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

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

This report covers the MODS discussions at what was otherwise a general instrumentation group meeting. Note that the main effort of our group at present is LBT Aluminizing, so much of the present MODS effort is focussed on working with vendors who are fabricating many of the key components of MODS since most of the design work is completed. Once Aluminizing is done, the assembly, integration, and testing phases of MODS1 will begin in earnest.

Optics

Paul has reviewed the first 2 collimators mirrors delivered from REOSC, and they are essentially perfect. Their shipping packaging, however, was not very good, and put the pieces at some risk. We got lucky they weren't damaged. REOSC has been told, and they will change what they do when shipping the other 2 collimators to us early next year.

The blue field flatteners have been delivered by SOML, and the red field flatteners should be ready soon.

Report from Steve Miller at SOML is that the grinding for the aspheric side of the blue correctors is complete (recall that the 2 correctors are off-axis pieces from a single parent, so we get 2 at once). The surface figure error is currently 9-microns peak-to-valley. Polishing on the asphere will begins soon. The spherical sides of the correctors have an rms of 0.73 waves at 632 nm. Once the blue correctors are done, SOML will start in on the red correctors, since they are already tooled up for doing them, then proceed to the remaining camera primary mirrors.

Paul has interferograms for the collmiators and the first camera primary mirror, and has been working to integrate them into his Code-V model to evaluate the predicted image quality with the delivered optics.

Paul is also working on quotes for the dichroic, fold mirrors, and calibration system optics. These are small pieces that will not take too long to fabricate.

We have an integrating sphere quote (12-inch diameter with a 4-inch diameter port), but it will need some modifications for the calibration lamps. We're working with the vendor to get the necessary adapters for the penray line source lamps and the quartz-halogen continuum lamps. We have taken delivery this week of the first set of calibration lamps, 5 penray lamps for Ar, Ne, Xe, Kr, and HgA, as well as three 10W quartz-halogen lamps. We have bench power supplies for all of these, and are exploring remotely programmable supplies for the quartz-halogen lamps.

Mechanical Fabrication & Design

Indian Creek Fabricators in Dayton has made excellent progress on the first of the two upper structures. Mark and Tom were out at Indian Creek earlier this week measuring the reference points and mountings to verify everything is in place before the final welding begins. All of their measurments were within 1-mm of what they should be (bear in mind that this is in a steel structure nearly 3 meters across). All the various subframes for inclusion in the structure are done (e.g., the subframe used to support the focal plane mechanisms), all that is needed now is for the frame to be welded together and the various parts "carpentered" into place. They're doing a terrific job, and we should see the first completed structure in January.

Mark Derwent has been doing the finishing work on the cart. This cart has three basic functions:

1. Transport MODS to the telescope for installation, and to store MODS when off the telescope.
2. Hold MODS in a convenient horizontal position for assembly and servicing when off the telescope.
3. During initial integration and testing, provide a pivot fixture so we can rotate MODS about its center-of-gravity for flexure testing (when deployed at the telescope, this fixture will be removed).

Mark reports that the parts for the instrument hatch (aka "darkslide") will be coming in this week from Columbus Machine Works, and that 30 of the 40 tapers being fabricated by Elyria Metal Spinning are done, we should get all 40 delivered during Christmas week. Each MODS spectrograph needs 16 tapers, so we've order 32 with 8 extras.

MDM4K Filter Wheel

The new MDM4K filter wheel was successfully deployed at the MDM 1.3m telescope last week by Darren, Jen, and Rick. This is relevant here because the MDM4K filter wheel is essentially identical to the MODS filter wheels, the main difference being that it has mounting points for 12 4-inch square filters instead of 8 rectangular filters in the MODS filter wheels. Otherwise the overall design, drive gears, drive motors, use of proximity sensors for position encoding, radial mounted filter cells, etc. are identical. The MDM4K filter wheel also lets us evaluate intelligent motor controllers that we are contemplating using in MODS, and to prototype the motion control software. We are also field testing an ethernet serial port server which we are contemplating using for MODS. This device (a Comtrol DeviceMaster RTS-4 for this test) lets us treat the filter wheel as a network-addressable device (TCP/IP), even though the interface into the intelligent motor controller proper is an RS232 serial connection.

During the deployment, we made two main tests of relevance to us here:

Filter Position Repeatability

Here the filter was stepped through 20 random positions, ending on a pinhole mask which was imaged against a flat field screen. This was repeated 5 times for each of a number of telescope azimuths (NSEW) and altitudes from 45 to 90-degrees (zenith-pointing). A total of ~1200 moves were done during the testing sequence. The filter position repeated to better than 10-microns from position-to-position on the sky, and at a single sky position repeatability was <5-microns, the limit of our ability to measure. The mechanism has a design spec of 10-microns, so we met the spec.

Motor Interference

Here we operated the CCD (a 1024x1024 Tektronix CCD operated with a Leach GenI controller) with the filter wheel operating in various configurations to see if running the filter wheel during readout would cause interference (appearing as either increased readout noise, fixed pattern noise, et al. in bias images). The CCD is mounted directly to the filter wheel, and the readout electronics is also bolted to the filter wheel, so all are in close proximity. Four sets of tests runs were made as follows:

1. 10 bias frames with the FW system completely powered off.
2. 10 bias frames with the FW motor controller powered on and the motor idling at 50% holding current (1.6 Amps).
3. 10 bias frames with the FW Motor running continuously at 3.3 amps drive current.
4. 10 bias frames acquired while the FW is setup to select a random filter position, which involves the motor switching between full current (3.3 Amps) and zero current every 2 seconds (i.e., pulsing the motor).

In all cases we saw no evidence of fixed pattern noise and could measure no increase in detector readout noise when comparing frames taken when the filter wheel motor controller was activated (continuous motion, pulsed motion on/off, and idle) to when there was no power to the controller. Since the filter wheel is mounted in direct proximity to the CCD dewar and its readout electronics, this implies to us that these controllers pose no interference problems. Note that we have take no noise suppression measures: none of the motor connections are shielded. For MODS we plan on using shielded connectors with all of the mechanisms.

Guide Cameras

We have not yet received the guide camera system from Steward Observatory. Delivery now looks to be sometime early in 2004. The holdup is a series of issues involving the Gen3 controllers and their control computers that need to be resolved by Mike Lesser's group before they can deliver the systems to OSU and Potsdam. One of the tests we intend to perform early on is to try our filter-wheel noise tests with this camera, using an identical system to the MDM4K that we are building for the Yale 1-m telescope at CTIO.