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

This report includes material from a brief meeting on Monday, Sept 25. There has been a long hiatus of reports due to a combination of the Ringberg meeting, summer vacations, and the fact that discussions of the optics bid packages are confidential in nature and so there are no reports here of those meetings devoted primarily to discussing them (most of August).

Finally, with this report we formally welcome Jennifer Marshall to the MODS team. Jen is an incoming OSU graduate student with her bachelor's degree from Northwestern, and is currently working with Darren on various slit design issues for MODS.

Discussion continues on the optics bid process. Needless to say this process is confidential and so our discussions will not be reported here.

Work Plan

Pursuant to starting work on MODS, we have been developing a detailed work plan and cost schedule. Jerry Mason and Bruce Atwood have been putting this all into Microsoft Project98, and presented a preliminary analysis. The analysis includes current personnel, commitments for the LBT aluminizing system, and reasonably realistic scheduling issues (available work days, vacation, etc.).

It is clear from this first run that Tom O'Brien is the "critical resource" for determining the schedule, primarily for design of the various pieces that go into MODS. The schedule gets better if we re-run the program inserting an as-yet mythical Tom₂ to share the work load. A less flippant way of putting this is that the scheduling exercise is already giving us a clearer picture of our personnel needs for MODS, and will help us determine what new personnel will be required.

Bruce and Jerry presented various scenarios, related to what a "first-light MODS" would look like and then ran the schedule through to see what we could realistically achieve by the project's current official M1 first light. The details are unimportant here, but the exercise has provided essential info to help us focus on what the minimal "first-light MODS" configuration should be with realistic personnel and schedule constraints.

The action item for next time is to iterate on the plan using various input tweaks suggested in discussion, and to try to develop a "first-light MODS" specification. More later. We need to submit a summary work plan and cost schedule to the NSF by the end of October to fulfil one of the conditions of our grant to build MODS.

Vacuum Camera

Paul Byard reported on various optical performance impacts of vacuum vs. air camera designs. In particular, there is no significant impact of having a separate field flattener and filter, instead of each filter optically bonded to its own field flattener as in earlier concepts. This opens the way to us using a single field flattener as the dewar window for the CCD in an air-camera design.

With no filter in the beam and just the field flattener, it is necessary to move the camera primary mirror ~4mm to refocus, at a cost of slight image degradation ("slight" was not quantified). After some discussion, it was suggested that having a 4mm "clear" filter in the beam, with suitable surfaces and AR coatings, should eliminate this.

Tom then reviewed the pros and cons of vacuum vs. air camera designs. Without diving into details, after some discussion it was decided to drop the vacuum camera concept and proceed forward with the air camera design. Given the outcome of Paul's optical analysis, there is no compelling reason to continue to pursue the vacuum camera design.

Acquisition & Guiding

Darren and Bruce presented two different concepts for performing target acquisition and guiding with MODS.

Darren started by discussing limiting magnitudes with one 8.4-m mirror in the MODS configuration for acquisition using a reasonable guide/acquier CCD camera. In a 3-minute broad-band (roughly wide-V) integration, you can reach S/N=10 on V=25 or R=24.5 magnitude stars in half-moon conditions. Except for objects which are dominated by large equivalent width emission line spectra, virtually no target that requires more than 5-10minutes to image would be a viable spectroscopic target.

For guiding, in the north galactic cap (an example of our sparsest fields), there are approximately 1445 stars/deg² with photographic magnitudes brighter than 22 (roughly V=21.3). This translates into 1 star per 2.5 arcmin², roughly equal to the number of galaxies of that integrated brightness. This means we should have at least 1 guide star, and perhaps even an occasional "guide galaxy" (if we can work out a good guiding algorithm) in a roughly 1.5x1.5 arcmin field of view. As has been said before, 8.4-m is a *big* telescope, so we won't lack for reasonable guide stars for simple guiding (we are not considering AO correction).

For the sake of this discussion, science observing with MODS can be broken into 6 categories characterized by "short" (up to a few minute) and "long" (many 10s of minutes) integration times in each of the three operating modes: imaging, long-slit spectroscopy (LSS), and multi-object spectroscopy (MOS). In table form, the methods of acquisition and guiding in Darren's concept are as follow:

Specifically, Darren proposes a 2-camera guide-acquire system.

