Two Mirrors Facing Up
Latest Photos from the July 28, 2007 "GMT1 Quarterly Review" at the University of Arizona’s Steward Observatory Mirror Lab show progress made preparing the front surfaces of two mirrors. The GMT1 primary segment is right-side up in preparation for front surface generation. The fold sphere mirror is already being ground in preparation for polishing.
Tucson, AZ. The Giant Magellan Telescope’s primary mirror – comprised
of seven segments, each 8.4 meter (27 ½ feet) in diameter – is being
developed and manufactured at the University of Arizona’s Steward
Observatory Mirror Lab.
We’ve been following the progress of the first mirror segment, called “GMT1.” This is the first of eight segments planned for the GMT: seven segments plus one spare. Successful production of this first segment is critical to the timely completion of the Giant Magellan Telescope.
In a previous report (April 2007), we saw the mirror upside down during preparation of its rear surface, concluding with the attachment of the loadspreaders: the triangular metal support brackets that will be used to secure the mirror into its holding cell on the telescope mount.
| Once again facing upward, the 8.4 M mirror segment reveals its concave surface formed by spin-casting. |
We now see the mirror right-side-up again, mounted in its polishing cell, in preparation for generating the front surface using a diamond wheel. Since the glass mirror is far too costly to practice on, a large piece of granite is currently being used to put the LOG (Large Optics Generator) through its paces; the actual generating of the front surface of the GMT1 mirror is expected to start next month.
| Surface Generation programs are first evaluated using a diamond wheel on a granite disk. |
Meanwhile, detailed designs of the equipment intended to test the mirror’s accuracy are being prototyped, and supplemental tests to verify the mirror’s figure have been developed.
| The 3.8 m fold sphere mirror surface is being shaped by grinding with a loose abrasive. |
We are also watching the progress of another GMT mirror being produced at the Mirror Lab: the 3.8 meter (12 1/2 feet) “fold sphere” mirror. This mirror, when completed, will be installed near the top of the Test Tower and become part of the arsenal of lasers, mirrors and holograms used for verifying the shape of the giant mirror segments. The fold sphere is shown here currently being ground into shape on the LOG.