Making giant telescope mirrors

MagAO and VisAO got a lot of press last week, when we announced our first-light results — demonstrating diffraction-limited imaging with 0.02 arcsecond resolution. This is the finest resolution of any filled-aperture long-exposure images ever taken! See the press release here.

But did we ever tell you where the Magellan telescope primary mirrors come from?

Steward Observatory Mirror Lab at the University of Arizona

They come from the Steward Observatory Mirror Lab (SOML) and it is the only facility in the world making giant self-supporting monolithic mirrors. Testing and polishing is also done right here in the facility. At the SOML the techniques were researched, developed, and executed by a team led by Roger Angel (who was awarded the Kavli Prize for his work). All of our beautiful images coming out of MagAO and VisAO would not be possible if the telescope mirror wasn’t producing an amazingly flat wavefront. The primary mirror must be stiff and strong, but also quickly responsive to changing temperatures for maximum stability. This is accomplished by an innovative hollow honey-comb structure for the glass mold that is both strong and lightweight. The glass is melted in a 2000-deg.(F) furnace spinning at 5 rpm to produce the proper shape, before being polished to a perfection of 20 nm rms.

Bear Down.

This past Saturday was the casting of the GMT3. The Giant Magellan Telescope (GMT), not to be confused with the Somewhat-Less-Giant Magellan Telescope, is a 25-m-diameter telescope being built for use at Las Companans Observatory in Chile. The GMT will be on a neighboring peak to our own at LCO. Once we get tired of MagAO/VisAO images (lol), GMT will be one of the next telescopes producing the highest-resolution images ever!

A model of the Giant Magellan Telescope

GMT3 is the third off-axis segment for the GMT. The first and second segments have been cast, and this past weekend marked the melting and beginning of the cooldown of the glass for the third segment. Here are some pictures and videos we took at the event. For the official stuff go here.

Here is the third segment, GMT3. It is spinning at 5 rpm in a 2000-deg. furnace to melt the glass.

And here’re some videos of the action, showing the spinning oven:

The first two segments have already been cast and are stored in the mirror lab:

The first segment of the telescope, called GMT1, cast in 2005. It is now complete, with a better than 20 nm RMS surface polish.

The second segment, GMT2, was cast in 2012. This is its backside, which was first smoothed to prepare for mounting in its cell. It will soon be flipped, and polishing will begin on the front optical surface.

And here is the fourth segment… we’re halfway there!

The fourth segment, GMT4, is in pieces here.

The participants in the casting event were members of the GMT consortium and their guests

The mirror lab is hard at work on other projects too, 24/7. During the tour the LSST primary mirror was being polished:

This is the large polishing machine, where the LSST is spinning around while the stress lap polishes it to perfection.

Peter Strittmatter explains the spin-casting process.
This is the test tower used to measure the wavefront of the mirror blanks during polishing. There at the top, that’s me! You can see my name tag reflecting off the 3-m spherical mirror used to test the mirrors — I was standing in the right place on the floor.
Roger Angel shows off his new furnace, for fabricating solar-telescope mirrors.