Effect of Dust on Optical Perfomance

Just wondering how much of an effect dust have optical performance. I often hear that a small amount has no effect. Is this true? How much dust is needed before having an impact on image quality. What about scratches or imperfection in the lens. I saw ads for refractors and for telephoto lens that states they have one or two small scratches on the lens but no effect in image quality. Could that be possible?

a little dust and or small nicks or scratches are not going to have any effect on the image. Its simply because the problem is well inside the near focus of the instrument in question. That is why you don't see the dust or scratches. It well however have an effect on contrast but for that to happen your going to have to have a lot of dust.

Excessive cleaning of optics can do way more harm than just leaving them dirty. Excessive cleaning can damage coatings even begin to alter the surface figure for those who can't resist the urge to really scrub

Clear Skies
Dwight L Bogan

Hi Yang
I had an 8" tube dob and had a cat get trapped in the tube and put hundreds of very tiny marks in the mirror along with 4 scratches about 2" long each. After cleaning the mirror a second time I was quite supprised that the view was still quite good and I couldn't actually see any of this damage in the view. Now I'm sure there was some degrading of the image but my point is that it was much less than I was expecting.
Buck