Sunday, November 1, 2009

Steward Observatory Mirror Lab Update

It has been over a couple months since I've posted about work. Doesn't seem that long, but it was regarding the 6.5 meter mirror we are casting for a Mexican Observatory. That project is still in the oven, it's grand unveiling in a few weeks when it finally reaches room temperature. Progress is also being made on the Giant Magellan Telescope, but it is a complicated project, and I'm waiting to blog about it...

A couple months ago we finished polishing the rear surface of the Large Synoptic Survey Telescope primary, and a crew has been busy bonding loadspreaders to it. The loadspreaders are computer-modeled to best support the mirror while being polished in the lab, and while in the telescope on a Chilean mountaintop. The supports are passive, hydraulic supports while being polished, but will be under active computer control to fine tune the mirror shape as the telescope moves to different parts of the sky. Here Dan is installing a "double" against the brown blocks, previously placed with double-sided tape. On the right he is pressing out the 2-part RTV adhesive until pressed out to the correct thickness (the string spacers).

The loadspreaders need to be located to accuracies of better than a half millimeter (.020"). In years past we went to a great deal of work to survey and mark the rear of the mirror with tiny diamond marks to lay out the supports. These days we use a "lasertracker" to lay them out - using a beam of light to measure not only angles, but the distance to a cornercube to a thousandth of a mm (.00004"). They are getting to be a popular tool around the lab - in fact, we currently have 4 of them, at well over $100K each (several are dedicated to particular projects). At left here, Robert waits patiently for Dan to finish the loadspreader layout. At right, he monitors the computer display readout of the lasertracker (lasertracker is visible as the squat black cylinder left of his head) and fine tunes the position of the support.

Loadspreader gluing was finished early last week, and LSST was moved into the Casting lab for an overall survey (the entire backplate couldn't be seen on the generator machine where layout occurred). The image at right shows the variety of loadspreaders (singles, doubles, triples and quads) used with the entire mirror backplate seen. I heard that the loadspreader survey indicated that only one loadspreader was located slightly out of spec, so had to be cut off and re-adhered. The only work left to be done is the installation and wiring of thermocouples to measure the mirror backplate and faceplate temperatures during fabrication and in the telescope. This work has already started and needs to be finished before flipping the mirror and installing it in the polishing cell for start of work on the front surface.

When loadspreader bonding started, I moved over and filled in as needed on GMT, as well as working on procedures and preparations for LSST faceplate generation. However, for a week or so in the midst of the above, I got an interesting assignment. Roger Angel, the original "Idea Man" of the Mirror Lab, has a current interest is in solar collectors and high power photovoltaics. The Lab, in conjunction with the University of Arizona and Raytheon has been working on a prototype collector. As described in the Arizona Daily Star the collector and the new PV sensors, when produced in bulk, will make electricity from the sun at comparable costs to coal. I needed to make a little prism from stainless steel - the goal is to electroform a low-mass pyramidal concentrator, so needed to bring it to high polish. Never working with polishing metals before, I needed to try a couple combinations before hitting one that worked. As you can see, the left unit came out pretty well, compared to the right one which shows the machined, heat treated surface. The solar collector was finished last week, and gets shipped out this week. This is a shot with one of our students, Jason, for scale. Always something going on at the Mirror Lab!

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