Re: [ATM] 7 inch sphereical mirror

["David Harbour" <stainless_steel@suddenlink.net>]

Hey guys,

The astigmatism should be negligible; invisible in the longest focal length 
eyepieces; or, if it is there, it is the eyepiece that puts it there.

R-101

----- Original Message ----- 
From: "Richard F.L.R. Snashall" <rflrs@rcn.com>
To: "ATM List" <atm@atmlist.net>
Sent: Monday, April 14, 2008 12:05 PM
Subject: Re: [ATM] 7 inch sphereical mirror


> Dominic-Luc Webb wrote:
>>
>> Did I miss something? My understanding was that coma is the
>> price paid when you make a parabola. Spherical aberration is
>> what you get with a sphere. At some point, the trade-off
>> between these two defects should be considered. Coma increases
>> as one progresses off axis. I think an F/10 can perform very
>> well. This scope is in the range of a lensless Schmidt, and
>> there are some very impressive images out there of objects like
>> Andromeda galaxy covering something like 3 degrees of sky.
> At 21 mm off-axis (0.6767 degrees), I get the Visual Strehl of
> about 0.34 and at the 70.7% point of that, about 0.58.  The
> "diffraction limited" field (0.80) is about 0.5 degree diameter.
>
> Of course the field is convex to the viewer, so there may be
> additional losses due mismatch with the eyepiece.
>
>
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