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

Camera Spacing

The main focus of discussion this week was the issue of the camera offset spacing. This design parameter has implications for the optical performance (vignetting and image quality), and the mechanical layout of the instrument as a whole. The current MODS design uses a decentered Schmidt camera consisting of an off-axis aspheric corrector and an off-axis paraboloid primary mirror. The "offset spacing" is the distance between the axis of the incoming center ray and the intersection of that ray at the detector in the camera focal plane, hence the distance the detector and its field-flattener are "offset" from the camera axis.

The question of the spacing to adopt for the final design has emerged as the most critical question we need to answer before we can proceed.

Paul Byard showed that a 220mm spacing produced no vignetting in the 6' field, but at a penalty of slight image degradation at the extreme ends of the wavelength range at the extreme edges of the field because we are more off-axis at the larger offset. A 190mm spacing can provide better imaging performance, but at a penalty of ~5% vignetting at one end of the reference 6-arcminute slit (but no vignetting at the ends of the 4-arcmin slit). The imaging performance is being evaluated as the change in 80% encircled energy with wavelength and position along the slit. The words "degradation" are relative, the differences are at most a few percent except in the blue camera at the extreme ends of the wavelength coverage.

There are, however, more important issues raised by the choice of spacing.

Will the vacuum camera concept work?

In the vacuum camera design, the corrector doubles as the window for the camera dewar. Under vacuum, the corrector will deflect by a calculable amount. Does this deflection result in unacceptable degradation of the images, perhaps so much so that the camera will not in fact work? Paul and Tom are iterating on feeding the mechanical deflection Tom has computed into Code V to assess this.

If we have to fall back upon an air camera design, this means we have to enclose the detector in a dewar with a window. How much clearance is required for this, and what does that imply for the camera offset spacing (i.e., to minimize vignetting by the detector dewar)?

In effect, this will largely define the minimum camera offset spacing. Bruce has been tasked with providing the specification.

Is it possible to design a cell for the corrector acting as a dewar window?

The corrector has an off-axis aspherical figure, so that the cross-section around the edge (where it will be held) changes. Can we make an adequate cell? Tom is tasked with answering this question.

Number of Gratings

Pat and Rick reviewed the question of the number of gratings raised in the previous meeting. The review was vis-a-vis both the proposed Baseline MODS and in the context of what other "peer" spectrometers on 8-10m class telescopes provide.

The bottom line is that the concept of each channel having a 4-position grating turret holding 3 gratings and 1 imaging flat will meet our requirements. The baseline MODS calls for 4 gratings total, 2 per channel (nominally R=3000 and R=6000). The full population at any one time will be 6 gratings; 3 red and 3 blue. This compares favorably with our closest peer spectrometers GMOS, DEIMOS, and VIRMOS (although the latter is a grism spectrometer, and does not work at the medium dispersions where DEIMOS and MODS will).

Action items:

Once the optical design has converged, Paul Byard has been tasked with generating a revised grating table, using the current catalog of Richardson Labs as the basis. What resolution, wavelength coverage, etc. is delivered by the current design for sensible choices of gratings near our baseline specification?

What new rulings might we consider to meet our specifications?

Software Issues

Software questions were raised at the LBT Software Telecon on Feb 10 which Rick brought to the MODS team:

How many optical fibers do we need at each Gregorian focus to support MODS?

Bruce is tasked with digging out the document we gave to John Hill a few years ago regarding this questions, reviewing that in light of current knowledge and replying to LBTPO.

What remote commands do we need/want from the TCS, AGW, and maybe the AO system for our data-taking system?

Rick and Jerry Mason are tasked with making up a command requirements list and forwarding that to Wagner at LBTPO.

R. Pogge, 2000 Feb 16