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Old 24-11-2013, 11:01 AM
clive milne
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clive milne is offline
Join Date: May 2011
Location: Freo WA
Posts: 1,443
I think it also worth adding to the discussion that field correction is a distinct issue separate from optical quality... if the component added to the image blur circle size by virtue of optical quality is below the sampling limit (on axis) then it (optical quality defects) will swell the image blur circle more or less the same at the edge of field.... the caveat to this is the usually unstated assumption that the location of the optical defect is coincident with the field stop. Unless you are talking about full aperture correctors located at the centre of curvature of the primary (lichtenecker, baker schmidt etc) then this is pretty much a given.

on somewhat of a tangent... but not totally unrelated... if you put the aperture stop at the centre of curvature of the primary mirror (ie, at twice the focal length) and leave the primary spherical, the system will have no coma or all! The only issues with it would be spherical aberration which would limit the speed to about f8 for a 200mm aperture, you would have field curvature (1600mm radius) and the system would be physically enormous.
It is however a very useful hypothetical instrument to help you understand the true nature of coma and astigmatism in geometric optics....
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