Introduction

This document summarizes the findings and conclusions of the MODS Preliminary Design Review (PDR) Panel, which met in Columbus, Ohio, on June 11, 2001. The panelists were:

Gary Bernstein*	University of Michigan, Ann Arbor
Dan Fabricant	CfA, Cambridge
Tom Herbst* (chair)	MPIA Heidelberg
Gary Hill*	University of Texas, Austin
Klaus Meisenheimer*	MPIA Heidelberg
Gary Schmidt	University of Arizona, Tucson
Walter Seifert*	Landessternwarte, Heidelberg
Paolo Vettolani*	Institute of Radio Astronomy, CNR, Bologna

Only the referees marked with an asterisk (*) were able to attend the Columbus meeting. The others supplied comments and questions beforehand. Additional participants at the meeting included:

John Hill	LBT Project Office, Tucson
Jim Liebert	University of Arizona, Tucson
Bruno Marano	Osservatorio Astronomico, Universita di Bologna
Mark Wagner	LBT Project Office, Tucson

There were also a number of additional, informal observers from the MODS instrument team.

The PDR Process

Approximately one month before the meeting, the MODS team distributed a document describing the instrument. The referees read this document and submitted questions and concerns to the panel chairman approximately two weeks before the Columbus gathering. The chairman forwarded this information to the MODS team on 29 May.

The responses to these questions and concerns came partially in the form of an online response and partially in the form of presentations held at the meeting on 11 June. After the general meeting, the review panel sat in closed session for approximately an hour and formed its recommendations. This document is the result of that session.

General Conclusions

The PDR panel unanimously concluded that the MODS PDR was successful, and that the instrument can proceed to the final design phase. No further meeting of the PDR panel is necessary, but the panel's conclusions presume attention to, and action on, the remarks and recommendations described below.

The PDR panel summarized its conclusions about the MODS design as follows:

"MODS is a good, fast spectrograph, with some unique features that should be supported and emphasized. The design was clearly optimized for speed (i.e. sensitivity) and has succeeded.

This success comes at the expense of field-of-view, but its broad spectral coverage makes it an excellent instrument for the identification of rare and transient objects."

More detailed remarks and recommendations appear in the following section.

Remarks and Recommendations

Major Issues

1. The PDR panel unanimously felt that the blue / UV channel of the instrument is the more interesting and unique capability, and that it should be the first implemented.

2. The low-resolution blue replica grating already available from Richardson Grating Lab (RGL) does not provide optimal performance in the ultraviolet. A new ruling is recommended. The panel noted concern about a possible shutdown of RGL in the coming years. Ensuring the best possible blue grating for MODS may require swift action.

3. The higher resolution mode with R~8,000-15,000 is very interesting, and should be implemented as soon as possible within financial constraints.

4. Recommendations 1-3 above are more important than the proposed CCD development plan. There is not yet a science driver to embark on such an effort. The Marconi chips are fine for the proposed instrument.

Minor Issues

1. The procedures for alignment, test, and setup of the instrument require more development. In particular, the proposed method of comparing measured images with Code V predictions may prove problematic, if there are degeneracies (i.e. different misalignments or decentrations producing similar output spots). A more complete alignment plan is necessary.

2. The area above the telescope focal plane needs to be defined. At the PDR, there was essentially no information about the calibration unit, atmospheric dispersion compensator (ADC), and integral field unit (IFU). While the ADC and IFU are not part of the initially delivered instrument, their interaction with the calibration scheme must be carefully planned, since they will occupy the same volume above the focal plane and also require calibration.

3. The PDR panel recommends that the MODS team exercise care and not "design out" potentially useful observing modes. The possibility of using partial adaptive optics correction at visible wavelengths seems particularly interesting. Implementing a deep depletion, enhanced red CCD may also make sense, once the technology has matured.

4. The MODS team must increase their planning in a number of areas:

   1. calibration procedures - there was no discussion of this at the time of the PDR

   2. commissioning and handover - there were a number of areas in which the instrument team said that items would be completed or refined during commissioning and early operations. This must be clarified with the LBT project.

   3. establishing and maintaining best focus - based on the ray-traced spot diagrams at the PDR, it was not obvious that there is a clear signature of best focus over the whole field of view. There must be an efficient procedure to establish and maintain best focus.

5. The PDR panel recognized that the Flexure Control System (FCS) is potentially an excellent solution to remove gravity-induced bending of the instrument. Because it is central to the success of MODS, the panel felt that a working prototype was necessary before proceeding with the final design.

6. There was significant discussion of the proposed carbon fiber technology for the multi-slit masks. While no immediate action is necessary, the MODS team should monitor the progress of the Gemini project in this area, and continue to work closely with the LUCIFER team in identifying a suitable solution.

7. The MODS team should investigate sealing off the camera unit from the rest of the instrument, to reduce the possibility of contamination and problems due to dust, humidity, and stray light.

8. A full thermal analysis must be done before freezing the final design (i.e. before the Final Design Review). There were concerns that temperature gradients in the steel structure could induce unacceptable tilts and decentrations of the optics.

9. At or near delivery of the first spectrograph, there must be software in place to allow scientists outside the MODS team to observe efficiently.