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MODS Team Progress Report
2001 February 27

Attendees: Pat Osmer, Darren DePoy, Tom O'Brien, Jen Marshall, & Rick Pogge,

The meeting was brief due to a number of absences and illnesses.

MODS PDR

The MODS PDR originally scheduled for 2001 Mar 12 has been postponed by Tom Herbst. Details later once things are worked out.

MODS Calibration

There was a brief discussion of the Draft Calibration Plan that Pat & Rick drew up. Overall the plan was deemed sensible, and some action items for further work are as follows:

  1. Tom will think about how to incorporate a calibration screen on the inside of the instrument cover for MODS. Issues include what to make the screen out of, whether it will be remotely operable at all position angles to permit on-the-spot calibration, etc.

  2. Paul is tasked with designing projector optics for the calibration lamps. The starting idea is that we will probably feed such a lens system from an integrating sphere, but we don't have especially stringent illumination requirements, only that there is not small-scale structure in the illumination of the instrument screen.

  3. Pat and Rick will look at what calibration lamps we need, especially emission-line spectrum sources, in order to cover the spectral ranges of MODS in the various proposed resolutions.

  4. Rick will write up an expanded Calibration Plan, including a "worked example" from actual practice as a demonstration of the procedure.

Flexure Control System

Work on Tom's concept for the flexure control system (FCS) for MODS was discussed next. The most important action item is that we need to draw up an short "experimental protocol" to be used to evaluate the FCS concept in upcoming lab tests. Pursuant to this, we have already ordered a Germanium Quad Cell sensor and are seeking to order an IR diode laser system to use as the light source. Jen Marshall is looking into the laser system, and will do the tests.

The two biggest questions to be addressed by the experiments are:

  1. What is the stability of the laser's beam profile as a function of time and temperature?
  2. What is the wavelength stability of the laser, also as a function of time and temperature?
Questions the draft experimental protocol needs to address are:
  1. Given our flexure specification, how well do we need to measure each of these parameters to determine if this approach will be suitable?
  2. What experimental design will we use in the lab?
  3. What kinds of data logging should we do? We have some equipment we think will be suitable.
  4. How to analyze and present the data that best shows whether this system will work or not?
The remain action item regards the grating angle "step" size. In the simplest implementation of the FCS concept, a single IR quad cell is used as the IR laser sensor, and the allowed grating tilt angles will be limited to a series of discrete steps by the spacing of the spectral orders of the bypass grating. The question before Pat and Rick is to determine what grating step size we need to satisfy our science requirements. For this exercise we shall assume a 1x2 CCD mosaic as the detector with a roughly 100-micron gap in the middle of the spectral range (this is a "build it today" consideration, a monolithic detector has no gap but such devices are not yet available). We need to have enough latitude to move spectral features of interest away from the gap. The most stringent "number of grating steps" problem will be our highest resolution mode of R=15000. Pat and Rick will go over the numbers and report these back to us.

MOS Masks

Darren reported on the continuing mask-making machine saga with LUCIFER, in this case email correspondence with Reiner Hofman at MPA, and what he saw on his visit to U Florida last week.

Richard Elston at U. Florida has a 20W YAG laser machine that has a 100mm X-Y working surface. The laser works at 1064nm (no doubling). They purchased this unit used for US$20K and are using it to fabricate masks for the FLAMINGOS IR multi-object spectrometer. FLAMINGOS requires 100-micron slits (compared to our 300-600 micron slits). So far, they have cut and evaluated slits made in graphite epoxy, aluminum, steel, Cromoly steel, tungsten, and titanium (to name a few). Tungsten slits were OK, Aluminum were marginal at best (the edges were blobby from melting), stainless was awful. Graphite epoxy worked only if the fibers were oriented 45-degrees to the direction of the cut; if aligned the slits looked terrible. This is similar to what the GMOS team found.

We are continuing discussions with Reiner Hofman to clarify what all the issues are, but so far it sounds as if the LPKF laser system would not work well with MODS given our curved slit needs. We still do not understand why there is a great need to adopt and purchase a particular machine right now since neither us nor LUCIFER seem to have worked out all of the MOS mask issues.

The next MODS meeting will be Tuesday, March 6 at 3pm in the Astronomy Conference Room.

R. Pogge, 2001 March 2

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