[ATM] Or over-correction?

["vladimir sacek" <vla@toast.net>]

A late reaction on the idea of deliberately making a mirror 
under-corrected. In the context of visual use, there is a 
reason to make it slightly (or more than slightly) over-corrected. 
The reason is the spherical aberration of the eyepiece, which 
increases rapidly as the F# goes bellow ~5. The eyepiece 
being a complex positive lens system, produces under-correction 
(this should include fancy wide-fields with the negative Smyth lens). 
The size of wavefront aberration is in proportion to the eyepiece 
focal length, and inversely proportional to the fourth power of the F#.

While the eyepiece manufacturers still enjoy the privilege of not 
providing any specifications in regard to the level of eyepiece 
correction, the raytrace reveals that the correction level of a 
5mm f.l. eyepiece at f/5 is likely to be between Lambda/11 
and Lambda/28 PV of under-correction. This doesn't sound 
as much, but if a mirror is, say, 1/7 wave under-corrected, 
adding, say, 1/15 wave from the eyepiece would result in 
1/4.8 wave PV total error. Not quite negligible.

At f/4, the likely range of a 5mm eyepiece under-correction is 
between Lambda/4.5 and Lambda/11.5 PV. Making a mirror 
~Lambda/6.5 over-corrected would likely ensure best possible 
visual performance.

Vlad
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