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

Attendees: Tom O'Brien, Dan Pappalardo, Jerry Mason, Jason Eastman, Paul Martini, Ross Zhelem, Bruce Atwood, Ray Gonzalez, Mark Derwent, & Rick Pogge.

Tom O'Brien started the meeting by giving us a report on his visit to SESO in Aix-en-Provence who is working on the rigid secondary mirror for the LBT.

MODS CCDs

Bruce did a brief show-and-tell of the e2v 3x8K CCD electrical sample, showing pictures of it in the e2v test camera in the UK. They are currently planning to ship this to us at the end of July. The detector is a front-side illuminated device that works electrically but is not suitable as a science detector. This is one of the milestone deliverables from e2v. They say we can expect to start receiving thinned, coated, and packaged science CCDs in November, and thereafter at a rate of about one device per month until we have received our 4 working devices (2 blue and 2 red). We greatly look forward to seeing them.

MODS Electronics

Dan reported on his further IMCS IR laser cold testing. After a few calls to ThorLabs, Dan finally found the RightGuy(tm), who provided us with lots of technical documentation, including unit design documents, not available on their webpage. These showed that there was a jumper on their TEC board that could be set to show the laser temperature instead of the desired set-point on the front-panel display. Dan rigged this jumper with a switch so we could toggle between set-point and laser temperature readouts remotely while the laser unit sits inside the freezer at the -20C testing temperature.

He started the test warm on the bench (ambient ~22C), and first set the laser TEC control to run at 24.0C. While still at room temp, the set-point readout would vary by 0.1C every now and then, and the laser Temperature was 23.9C, again with a 0.1C jitter. This apparent offset in the readout (and the 0.1C readout jitter) is within the design spec.

He then put the laser in the freezer and cooled it to -20C. During the initial cooldown from room temp, he monitored the set-point and actual laser temperature, and saw no change other than the 0.1C jitter (e.g., display read 23.8 and 23.9 intermittently). After 36 hours of continous cold-soaking, the temperature display still read the same. This was then run for a few days without changes. This verifies that the laser TEC system can maintain operating temperature well outside the manufacturer's rated 15-35C range, which is essential for us on MODS.

While this was going on, Rick got the manufacturer's part number for the laser diode. The ThorLabs S3FC1550 laser uses a Mitsubishi ML925B11F DFB (Distributed Feedback) laser diode. The peak wavelength changes with temperature with a characteristic temperature coefficient of 0.09nm/K. Given a temperature control of +/-0.1C, this suggests an RMS change in central wavelength of +/-0.009nm relative to the nominal central wavelength of 1550nm, or 1 part in 172,000. The actual temperature RMS is likely less since the LED indicator only ready a 0.1C change (occasional jump to 0.2). This is too small to cause a systematic wavelength shift that would be mistaken by the IMCS for flexure, and is well within the 1.5-micron RMS measured compensation at the CCD plane.

Dan will be working out a way to readout this temperature with the WAGO units so we can remotely monitor the IMCS laser temperature in operation. The good news is that we can run the ThorLabs laser for the IMCS at the ambient temperatures expected at the LBT and have the unit's TEC can maintain a constant temperature.

This means we can mount the laser in the lamp control box without having to use active heating of the box, and all the nasty complication and bother that would entail. Dan is now proceeding to the final detailed design of the lamp box, using a standard NEMA box and WAGO control hardware.

In other electronics news, Dave Brewer is making the MicroLYNX mounting panels for the 4 remaining IEBs, probably done middle of next week, and Ed it wiring up the second AGW/Calibration interlock box and wiring harness.

MODS Optics

Ross reports that SOML is ready for optical testing of the red corrector lens parent. They are doing the last rubs with loose abrasive to push down the last remaining circular zones on the part. They are still at 6-microns, but they are confident of their material removal rates, and should be able to reach their goal of 2-microns. Steve Miller also reports that he is seeking quotes from QED for an MRF run and should know more by the end of the week. They are looking to do a run in late August. We can expect the first optical tests in the next couple of weeks.

Other optics briefs:

• CSIRO has started work on the dichroics this week, and we can expect delivery of the two completed parts in August.
• The blue corrector lenses have been shipped to ZC&R. We can expect them back coated in 6-8 weeks.
• The red mirror coating quote is being refreshed, we expect to have it soon and then begin packing the mirrors for shipment

MODS Enclosure

Mark reports that the MODS1 enclosure framework is now fully assembled in the McPherson shop. Everyone should go down for a look, it gives you an idea of the size and volume of this beast.

The PO for the MODS1 side panels has been sent out, we can expect the panels in 3-4 weeks from receipt of the PO. The panels are going to be painted black on one side and red on the other. The black sides face inwards, and red outwards, so combined with the brushed aluminum of the framework, the general color scheme of MODS will be scarlet and gray.

As it should be.

The remaining areas of work on the enclosure before arrival of the panels is:

1. The transition "cone" between the hexagon and dodecagon is tricky. They're working with cardboard models now to determine the best strategies for covering the region and ensuring light tightness, etc.
2. The tie rods for the cross-bracing as some issues with the tie points close to the main mounting balls for MODS - the region is tighter than we had anticipated from the engineering drawings, so we will need to pre-install the mounting bolts. This does not require we dismantle the framework as these sections were designed to be removed in sections of 2.
3. Mark and Justin are completing an inventory of the parts needed for the MODS2 enclosure and will get a quote out by the end of the week. In the meantime, we have all the parts needed for the fixed hexagonal ring, and Dave and Justin will get started on that out at the West Campus birdhouse next week.
4. Justin is about done with the framework assembly documentation, and has completed the parts and fastener spreadsheet.

MODS Software

Ray reported on progress with the multi-mechanism testing, using drive power supply voltage telemetry from the WAGOs. With 3 or 4 of the big motors in action, it looks like they consume about 50% of the rated drive supply current. Parts have been ordered to build proper current readouts, and these will be part of the housekeeping and engineering telemetry of the final system.

During multi-mechanism testing, Ray ran the Blue and Red camera filter wheels for about 6000 cycles. After 5000 cycles, the blue filter wheel's drive pinion gear came loose. Inspection revealed that the bonding on the gear to the drive shaft gave way. Tom and Mark have a solution they will implement soon to prevent this. No similar problem was seen on the red filter wheel. This is why we endurance test all the mechanisms, kids. Better here than on the mountain...

During this test, the particular filter wheel failure revealed a previously unexpected vulnerability in the error checking code. Ray has addressed this problem, and the error checking is now very robust. Upcoming testing will test the error checking, either using actual (unplanned) faults, or inducing faults by disconnecting sensors, etc.