Thursday, May 14, 2009

More absolutely wicked optics :)

The current priority is to try out the Computer Generated Hologram (CGH) test that will eventually be used to align M3 with respect to the other mirrors & hence to check the optical alignment of the whole SAC system.

At first glance, the CGH looks like a thick CD without the hole in the middle, but it's quite a bit fancier than that... It's specifically designed to generate all the spherical aberration that SALT's primary mirror array would ordinarily impose on the incoming wavefronts - exactly the aberration that the SAC's required to clean up. By putting that aberrated light through the SAC & evaluating what comes out, we can test whether the 4 SAC mirrors are working as they should.

The CGH has been bonded to a base that will attach to the stage assembly & a set of safety hooks was installed around the edges for additional peace of mind.

The CGH consists of 23590 concentric chrome rings of varying thickness & separation deposited on a 15 cm wide flat piece of Sitall (the same low-thermal-expansion material used for SALT's primary mirror segments). It behaves like a diffraction grating & is thoroughly spectacular when illuminated & viewed on-axis!

As for the pinhole in the M4/M5 test, the CGH stage assembly lives in a tube that will be inserted into the hole in M3 to locate the CGH at the appropriate axial position.

Wires, wires - everywhere! It's all got to be controlled somehow...

Craig had to test some of his gear so the interferometer, NRS simulator super-structure & SAC cover had to be removed, leaving the Faro stranded without its usual hitching post.

Not an extractor fan for the SAC - this plate will serve as a drilling jig for improving the way the M4 mirror cell is secured to the 3 vanes of the mounting structure.

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