-arcmin FOV, we will pay by compromising the image quality in the center "zero" field where we would get our best performance for eventual AO work. It was generally agreed that we would continue to design to the 4-arcmin unobstructed FOV, and try to understand the amount of field vignetting in the 6-arcmin extended beam. At present this which this looks to be only a few percent of "snipping" at the edges of the beam. Since changing the camera offset feeds back into the mechanical design, it is one of the main issues in the final convergence of the design. There is also an interaction between the offset and the detector that also has to be considered. More work is required.

The next MODS project meeting will be the week of Feb 14-18, TTBD.

R. Pogge, 2000 Feb 4

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[rwp]