Re: Cleaning and a Question

Richard Schiek (richard@chemeng.Stanford.edu)
Thu, 6 Apr 1995 13:58:22 -0700

Jaakko Oksa wrote: | Since then I haven't paid much attention to local defects
| which are not near the focal plane...dust, scratches and
| such are in my opinion insignificant compared to things that
| really do matter, i.e. optical and mechanical quality of an
| instrument.
|

This is something I've never really understood. I know from experience that some dust or scratches on the primary don't degrade the image significantly. However, a quarter wave problem on the primary will significantly degrade an image. Since dust particles are definitely larger than a wavelength of light why don't they cause significant errors in the image? My guess is that a figuring error in the primary is a systematic error that will degrade the entire image while dust and scratches are random errors that will tend to average out on the focal plane (but possibly reduce contrast?) Can anyone help on this?

Thanks,

Rich Schiek richard@chemeng.Stanford.EDU