Tuesday, October 23, 2012

One Down...

The press release came out today - WE FINISHED A MIRROR!  Melinda had been chiding me recently that since she has known me (going on what, seven years now), that we've never finished a mirror project.  Well, the press release explains a little of what went on.  This first mirror of the Giant Magellan Telescope has been the most difficult mirror to finish OF ALL TIME!  Really!  What makes it difficult is that it is an off-axis parabola, is very large (8.4 meters, nearly 28 feet diameter) and very aspheric.  Its asphericity is a measure of how far the surface differs from a sphere, which has a constant curvature, but isn't very useful in a telescope.  Most large telescopes have an aspheric departure measured in tenths of a millimeter.  The Large Binocular Telescope likely held the previous record at over 1mm of departure.  The outer GMT mirrors have 14 millimeters of asphere, and unfortunately, it isn't rotationally symmetric, which makes it even harder!  We have been polishing on it seemingly for years, and we really have - it took a lot of work to develop tools to polish and test the surface and the result is that it was completed to 29 nanometers rms (millionths of a millimeter).  Note that the diameter of a human hair is about 75,000 nanometers diameter!

The image at top left is from the LBT site linked above and shows the 7 mirrors that make up the surface.  The plan is for each mirror to have its own adaptive optics secondary that will correct atmospheric turbulence, and the yellow lasers are used to make artificial stars to control the flexible secondary mirrors to do that correction.  I like how they've been recently including semi trucks into the scene to provide for scale!

We finished the polishing a couple months ago and had a testing campaign of all 4 different methods of testing the mirror shape (thanks to the Hubble controversy of 25 years ago!).  The photo at left shows some of the process of the SCOTS test.  There is an array of fiducials placed on the mirror that are used to accurately align the error mapping to the mirror surface.  With testing complete, the last few weeks we've effectively put the mirror in "cold storage".  Since this is the first mirror completed, when the second one rolls around, we'll repeat the testing of this first one - effectively using it as the standard for completion of the rest.  This mirror may be around for a while, it might even be the last delivered!  So we've put a protective Opticote layer on the optical surface, covered it with 4" of foam, a layer of plywood, then a 10 mil layer of polyurethane.  So not only is it safe from dust, but also falling tools!  The photo at right shows it safely parked in our integration lab.  The mirror substrate in the rear is a 6.5 meter for the San Pedro Mártir Observatory,  to be eventually located in Mexico's in Baja Norte.  It is up next in the fabrication queue.
And speaking of what we're working on next, besides the ongoing LSST project, we've already cast the second mirror for GMT, and just yesterday has been exposed after mold clean out from the cast substrate.  If you think these mirrors look big standing next to it, try standing under it like I am here!  We'll be starting diamond generating of the rear surface along with the SPM scope mentioned above in the coming months, giving the second GMT mirror a bump on the progress curve.  Meanwhile the casting crew has already started the mold for GMT3!  No rest for the weary around here!


Astroweis said...

Hi Dean

Wow, great news! So, how long are the other 6 mirrors going to take? Hope, you have a big party.


Anonymous said...

Hi Dean

Thanks for the excellent descriptions on how the mirrors are made. These great telescopes will be among the greatest achievements that humankind will make. Many wonderful discoveries to be made by us and future generations.

Thanks, Tim C