Quote:
Originally Posted by Ian Robinson
Nova Optical did potatoe chips up to 30" in 2" and 1.5" substrates.
|
30" X 2" mirrors shouldn't show any image defects at the horizon if the sling is designed and fitted properly, and the seeing is pretty lousy down there anyway.. FEA analysis only suggests 6nm RMS error for a correctly positioned sling. I suspect most of what people see as potato chipping are actually scopes that have stretching slings that allow the scopes to go out of collimation when they are pointed low. Warm air pooling at the top of the mirror box can also give an astigmatic effect.
The tolerances for angle of the sling support and edge position are very small though: a few mm off the centre of gravity at the edge of the mirror and things will go pear shaped pretty quickly. A 2mm positional error on th edge for a cable sling on a 30" X 2" mirror results in a Strehl ratio reduction of 7%. On the test bench, I can still see a quite recognisable airy disc with a 24" x 1.75" thickness.