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

[vladimir sacek <vladis.2@juno.com>]

<...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
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