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MODS Team Progress Report
2001 January 31

Attendees: Pat Osmer, Darren DePoy, Paul Byard, Tom O'Brien, Jerry Mason, Jen Marshall, & Rick Pogge.

This is our first full meeting for 2001 devoted to MODS.

Correction (2001 Feb 6): John Hill informs me that the guide camera for LBT will be a Steward-designed CCD camera (512x512) using guide software provided by Steward. I'ved edited the text accordingly below [rwp]

Until further notice, we will meet every Tuesday at 3pm in the Astronomy Conference Room (MP4054).

A reminder: two email lists are available for MODS:

mods-osu@astronomy.ohio-state.edu: all MODS team members at Ohio State. This should be used for meeting annoucements, internal discussion, and other messages we want to stay in-house.

mods-lbt@astronomy.ohio-state.edu: all members of the LBT Optical Spectrograph Working Group outside of Ohio State (including OSU people now attached to the LBTPO), and any interested potential users of MODS who want to keep up with progress.
These two lists are mutually exclusive, so if you have something that needs both local and global distribution, you must send email to both lists. The use of personal email lists is discouraged because they can be out of date (team members have been accidentally excluded from some discussions). mods-osu is guaranteed to reach all current team members.

Sharing is Good, Part I: MOS Mask Making Machine

Darren handed out copies of an email from Naranjan Thatte (MPE Garching) regarding interest by the LUCIFER team in the Nd:YAG laser mask-making machine being used by the VIRMOS team at the VLT. Since we have discussed having a common mask-making system for both MODS and LUCIFER, we should start considering this in more detail.

Jen Marshall was tasked with reading the ESO Messenger paper mentioned by Naranjan (Dec 2000, no 102), and an astro-ph paper describing the VIRMOS mask-making system and reporting back to us next week. Out of this discussion should come a set of requirements for our MOS masks. Sharing this system and its costs would be good. We need to know, however, that it will meet our needs.

Optics

There was more closed-notes discussion of progress on revising the optics bid package. We should be ready to start talking in public soon.

Sharing is Good, Part II: MODS and PEPSI

Pat Osmer discussed an email he circulated from Klaus Strassmeier at Potsdam regarding how MODS and PEPSI (their concept for a high-resolution spectropolarimeter) will share the LBT Gregorian focus.

MODS uses up the head space above the focal plane, so making the two instrument co-existant is impractical. MODS will have to be removed from the telescope when PEPSI is begin used. The PEPSI team should proceed with its design under this assumption.

Calibration

Pat and Rick have been tasked with drawing up the calibration requirements for MODS in all of its major operating modes (direct imaging, long-slit spectroscopy, and MOS). They should present a draft calibration protocol and set of requirements at next week's meeting.

Acquisition & Guiding

Darren presented details of the Acquisiton and Guiding (A&G for short) concept that he and Bruce and other hammered out over email last month. After debating the relative merits of behind-the-slit viewing and off-the-slit viewing modes, the concept converged on a hybrid "do both" approach.

The proposed MODS A&G system would be implemented in 2 phases:

Phase 1: Behind-the-Slit (BTS) Viewer
A pickoff mirror and optics mounted on an XY stage travels behind the 6x6-arcmin science field and a 1x6-arcmin offset strip outside it. There are two basic operations:

Acquisition & Peak-Up: This is accomplished by taking the slit mask out, putting the BTS pickoff into the science field center and acquiring the target. The mask (long-slit or MOS) is re-inserted and the object is peaked up by viewing it through the slit.

Offset Guiding: After acquisition & peak-up, the BTS pickoff is moved into the offset guiding strip and a guide star is acquired.

Residual paranoia about target motion between target and guide star acquisition can be addressed by doing a sanity check back to the slit position to make sure the target has not drifted out of the slit, then back to the guide star to start guiding.

We would intend to deliver the BTS system with the first-light MODS.

Phase 2: Slit Viewer
This system uses a special reflective long-slit unit and a fixed slit-viewer camera and associated optics. It would only be used with the long-slit mode (it is impractical with MOS mode).

The particular advantage of the slit viewer system is for rapid acquisition of many targets, e.g., programs with a large number targets requiring relatively short integrations. The BTS system could be used in the offset field to provide guiding support in some cases.

There was then discussion of the A&G concept. Paul Byard presented a design for the relay optics for the BTS viewer. Tom noted that we have plenty of room behind the slit to accomodate this system, but not that much. The volume behind the slit is clear in the science field proper, but constrained on 3 sides by the grating turrets and the dichroic/flat changer mechanism. The science and guider fields will not be not exactly contiguous due to the finite width of the slit mask frame.

Precise details will be developed and better engineering-quality drawings posted as things get worked out.

Open A&G Issues:

Camera System
We should coordinate closely with Steward/LBTPO to see if we can use their A&G camera design for the BTS and Slit viewers. Further, even if we build our own cameras, it should be able to provide data to the guiding software as if it were an LBT Guide camera, so we don't have to write our own AG software.

Secondary Mirror Setup (wavefront sensor)
After some discussion, it was noted that things are pretty tight as is with the BTS viewer, and so it would be undesirable from a lot of perspectives to use the BTS system to couple to a wavefront sensor to setup the secondary mirror figure using a bright star.

Tom suggested that we should consider a dedicated system, for example a pickoff mirror on a swing arm above the slit that is used just for looking at bright stars for setting the secondary mirror figure. This would presumably feed by a [Steward] designed/built camera. The modularity advantage of having all of the wavefront and acquisition/guiding cameras be the same all over the telescope is pretty clear, and should be a goal.

Baffling
A behind-the-slit viewer means that we now have additional baffling issues given the open "guide" field. Care will need to be taken to design adequate light-trap/baffling for the BTS unit from the start. Generally, it was agreed that this baffling issue is probably minor compared to others we face in the system regardless of our final A&G decision, but it should still be factored into the design and manufacturing plans now so it doesn't risk getting lost and become a problem later.
For next time, Rick and Pat have been tasked with drawing up a draft A&G protocol for each of the three major instrument modes from the observer's perspective. This should help us define the final requirements for the MODS A&G system.

MODS PDR

The MODS Preliminary Design Review (PDR) will be on 2001 March 12 in Columbus. We need to present Tom Herbst with our PDR documents by mid February. Darren divided up the tasks as follows, giving a basic outline of our PDR presentation:

Science Goals: Osmer
Instrument Overview: DePoy
Optics Design: Byard
Mechanical Design: O'Brien
Electronics:
Software:
Management: DePoy
Rick was asked to put together a common PowerPoint template that we should all use for our presentations (both at the PDR and to be distributed as part of the PDR packet to the committee being assembled by Tom Herbst). [This latter has been done at this writing. You can get it off the internal network, or talk to Rick]

R. Pogge, 2001 February 3
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