Quote:
Originally Posted by Satchmo
It does surprise me with these premium intruments that optical defects ( ie astigmatism , corrector lens wedge etc ) can slip through . It suggests that they are not given a thorough end to end artificial star test upon final assembly, to make sure it is meeting spec and that the customer is not going to see optical defects on the real sky. I would have thought that that kind of thing was what you were paying extra dollars for.
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The off-axis astig doesn't show up on a star test, only on really big CCDs. The wedge issue was only apparent with a one-shot color CCD in very good seeing on a star near zenith. I might have been able to see visually, but probably would have had to know it was there to distinguish it from atmospheric dispersion. In fact in the normal star testing I did the correction\roughness\on axis astig looked really good.
Quote:
Originally Posted by Satchmo
It is not rocket science to set up a collimating telescope of larger than the aperture to view an artificial star upon final assembly and tweaking. Celestron for example in their heydey used a 22" of long focus telescope to produce a collimated beam and final checking on hundreds of thousands of telescopes.
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For a big telescope the artificial star need to be <very> far away (impractically) to be of any use, or you swamp the test image with S.A., so the collimating scope is not the problem, it's the <very> long hallway one needs, see here for example:
http://www.telescope-optics.net/two-mirror2.htm#Close_objects_error
Regards,
EB