College of Mathematical & Physical Sciences
Department of Astronomy
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The Ohio State University College of Mathematical & Physical Sciences Department of Astronomy |
Attendees: Darren DePoy, Pat Osmer, Bruce Atwood, Dan Papallardo, Jerry Mason, Tom O'Brien, Paul Byard, Jen Marshall, Rick Pogge, Mark Derwent, & Dave Steinbrecher
This is our first meeting after the start of Spring Quarter.
Optics
Paul Byard reports that the Ohara BK7 blanks for the red camera correctors have been shipped to Steward. Corning is about to ship the fused silica blanks for the blue camera correctors to Steward, but there are still some loose ends to tie up. The Hextek collimator blanks are ready for shipping to REOSC, pending completion of the final paperwork.
We will be sending Steward a change order regarding the glass for the red field flattener. Steve Miller (SO) says he has an optician he wants to put on the small optics right away, so we have to move forward on the CCD tests so we can get an order in to Schott for the NZK7 glass.
Paul is talking to possible vendors for the bypass gratings used in the IMCS (ex-FCS), working out issues regarding the size of the grating, energy distribution, etc. We need about 20 of these gratings for the IMCS. The current optic Paul is working with creates 100 orders with a 0.3-degree separation of the beams (30-degree spread). Open questions are the minimum size of the beam, the spot size on the detector, and the distribution of energies among the orders. The range of the grating is determined by the range in angles we need to go from 0th order to largest anamorphic angle, or 40 degrees. So far he has been talking to a Finnish firm that specializes in diffractive optics, but we clearly need to learn more about this technology before we can proceed to our final choice of bypass grating.
Mechanical
Tom and Mark paid a visit to a steel fabricator in Dayton and delivered a bid package for the upper structure of MODS. The facility was impressive and well equipped for massive steel fabrication. We are waiting to hear a response (they called to say their bid would be a few days late). We're also speaking with a firm in Chicago, and looking for a couple of more vendors to send the package to. The bid package is very complete, so bids should be accurate.
Hextek finished the 4th camera blank, which replaces the one that cracked during slumping, and will have it shipped to Steward for polishing. Tom has finished measuring the cracked camera blank we had in hand, and made changes to our mounting design to accommodate the camera primary mirrors as-built. The collimators required no change in the mounting design as they were built.
Ralph Belville has prepared a set of drawings detailing the MODS shutter mechanisms. We will be sending these to ASU to see if their shop is interested in making these for MODS.
Jerry Mason has all the parts we need to test how we will bond the invar flexures to pyrex (the glass in the camera primary mirrors and collimators), so we're ready to start those tests. We don't have any zerodur samples yet to test the invar bondings for the gratings. Keck did a fair amount of work on this for their primary mirror segments, but we found that we can't use the same epoxy because it has more like a syrup-like consistency, whereas with our mounting bosses an epoxy with a paste-like consistency will be easier to work with. We found a paste epoxy in the same family as the Keck epoxy and will be testing that.
We received the linear encoders that will be used to measure the collimator actuator performance. Position repeatability tests will proceed shortly.
Mark Derwent has all of the parts he needs the grating select assembly, except for the last couple of flexures still being finished. He spent last few days hooking up the cell for the imaging flat (doesn't have the same tilt mechanism as gratings), and things are going well. We have the grating turret handling carts in hand, and after some finishing work they will be ready to use. The cart design is a close analog of how we intend to support the turrets in the instrument, so using them is a good test of that design. The last bit to be designed is a lifting fixture for how to get the turrets on and off the carts (and analogously, how to get the in and out of the instrument).
The covering for filter wheel has been designed and is out to bid.
Progress is also being made on the design for the dichroic select mechanism, the main points now are working through issues with interference with other parts of the instrument and the light path since the dichroic is in a tight part of the instrument. Mark and Tom are continuing to iterate on this.
[IMCS = Image Motion Compensation System, aka the "Flexure Compensation System" as named by engineers. "Image motion" in this context is unwanted motion of the spectral image on the detector due to gravity-induced flexure of the spectrograph structure as it tracks across the sky. It is not to be confused with image motion as usually applied to motion of images on the telescope focal plane due to atmospheric turbulence, tracking errors, etc.]
Jen Marshall is currently working on the prototype of this system, which currently in the definition phases. One question that has arisen is how much observer interaction do we want? There are two choices:
It turns out that #1 (on/off/setup) is sufficient for most uses of the system. Unlike a guider, the low-level setting will really only be needed by the support people to diagnose problems or recalibrate the system. From the observer's point of view, it should just work transparently, and what is needed are basic "toggle" commands to (a) initialize the system, (b) turn it on and close the loop, (c) open the loop and disengate it. Letting observers at lower-level settings would just be asking for trouble (no user servicable parts inside), but we would of course provide a full engineering interface for diagnosing problems, separate from the simple on/off/setup interface provided to the data-taking software proper.
As for also tweaking the grating in real-time with this system (option #2), this turns out to be a red herring. The grating tilt and collimator steering resolution is about the same, about 1-micron on the CCD (which has 15-micron pixels). If we include both grating and collimator control, it amounts to trying to control 3-axes of motion with only 2-axes of feedback (x,y position of the spot on the IR array), which would way over-complicate the control system. The decision is that the IMCS only works by steering the collimator mirror and leaves the grating out of the control loop.
Jen, Rick, and Darren will continue discussing what the FCS looks like to the observer, and developing specifications leading to a prototype setup protocol to be reported later.
R. Pogge, 2002 April 11