Re: [ATM] Re:ATM Digest, Vol 10, Issue 17

[Ken Hunter <atm_ken_hunter@yahoo.com>]

OR...

Since coma is related to the distance OFF-AXIS, how
much coma can you generate by being only 3-4 inches
off-axis? These are probably f-long enough that you
are going to be mainly looking at narrow views near
the center of the focal plane so most folks won't
recognize the coma because they aren't looking for it
and for the above reason also.

Ken Hunter


--- vladimir sacek <vladis.2@juno.com> wrote:

> <...Very little coma has been reported by DGM
> off-axis Newt owners
> however, so there must be some way to reduce coma in
> this scope design;
> perhaps the off-axis primaries are refigured
> slightly after being cut
> from
> the parent mirror?>
> 
> OA coma reduction is inherent to the design.
> Oversize parent mirror
> wouldn't 
> have an effect if the f.l. of the OA mirror is fixed
> (unless you move the
> cut towards
> the edge of the larger parent mirror, which would
> make coma worse).
> Refiguring
> of the OA section also wouldn't have effect on coma,
> which is independent
> of mirror 
> conic (but would induce form of spherical
> aberration).
> 
> Comatic blur of parabolic mirror is formed as each
> zone of the hight "z"
> on it focuses not into a point, but into a circle of
> diamater hz^2/2f^2,
> where "h" is the
> off-axis distance in the image plane, and "f" the
> mirror f.l.. Each
> comatic circle has also 
> its center shifted as much (for the diameter) from
> the point formed by
> the rays from the 
> very center of the mirror. Comatic blur emerges from
> these spreading
> circles into a final blur, 
> whose max width is equal to the diameter of the
> circle formed by mirror's
> edge, and 1.5 times
> as much in its length.
> 
> >From this simple geometry one can easily figure out
> what an off-axis blur
> of an OA section will
> look like. For a 6" section out of a 16" f/4 parent
> mirror, which would
> be 1.5" from the center and 
> 0.5" from the edge, the new blur would be about 4/5
> as long and 1/4 as
> wide, at its widest. It would 
> also have different ray distribution and different
> way of overlapping
> diffraction pattern. So where the parent 
> mirror would begin to slip bellow "diffraction
> limited", with the coma
> length reaching 1.6 times the Airy disc 
> diameter, the OA section would be better than
> diffraction limited, with
> its blur length being 1.6*0.8/2.7=0.5 times 
> the Airy disc diameter. However, I wouldn't expect
> it to have three times
> larger quality field than 
> the parent mirror, due to the rays more evenly
> distributed throughout the
> blur. My guess is that OA mirrors
> typically have 2 to 2.5 times wider quality field
> than parent mirrors.
> 
> At low-power wide-field observing what matters is
> the physical blur size.
> The OA blur is 2-3 times smaller by the area, 
> and its dense (roughly) half is not just smaller,
> but also much more
> round-like than familiar spreading tails of sagittal
> 
> coma in fast parabolas. That makes it less
> noticeable, and definitely
> less coma-like.
> 
> Vlad
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



		
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