When reading Why was StDr56 discovered only now? and its answers and comments, I realized that refractor telescopes, and in particular the Dragonfly Telephoto Array are a great tool to find nebula and other faint objects. The proud parents at the Dunlap Institute summarize their technique nicely:

Dragonfly images a galaxy through multiple lenses simultaneously—akin to a dragonfly’s compound eye—enabling further removal of unwanted light. The result is an image in which extremely faint galaxy structure is visible.

Why are Earth-bound mirror telescopes not able to see such objects? To use uhoh's words:

[T]he back story there is that refractors were better suited than reflectors because nano-roughness of silvered mirrors produces a faint haze of scattered starlight that competes with natural skyglow to "fog" exposures and hide objects with low surface brightness.

This finally brings me to my question: How can this haze originating from the mirror telescope itself (and not from atmospheric effects) be quantified? Why does the nano-roughness of a mirror disturbes the image more than optical abberation of the lenses in a refractor telescope? Is it really only the nano-roughness? Or is it maybe also the support of the secondary mirror which is responsible for the "spikes" in pictures of stars?


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    $\begingroup$ @uhoh This is the question I promised to write :-) $\endgroup$ – B--rian Jan 22 at 23:20
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    $\begingroup$ Can anyone provide a reference for the idea that mirrors are necessarily rougher than lenses? $\endgroup$ – Mike G Jan 22 at 23:29
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    $\begingroup$ @MikeG The last reference cited in the question, page 9 for example, and references therein. $\endgroup$ – uhoh Jan 23 at 1:35
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    $\begingroup$ I guess Abraham et al. 2017 meant to cite Sandin 2014...which cites Abraham and van Dokkum 2014...hmm. Still not seeing justification for a sweeping "refractors > reflectors." $\endgroup$ – Mike G Jan 23 at 2:40

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