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

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

This was a brief meeting to hear reports on Jen Marshall's visit to the Gemini-North HQ in Hilo during her recent observing trip to Hawaii, and Paul Byard giving an update on MODS optics fabrication progress.

GMOS Mask-Making System

The Gemini Multiobject Spectrograph (GMOS) has many features similar to MODS (FOV and focal-plane scale are roughly comparable). In particular, they have a lot of operational experience with laser machining multi-object slit (MOS) masks in carbon-fiber epoxy, a technology we are considering for MODS. During a recent observing run at Mauna Kea, Jen Marshall had a chance to visit the Gemini-North HQ in Hilo, where she met with John Hamilton who described the mask-making process and showed her one in progress.

Her report may be summarized as follows:

• The mask cutting machine is located in a small closet in the Gemini offices, and is used to create masks for both Gemini North and South. The machine is a Laserlith Model 104C that was custom-made for Gemini by ART Technologies for $120K (ART assembles parts obtained from other vendors, but they seem to be getting out of this business). The machine has a Nd:YAG laser that cuts the masks, and the masks move on an x,y stage (although an x,y,z stage could be incorporated easily).

• The mask material is manufactured for Gemini by Kinetic Composites, Inc. of Oceanside, CA. It is made of three layers of carbon fiber sheets epoxied together, the middle one oriented 90 degrees from the outer layers and all of them oriented 45 degrees relative to the edges of the square mask. Gemini originally made masks from sheets with the carbon fibers aligned with the slit axes, but this resulted in "hairs" of material dangling into the slits. The new 45-degree rotated material eliminates this problem.

• Originally, Gemini bought just the material and cut the masks themselves with the laser, but now they order the mask templates cut-to-shape from the vendor. The masks cost $9 each (the price recently dropped from $12). The masks are mounted in the frames that are used in the instrument during cutting.

• The software to run the laser is a commercially available and widely used machining software called Acroloop. The procedure is relatively straightforward: put a mask blank in the machine, turn on the laser, load the slit pattern, and hit GO. The laser moves to the position of a slit, starts cutting in the center of the slit to burn out the undesired material, and traces the slit edges three times to ensure a good cut. It typically takes 20 minutes to cut a mask, but this could be reduced by optimizing the order in which the slits are cut (now they are cut in a random order and most of the time spent in cutting a mask is in moving the laser between the slit locations).

• Pre-observation targeting images of fields are executed in queue mode at the beginning of the semester, and delivered to the PIs who then use Gemini-provided software to select the targets and design the masks. Any size or shape slit (tilted, curved) may be used. Other Gemini software is then used to translate the observer's design into x,y coordinates for the laser-machining program.

• Once the masks are cut they are labeled with a bar code to be read inside GMOS at the time of observation. GMOS can hold ~12 masks at a time in its cassette, more if the IFU module is removed. The bar code is scanned, the mask is moved into place, and the observer aligns 3-4 alignment holes that have been cut in the mask with stars in the field.

Overall, this sounds very promising, and we will continue to explore emulating certain aspects of the GMOS system for MODS.

Optics Fabrication Progress

Paul Byard reported on progress being made on various fronts with fabrication of the MODS optics.

• SAGEM/REOSC has completed 2 of the 4 MODS collimator mirrors. We have accepted a small variance from our spec: a very slight mis-positioning of some of the holes for the flexure mounts of <300-microns. Tom analyzed this and found it acceptable. They are drawing up the acceptance report for delivery after which we will expect an invoice. If all goes well we can expect delivery of the collimator mirrors for MODS1 in late August/early September.

• The NZK7 substrates for the red field flatteners have been delivered by AG Schott, and have been shipped on to Steve Miller at SOML. The melt data for the indices was within 1 part in 10⁴ of the prescription values and the prescription did not need changing. The cost was also less than estimated ($6300 vs. $10000).

• Steve Miller at SOML reports that they have completed the shaping of the first two camera primary mirrors (one is polished, the other is being readied for polishing). Prior to shaping, they tested the first primary at center of curvature and it meets the the specification, with one qualification.

  The interferograms show a pronounced high spot of about 3-microns on one corner. Initially they thought that this was was due to relieved stresses from the edge cutting, since they needed to remove about 1/2" of the faceplate in the corners to get to the final dimensions (they removed much less than that along the other edges). However, closer inspection revealed a fracture running just below the faceplate.  After consultation with Rich Wortley from Hextek and Blain Olbert (their materials scientist), they found no other cracks. They then inspected the second camera blank (which has not yet been polished), and found a similar fracture in a similar location.

  They think that the fracture is the result of some force, either leftover from the fusion and annealing process or, more likely, from the edging process. They suspect that the vacuum chuck is getting twisted from its spherical shape when being bolted onto the mill. Sucking the mirror down onto the twisted chuck and then milling the edge could be enough to cause the fracture.

  They have come up with a repair plan, (using UV curing cements they have used before on MMT mirror and other projects to deal with fracturing), and will polish the upturned corner back down to spec. Steve will proceed to try this to correct mirror 1, and will get back to us with the results. He will also present us with a risk mitigation plan before proceeding with edging the remaining mirrors.

Overall, optics progress has been excellent, and we await the outcome of Steve's work on camera primary #1.