Friday, May 15, 2009

New Mirror Lab Project!

Work at the Steward Observatory Mirror Laboratory continues on the Giant Magellan Telescope (GMT), as well as the Large Synoptic Survey Telescope (LSST). In recent months I've posted numerous times about LSST as that is the project keeping me busy. But now, the entire casting lab is humming with activity as they prepare to cast another telescope mirror - this time a 6.5 meter diameter mirror for the San Pedro Martir Telescope for the Observatorio Astronomico Nacional (Mexico's National Astronomy Observatory).

Work has actually been proceeding for a couple months now, but just yesterday, the casting crew started installing cores into the mold, and progress is visible from hour to hour. In the top image, among the dust collection system, pneumatic lines for tools, gangplanks for access and materiel transfer, is the rotating oven and the beginnings of mold construction.

The lab is full of the hexagonal solids that are used for the mold. Made of an alumina silicate ceramic, these "soft" refractories are machined in a computer numeric control (CNC) mill to shape and length, depending where they go into the mold. Because of 6-fold symmetry, there still needs to be 170 program routines written for the 1020 cores for this casting. After a visual inspection, they are individually subjected to a vacuum test to verify there are no flaws or defects that will affect the strength of the mold from the stresses of casting.

Phil runs the CNC mill. After loading the blank into the mount and initializing the program, the computer takes control going through 7 or 8 tool changes, cutting the hexagon to the proper diameter, radiusing the sharp edges, cutting the crosspin holes and mounting post to the right diameter and length, and then cuts the hex to the length and angle for it's proper position in the mold (the height of the mirror surface changes about 30cm (12") from edge to center). After another inspection and another step or two, the core is ready for installation.

While the "standard" core shape is hexagonal, there are some unusual shapes needed to transition to a round inside diameter, as well as the round outer diameter. The strange shapes also require custom blanks, some handmade, so those are made earlier, rather than later. Here is a shelf unit filled with some of the stranger shapes required around the outside.

While the start of core installation starts in the center, you can see here that a considerable amount of work has already been done. Inner and outer tub walls (all seen in the top picture)were installed a couple months ago, wrapped with inconel bands for strength at high temperatures, and prefired to "seat" the bands. The "hard refractory" walls and floor were then lined with soft refractories which were then machined to form the outline of the back and sides of the mirror blank. These soft refractories are easily damaged, so there is a double layer of a working surface to help distribute the weight of workers.

The general form of the lightweight mirrors produced at the Lab is that of a faceplate and backplate about 2.5cm (1") thick, with a rib structure about 1cm (.5") thick, not unlike an I-beam - stiff and relatively lightweight. Of course, the inner tub forms the inner hole in the mirror to allow the light to reach the Cassegrain focus of the telescope. You can also see the gaps between the cores that will form the ribs, once the mold is filled with glass. The tops of the cores are installed last, as the cores must remain open to machine and install the cross-pins between the cores to help the mold rigidity during casting.

Here Randy installs a core into a spot defined by a guide frame, insuring that it is located correctly, and square to the optical and mechanical axis of the casting. The core is held down by a bolt made of hard refractory (that is what the torque wrench is for). And the pneumatic tool is actually a reamer that opens the crosspin hole to the right taper diameter for gluing in the crosspin, which Randy is getting from Jim at the photo at right. For a practiced team, the work proceeds rapidly - the 18 or so cores installed here represents just a few hours effort, though for the 900 cores, likely over a month will be required, followed by a cleaning, prefire, and another inspection before glass loading and the actual casting, currently scheduled for early fall. Stay tuned!

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