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

Attendees: Ross Zhelem, Bruce Atwood, Dave Brewer, Jerry Mason, Paul Martini, Ray Gonzelz, Dan Pappalardo, Mark Derwent, & Rick Pogge.

MODS Optics

Ross has received and analyzed the latest round of CMM measurements of the red corrector lenses from SOML, showing us two overlapping maps. We are now at the point where we need to decide to go on to polishing and full-aperture optical testing. The overlap regions agree well, but there is still some residual astigmatism at one edge. The largest features are still ~7-microns, so the plan is to go ahead with another round of loose-abrasive work to reduce these to ~2-microns, and then proceed to polishing and optical testing. Surface maps derived from full-aperture optical tests will be needed before we can proceed to pressing for Magnetorheological Finishing to complete the part.

The blue corrector lens shipment for coating has been delayed by OSURF who held up the purchase order processing because of the backlog of end-of-the-fiscal-year closings. We hope to get this clear early next week (so OSURF purchasing promises) and get the lenses off to ZC&R.

The integrating sphere exit port optics are delayed until the end of June because the vendor has moved adn they have taken longer to get their production lines back in operation than expected.

MODS Enclosure

Mark reports that the hexagonal upper framework of the MODS1 enclosure is now done, and assembly of the lower dodecagon section is in progress. The framework should be completely assembled in about a week, well ahead of schedule. Testing of the cross-bracing tie rods has been held up because a minor part was made incorrectly (the center support ring), but this will be corrected in a couple of days. If this works OK, they will proceed to order and fabricate the remaining cross-bracing parts.

Once the assembly of the framework is complete, they can make the final measurements and put in an order for the siding material. Justin has also been documenting all of the assembly steps, and is work is very thorough. Justin has been a huge help this summer, and we would not be as far along as we are with the enclosure assembly without his efforts.

MODS Electronics

Dan reported on the outcome of cold testing of the components of the lamp control box. The pen-ray and QTH lamp power supplied passed the -20C cold-soak testing with flying colors. All supplies work fine at -20C and light up right away. We will proceed to order all the power supplies we will need for both sets of calibration spheres for MODS1 and MODS2.

The IMCS IR laser cold tests are still incomplete. The initial run was to cold soak it off until it reached -20C (24+ hours in the freezer), then start it up cold and verify that it lit the laser and the controls worked. The ThorLabs laser system has a front-panel LED display that can show either the laser power (measured with an internal reference sensor integrated into the laser diode) and the set-point temperature of the TEC cooler for the laser head. The laser head has to be kept at a constant temperature (+/-0.1C) to keep the wavelength from changing (the laser cavity changes size). Oddly, the laser control box does not give you the option of viewing the laser head temperature proper, only the desired set-point of the TEC. Dan will contact ThorLabs tech support to find out if it is possible to read the laser temperature proper.

So far, however, the laser appears to operate correctly when cold. The question is whether or not the laser temperature control is operating effectively. If the laser changes temperature, the wavelength of the laser line changes, which would look to the IMCS like a mechanical "drift". We need to understand and measure this better. The available documentation from ThorLabs is very sketchy, so we are hoping that by reaching a qualified engineer at ThorLabs we can get better documentation that helps us address this issue. More later...