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

Attendees: Darren DePoy, Jen Marshall, Tom O'Brien, Paul Byard, Ray Gonzalez, Dan Papallardo, Mark Derwent, Rick Pogge, & Pat Osmer.

Space (the kind in buildings)

Darren reported that the MOU for use of high bay space in the Food Sciences building on West Campus has been signed by our College and the College of Biological Sciences. We can move in on around August 1, and can have the space for at least 2 years. This space will be used to house the MODS2 structure while MODS1 occupies the McPherson Lab shop.

MODS Optics

Paul reports that we now have two MgF₂-coated field lenses in hand from JML. The coatings are adequate, if we want to get something better we'd have to consider someday getting SolGel coatings, which currently are not practical.

We have also taken delivery of two coated red field flatteners from JML. These have broadband dielectric coatings that make them look slightly reddish in transmission, and bluish in reflection (they are tuned to work in the red channel, and so are 97% transmissive at 550nm, falling of rapidly towards the blue).

The two blue field flatteners have been returned to us by JML. Their surface quality is unacceptable for coating. After some discussion, we have decided to junk these parts and have JML figure and coat two brand-new blue field flatteners. We will use the SOML parts as test windows for Dewar testing and integration, but they are unacceptable for use in MODS.

Paul is also seeking quotes for the low-dispersion double-pass prisms. We need quotes from at least 3 vendors before we can proceed with an order. We already have one firm quote in hand (fabricated and coated).

Paul also read an email from Steve Miller at SOML giving his prognosis on the blue corrector. Bottom line from the draft progress report is that they are at least 1 month behind and still have much work to do.

MODS Assembly

Tom reported that the laser alignment tool for MODS has been assembled and installed in the instrument. This is a laser and pentaprism system that is located in a custom slit mask, and used to align the plane of the other MODS components relative to the instrument focal plane, as well as to project the centerline of the optical axis back through the instrument for alignment.

Alignment of the focal plane is actually quite close to the spec from the start, but the final zeroing-in process will be straightforward if tedious. It is looking very good, he is very encouraged both by the performance of the alignment tool, as well as by the process they've developed to employ it.

In the process of making detailed measurements of the MODS1 structure, Tom found that one of the mounting pads on the cart is out of plane by several millimeters (in the sense of a gap not an interference), so some minor structure rework will be required to correct this (either mechanical rework or fabrication of a shim).

MODS Slit Masks

Tom has been revisiting the question of good substrates for the MODS multislit masks. To remind our readers (since it has been a while since this has come up), MODS slit masks are bent into the 3D shape of the LBT f/15 Gregorian focal surface (a spherical surface). By comparison, the current LUCIFER slit mask design has their slits machined flat then bent into a cylinder, which they can more or less get away with vis-a-vis focus because of their smaller field size (2.5x4 arcminutes compared to 6x6 arcminutes for MODS).

Tom has come across what he considers the "ideal" substrate: a thin electroformed alloy of Nickel-Cobalt that can be bonded with copper and blackedned on both sides. For LUCIFER, one side can be coated with gold, giving it excellent radiative properties. The material is very thin (~100-microns), and is easy to machine with any commercial laser-cutting machine. The main disadvantage is a higer cost, ~$100 per blank, compared to shaped carbon-fiber epoxy, though we don't yet have firm quotes on shaped CFE, and those could easily run $50/blank (GMOS gets them for $10-15/blank, but flat). He as two vendors he's talking to about getting sample production runs made so we can review the material, have some slits cut at an on-campus laser machining facility, and evaluation more quantitatively. It could prove to be an ideal medium for both MODS and LUCIFER.

Tom also reported on investigations into modern laser machines. It turns out waiting to buy has been a good thing, as laser technology has advanced and costs (both unit costs and maintenance costs) have gone down, and lasers have gotten better. Modern laser machines are now using diode-pumped 1-micron Nd:YAG lasers as opposed to older flashlamp-pumped lasers. These have much longer lifetimes (10K hours), essentially zero maintenance compared to flash-lamp systems, the ability to partially (or totally) aircool (all flash-lamp systems are water-cooled), and they are relatively cheap. A stock 20W single-mode laser head runs about US$35K. When their life is up, laser heads are typically shipped back to the vendor for maintenance at less (half to third) the cost of a new head. A number of good vendors exist.

This laser head would be mounted in a 3-axis table-top machine. Haas, who made our big CCM mill downstairs makes a small table machine wiht the right travel, 3-axis controller, and enclosure that would be close to ready to go for about US$30K, but we'd need to invest effort to integrate the laser cutting head (or pay quite a bit extra to an outside integrator). Tom will continue developing these options into a specific proposal for a joint MODS/LUCIFER laser mask-cutting facility.

MODS Mechanism Controls

Dan is assembling the single-axis motion control box to be used in the lab, and has ordered the parts for the 4-axis version. We should be able to start testing the 1-axis version soon.

Other Stuff

Andy Krygier has been working on a design for MODS tipping mechanism that will allow us to do flexure testing of the instrument in the lab by driving the spectrograph on its cart from horizon-pointing to (and through) nadir-pointing orientations. This will be used for acid-testing the IMCS system during and after instrument integration.

Darren reported on his investigation into the current status of the Gregorian instrument rotators for LBT. MODS mounts to these rotators. It turns out that the rotators have not been built, the basic design is not complete, and nobody seems to be working on it at the present time. Overall, it is not unlikely that 12-18 months of work would be required to get them to the telescope. They're not our reponsibility, but if they're not ready they are a problem as we won't be able to mount MODS on the LBT without them. The LBT Project Office has been made aware of our concerns.