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

Attendees: Darren DePoy, Dan Pappalardo, Jerry Mason, Ray Gonzalez, Bruce Atwood, Mark Derwent, Dave Steinbrecher, Phil Covington, Ed Teiga, Pat Osmer, Tom O'Brien, Ross Zhelem, Paul Byard, & Rick Pogge

MODS Corrector Lens Progress

Darren and Ross reported on their visit to SOML on June 22 to investigate progress on the MODS corrector lenses. At SOML they met with Steve Miller and Mike Tuell. JJ Valenzuela, the optician doing most of the work on the correctors was away. Darren and Ross saw both the Red and Blue corrector lenses. The Red lens was on the CMM, and the Blue lens was in the optical testing area.

Frank and beneficial discussions were held on the MODS corrector lens progress. The coordinate measuring machine (CMM) test and optical tests are going better, but there is as yet no quantitative confirmation that they are producing consistent results to within their respective precisions (about +/-1-micron for the CMM, as yet unstated for the optical tests which are pending detailed data reduction). This is not quite where we want to be, but the progress in this regard is encouraging.

It was agreed that Ross and SOML personnel would hold weekly telecons, with monthly in-person visits to Tucson to monitor progress until the correctors are done.

An issue arose as to what constitutes "done" for the correctors. It was decided that we would need detailed surface maps that could be fed into Zemax to predict the MODS imaging performance (e.g., spot diagrams and encircled-energy metrics). These would determine quantitatively when we had achieved our desired specifications, provided that these quality estimates do not turn out more stringent than the structure function specification in the original contract. Mike Tuell is working with Ross to format the surface maps so that we can use them with CodeV.

To date, the new hologram appears to be giving a meaningful test of the blue corrector lens' aspheric surface, which at least in the preliminary analysis looks broadly consistent with the CMM data, but there are issues with the software SOML uses to stitch the data together having problems (the size of the optical test beam is smaller than the work piece, only covering about 80% of the diameter at a shot - enough to cover completely one of the sections of the parent with an off-axis lens). The current prognosis is that once they get a confirmed, consistent surface map, one more run at QED for Magnetorheological Finishing (MRF) should complete the figuring. The optical tests, once stitched together and confirmed are needed to generate the MRF instructions.

A proposed schedule for work needed to complete the aspheric side of the blue corrector lens is as follows:

• About 2 weeks for the optical test, including data analysis
• About 3 weeks to iterate with QED for MRF (including shipping to Rochester)
• About 2 weeks for a post-MRF optical test

To complete the corrector, the off-axis lenses need to be cut from the parent (about one month), and we need to address the problem of the craters on the spherical side caused by the ball bearings that were dropped on the part (see the 2004 Sept 8 Report). These craters are about 3-5mm in diameter, and about 2mm deep. There are two basic options:

• Re-grind and re-polish the spherical side to remove 2mm of material to eradicate all of the craters. This will take "at least 1 month" for the basic work.
• Drill out the craters to relieve stress and blacken them, accepting the loss in throughput (blocking) and scattered light.

We need to decide which option is best. The first (re-grind/re-polish) adds significant time to the schedule, and optimistically we might not receive the lenses until late January 2007. The second, living with the craters but treating them as described above, could have the lenses to us sometime in October 2006. We note that at this point the blue corrector delivery is significantly behind schedule regardless of which option we pursue, since the original delivery target for the blue corrector lenses was mid-July 2005.

The Red Corrector Lens so far has had both surfaces generated using diamond milling. There are some residual bad tooling marks on the aspheric side of the parent, roughly dead center. This is the same place as analogous tooling marks on the blue corrector lens parent, but worse (larger and deeper). How we cut the off-axis lenses from the parent will determine the impact on the final optics, and so at this point it is hard to tell how bad (or not) this is.

The first run with the CMM on the red corrector has produced inconsisent data that is confusing, and re-analysis is underway to understand this.

At this stage, no plan has been put forward by SOML for how to complete the red corrector lens, so no schedule has been proposed. The use of QED, Inc. to do MRF like on the blue corrector is apparently not an option anymore, so SOML and we will have to explore possibilities for getting the part finished. We therefore do not have a good idea as yet of the schedule impact on MODS assembly.

Ross did a preliminary calculation asking what would happen if we decided to use the Blue corrector in place of the Red corrector just to move MODS1 along to commissioning. The answer in a word is "bad". The correctors are not remotely interchangeable.

We await news of further progress, especially from the weekly telecons with SOML, and the July visit.