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Straylight scattering and mirror polishing

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#1 Lucullus

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Posted 11 August 2020 - 04:04 AM

When reading about straylight affecting optical systems, I read that refracting systems are much better than reflecting systems, because in the latter straylight bounced back and forth between the mirrors creating kind of a straylight fog leading to a brighter picture once all those rays reach the focus - this is the picture in my mind of the process.

Here is a good paper about the straylight reduction effects from baffling improvements:

http://articles.adsa...A&AS..116..367G

In one of the Dragonfly Telephoto Array articles I read the statement concerning lens and mirror systems.

 

Ok, you build a Cassegrain-like telescope with two mirrors and a whole in the primary mirror; like in the link.

And you can only polish or have only one of both mirrors extraordinarily smoothly polished.

Which mirror would you want to have a smoother surface causing less scattered light, the large primary or the smaller secondary mirror?


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#2 Benach

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Posted 11 August 2020 - 06:39 AM

Secondary mirror should be smoother. Scatter is basically a shape error of the mirror. The secondary mirror magnifies the shape error of the primary mirror whereas the primary mirror does not magnify the errors of the secondary mirror.



#3 msheald

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Posted 12 August 2020 - 06:31 PM

Hello! I've heard that the secondary should be twice as smooth as the primary in order to keep the primaries error at the visual or photographic plane.

 

    So, for a 1/4 wave primary, the secondary should be 1/8 wave. Best regards.

 

Mike



#4 Lucullus

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Posted 13 August 2020 - 12:54 AM

Does this depend on the secondary's magnification factor of is this general?


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