MODS Team Progress Report for 2001 September 18

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

MODS Team Progress Report
2001 September 18

Attendees: Pat Osmer, Dan Papallardo, Bruce Atwood, Paul Byard, Tom O'Brien, Mark Derwent, Darren DePoy, & Rick Pogge,

Mechanical

Tom reviewed progress on designing the MODS structure. MODS will use an open spaceframe truss structure for its main structure. Some design parameters:

Needs to mate to the Gregorian focus support ring.
Needs to allow easy removal and installation of the Red and Blue cameras when off the telescope.
Need to be able to remove the collimator assembly from the bottom of the spectrograph.
Designed to minimize image motion due to collimator decentering as the structure flexes in response to gravity, or expands/contracts in response to temperature changes.

A number of options were explored, using the AnSys finite-element analysis (FEA) program to evaluat designs.

Analyzed the amount of image motion due to flexure. The main items in the mechanical flexure budget is, roughly in order:

self-weight of the collimator support members (long, >2-3m tubes)
collimator assembly "payload"

Analysis shows that the dominant mechanical distortion is tilting of the collimator assembly. If uncompensated, can amount to as much as +/-150 microns of image motion at the detector. The amount by which the collimator mirrors would need to be tipped/tilted to compensate is well within the current actuator design for refocus with lots of head room.

Work to be done includes

Trade study of using stainless steel versus carbon-fiber members for the collimator support arms to reduce the self-weight.
Work out details of the endpoint attachments for the support members.
Analysis of the ball and cone joints in the structure, including where the instrument mates to the Gregorian ring to analyze for ticks and jumps. The question is whether the joints should be free or fixed.

Tom is pleased with how the structural design is proceeding, but there is still much to be done.

IR Laser

Darren reported briefly on results Jen Marshall has been getting on the IR laser we are evaluating for use in the Flexure Compensation System (FCS). The laser is an Agere Systems solid-state InGaAs laser emitting 40mW CW at 1.55 microns.

One early mystery was why we could detect the laser directly with a lab CCD camera (an Apogee AP7 with a SITe 512x512 CCD) when silicon is not supposed to have any QE at 1.55 microns.

After much fussing about in the lab to build a simple spectrometer on the bench, we found that indeed, if we estimate the laser power correctly, a silicon CCD has a quantum efficiency of about 1.2x10^-10 electrons/photon at 1.55 microns! Not a lot, but when we focus all of the laser power onto the CCD, it is enough to detect it. This actually simplifies tests of the laser stability with time and temperature, as we can rig up a simple grating spectrometer in the lab and use the CCD to measures laser spot motion. In an actual instrument, we wouldn't need to have the laser so cranked up, and good baffling would keep stray light out of the science beam (we're not focussing the laser spot onto our science arrays).

Another curiosity is that the CCD's detection of the IR laser decreases as we decrease the CCD's ambient temperature (the AP7 camera is thermocooled to 40C below ambient). Could it be that part of the 1.55-micron QE is due to thermal (tunnelling?) effects? We don't know, but we don't mind exploiting it for future tests Jen will run.

The next meeting will be on September 25, same time same place as usual.

R. Pogge, 2001 September 20

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