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

Attendees: Darren DePoy, Dan Papallardo, Jerry Mason, Bruce Atwood, Tom O'Brien, Paul Byard, Jen Marshall, Mark Derwent, and Rick Pogge.

Image Motion Compensation System

Darren and Paul reviewed the current state of design for the image motion compensation (aka flexure compensation) system.

The optical path of the laser is shown in the figures below, depicting [left] the path through the red channel (including dichroic), and [right] the path through the red camera optics.

[Click on the thumbnails to view full size]

The laser is launched from below the slit using laser projector optics, through the dichroic towards the center of the grating. (An open question is whether the dichroic coating is sufficiently transparent at 1.55-microns to allow us to shine straight through, or do we need to make a "hole" in the coating for the laser.) At the grating center there is a 20-mm hole within which the fan-out element is mounted. The current design is using a 5mm diameter fan-out element. A small gamma angle is given to the fan-out element to steer the dispersed laser beams to a point above the science detector, through a hole in the filter wheel.

Thermo/RGL says there will be a 2-mm margin around the hole where the grating will not perform well. Because the LBT secondary mirror obscuration is about 25-mm in diameter at the grating, 20-mm is the largest hole we can accommodate without starting to impact grating performance. A larger fan-out element in the system could be used, but it would be more expensive roughly as the square of the aperture.

The fan-out element at the center of the grating disperses the laser beam into a series of spots. For a 5mm diameter grating, a camera focal length of 700mm, and a wavelength of 1.55-microns, the spot diameter at the detector focal plane will be 512 microns.

With one fan-out design with 3.6 grooves/mm, the fan-out separation angle is 0.31 degrees, which for a 700mm focal length camera gives a spot separation of 3.8-mm.

If we were to use a quad cell detector for the IMCS, the grating tilts would be restricted to discrete intervals of 0.31 degrees. For the R=2000 red and blue gratings, this translates into 4 or 5 fixed tilt positions that cover the respective spectral ranges of the gratings. A question to be addressed by Pat, Rick and others is whether this is a serious limiting factor in terms of potential science use of the instrument, or if it presents operational difficulties.

[Left: Red R=2000, Right: Blue R=2000; Click on the thumbnails to view full size]

With the Red R=8000 grating, much finer tilt resolution is possible with a fixed quad cell since the range of tilt angles required to cover the entire red-camera spectral range (~7 degrees) is large compared to the 0.31-degree fan-out angle of the laser system. As such, allowing only discrete tilt angles presents less of an issue, although this should be addressed in detail.

[Red R=8000 grating discrete positions in second order; click on the thumbnail to view full size]

Paul presented an optical design for a laser projector system that will insert the IR laser beam into the system along with a co-aligned optical laser (670nm diode laser) that can be used to visually align the system. It can be built with all off-the-shelf components from Edmund Scientific.

[click on the thumbnail to view full size]

As presented last time, we have three detector choices. Jen Marshall spent more time talking to possible vendors, and we can buy InGaAs arrays (128x128-pixels/60-microns) from one source, at a cost of about $20K per MODS spectrograph, not counting electronics. The pros of this approach are that we can then enable an essentially continuous range of grating tilts for all gratings and modes. The cons are more complex software (image analysis of centroids), a question of whether having additional clock signals from the IR array in the same detector enclosure as the science CCDs would introduce cross-talk into their readout. There is also some risk that the array we are interested in could be an orphan technology (this is unclear, Jen is tasked with getting a clearer read on this from the prospective vendor). There are also questions about whether the vendor cameras, designed for room-temperature operation with integrated TECs would survive in an LN₂ environment in the CCD dewar.

A second option is to use an IR quad cell. This will be low cost option (<$5K for both MODS instruments), and have relatively simple software for control. The cons are the issue of grating tilt "quantization" which as described above gets fairly coarse-grained for the R=2000 modes.

The third option is to use regular CCDs coated with anti-stokes phospors as discussed during the previous meeting.

Another issue that was raised is whether the fan-out gratings are the right technology, compared to, for example, ronchi rulings that might perform the same job. Paul has not looked into this in detail but will do so.

Various people have been tasked with addressing the technical and scientific issues raised, and will report back at future meetings. Specification, design, and prototyping of the IMCS is the primary group task for the upcoming summer.

Mechanical

The first of the spherical end pieces needed to mount the instrument to the Gregorian focus rotator clamps are now in production in the shop. These are being machined out of steel.

Bids have been arriving for the steel structure, we are still waiting on one before proceeding to the next steps. There was some discussion of possible vendors we have investigated to date, but these discussions must remained closed for now.

Tom still needs to generate bid packages for fabrication of the steel tapers for the ends of the carbon-fiber collimator tubes, as well as for fabrication of the tubes themselves. These will go out to possible vendors soon.

R. Pogge, 2002 May 22