Re: [ATM] Looking for information on a telescope design

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

Scott,

>(...) From this standpoint, the sole impact of the
> secondary magnification is to reduce the angular field of view in the sky
> that is projected onto a given linear field size, with field curvature 
> being
> the only aberration that is aggravated in combination.

Yes, if we follow the logic of the increased incident angle being
cancelled by as much smaller angular field in the Mersenne
arrangement. But things don't seem to be that simple. The widely displaced
stop is a wild card in that it can significantly affect off-axis 
aberrations.
I've put two 4" f/10 apos from ATMOS, one ED doublet with low residual
third-order coma and astigmatism, the other triplet with corrected 
third-order coma
and low astigmatism into a 400mm aperture Mersenne with 0.25D
minimum secondary size,  at half the f.l. of the f/5 primary from the 
secondary
(in Oslo).

At 20mm off-axis (1.2 degree), refractors alone have about
1 wave  and 1.3 wave p-v, mostly astigmatism on best curved field,
-340mm and -380mm, respectively. In the Mersenne, at the same 20mm off-axis
(0.3 degree), the doublet has 3.7 wave p-v of the combined third- and 
fifth-order
coma and astigmatism, on the best surface radius of -120mm. The triplet has
4.7 wave p-v of mainly third- and fifth-order coma plus some fifth-order 
astigmatism,
on somewhat more strongly curved field. Average blur size in 20mm eyepiece 
is
roughly 20 arc minutes, which probably doesn't appear too offensive, but 
leaves
something to be desired.

Not only than the displaced Mersenne stop can cause significant off-axis 
aberrations
in refractors, it seems that their level cannot be even approximately 
predicted.
It would require raytracing to find out what the field quality is.

Vlad 

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