1. Reflective long-slits to a fixed slit-viewing CCD camera
2. Fixed off-axis pick-off mirror with a 2.5x2.5 arcmin FOV viewed by a second fixed CCD camera

Bruce presented a second AG system proposal. The centerpiece is a guide/acquire camera mounted on an X-Y-F stage that is located behind the slit plane. The camera would be designed by Bruce and use 512x512 4-amp, back illuminated, thinned TEC cooled CCD delivering a 2.5-arcmin FOV. The stage will be able to run over a 10x8 arcmin field, composed of the 6x6 arcmin science field plus a 2-arcmin wide margin around three sides. The WFS mode would be provided by adding out-of-focus capabilities to the guide/acquire camera to provide Roddier-type curvature sensing instead of the Hartmann mask WFS proposed by Potsdam. OSU would have to provide all hardware and software for this system.

The Acquisition/Guiding matrix is something like this:

Imaging Mode:

Acquire targets by running the pickoff mirror to field center.

Guide by retracting the pickoff mirror to an off-axis position and selecting a guide star.

LSS Mode:

Acquire in 4 steps:

1. Retract long-slit mask
2. Send pickoff mirror to field center. Note target position when telescope reports "on target"
3. Deploy the long-slit mask, view sky through the slit, compute the target/slit offset.
4. Offset the telescope to put the target down the slit, viewing again to verify a correct offset.

The first 2 steps may be done while slewing.

Guide by retracting the pickoff mirror to an off-axis position and selecting a guide star.

MOS Mode:

Acquire as follows:

1. Retract slit mask
2. Send pickoff mirror to the location of a pre-selected alignment star (minimum of 3 required in the mask field) and image the alignment star.
3. Deploy the slit mask, view sky through the first alignment star aperture. Compute the target/slit offset.
4. Offset the telescope to put the target down the slit, viewing again to verify a correct offset.
5. Travel to alignment star position 2 and verify that it is down its aperture.
6. Iterate through each of the N alignment star positions, refining the telescope pointing (Alt, Az, PA) to align the mask.

As before, the first two steps may be done while slewing.

Guide by retracting the pickoff mirror to an off-axis position and selecting a guide star.

A suggestion that the X-Y stage might be located in front of the slit was summarily dismissed by Darren and Bruce.

Darren attempt to summarize the various salient features in the following list (based on a viewgraph presented at the end of the meeting):

Fixed slit-viewer and off-axis viewer system:

• Two cameras of Potsdam design/manufacture in a fixed geometry.
• No moving optics (except for camera focus and a filter wheel).
• Requires 2 sets of optics.
• On-axis acquisition of bright targets in long-slit mode using reflective slits requires no change of instrument configuration (i.e., slit, grating, etc. are fixed).
• All optics are located above the slit plane, so no additional baffling required.
• Since guide & acquire cameras are decoupled, there is no deadtime between end of acquisition and start of guiding.
• Uses Potsdam provided cameras and software. No additional design or programming by OSU. User/telops interface will be the same as that at other focal stations using the Potsdam AGW.

Behind-slit guide/acquire system:

• Single camera on an X-Y-Focus stage.
• Camera and software of OSU design/manufacture.
• Provides more off-axis field possibilities for guiding because the pickoff mirror can be moved in X-Y.
• Long-slits simpler and easier to make since they don't need to be reflective.
• Requires additional, moving baffling to keep light from the off-axis positions from entering the spectrometer.
• Arguably faster MOS mask alignment, as we do not need to reconfigure the instrument (switch from imaging to spectroscopic modes) and readout the "science" CCDs.
• More software must be provided by OSU, and presents the user/telops with a different guide/acquire interface, but that can be mitigated with more software to mimic the look-and-feel of the Potsdam system.

This is not the last word, but we need to put closure on this soon, as both have design impact on the "top" of the instrument. Additional comments from the rest of the LBT community would be greatly appreciated.