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

Discussion focussed on resolution of the camera spacing issue raised at the Feb 16 meeting.

Bruce addressed the question of how much space is needed for the detector system, as this in effect largely defines the minimum camera offset spacing that will accommodate the detector dewar for an air camera design.

Bruce gave an excellent summary of why we want to hang on to as much detector-area real estate for future detector possibilities. The working detector space is an 8Kx4K detector with 15-micron pixels. The full 6-arcmin slit extends for only about 2800 pixels, and dispersion runs along the 8K pixel axis. With a 220-mm camera spacing, there will be 23-mm between the edge of the detector and the edge of the beam. The edge of a 6-arcminute field flattener will eat up a little of this 23-mm margin.

Bruce reviewed a number of possible ways to pave the 8Kx4K detector space, from mythical 8Kx4K monolith detectors with skipper amps and 2-way frame transfer mode (and 64 amplifiers), to our Zeroth Order Solution of two 2Kx4K 3-edge buttable detectors we can walk out and buy today that are butted end-to-end to give a 2Kx8K detector that gives us only a 4.4-arcmin long slit. Bruce made a persuasive case that multi-amp, frame-transfer readout will offer the best readout noise performance, and so is the ultimate goal (if not the first/second-light goal). There are clearly technology issues that need to be resolved to make this work.

The bottom line, however, is that all of these solutions, from the "buy tomorrow" to "idea" dictate a minimum amount of real-estate at the detector plane.

After discussion, it was agreed that the 220-mm camera spacing gives us acceptable clearance without vignetting the full 6x6-arcmin field. It also provides us room for future possibilities we haven't even thought of for detectors (e.g., IR array-like hybrid devices that permit direct readout, or whatever).

We will design to a 220-mm camera spacing

A definitive bid package for the optics will be ready for distribution on April 15, following an independent design study by Brooke Gregory in early April. Paul Byard will communicate with the vendors before hand, and persons interested in bidding on the optics fabrication who want more information can contact us in advance with questions.

Paul Byard showed that the image quality in the red camera for the full 6-arcmin field is acceptable after refocusing to an optimum near-uniform focus across this field we can achieve D80 (80% encircled energy) at 550nm of 0.8-arcsec over most of the 6x6-arcmin field, with 1.2-arcsec at the extreme corners. For AO work, we can refocus and get D80=0.3-arcsec in a 1x1-arcminute field. The blue camera would perform worse, but not out of line with degradation in performance from the mirrors or the atmosphere in the blue.

Now that we have a firm camera offset specification, Paul will be able to generate a more complete image quality assessment for both imaging and spectroscopic modes. This will be presented on his poster at the SPIE in Munich in March, with an illustrated report to become available on these web pages at about the time we finalized the design for the optics fabrication bid package.

Regarding whether the vacuum camera concept will work, Tom is betting "not", but Paul will continue to work on it. It is clear we have a soft fall-back position now to either an air camera with a detached detector dewar, or some variation on the vacuum camera theme in which the corrector is not the dewar window. Other ideas and inputs are welcome from the rest of the LBT community.

Tom continues working on the packaging of the instrument (see our new computer model). The dichroic changer, grism changer, and slit-mask cassette mechanisms are all easy to package. No problems there. The folding angle of the red beam is now 5.7-degrees, down from 5.9-degrees, both of which are good mechanically and optically.

Two problems remain: The red camera's primary mirror is still knocking against the design envelope of the instrument and needs work. The red grating turret overhead is still too high. Tom plans to contact Jesper Storm at Potsdam to start discussing this with the AGW team.

The action items so far are:

1. Paul will design to the 220-mm camera spacing, finalizing the optical design preparatory to:
   1. Brooke Gregory's independent optical design review.
   2. Preparation of the optics fabrication bid package.
   The bid package should be ready for distribution on April 15.

2. Tom will continue working on the instrument packaging, iterating with Paul as the optical design approaches finalization. Tom will also contact the AGW team in Potsdam to start resolving the issue of the red-channel grating turret mechanism which protrudes into the volume above the MODS focal plane.

3. Work on our posters/talks for the upcoming SPIE meeting.

R. Pogge, 2000 Feb 23