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

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

Ken Hunter wrote:

< 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.>

It is reasonable to expect that OA's tend to be used more for
high-power observing than large Newtonians, which would make
coma - even if it would be of the same magnitude - noticed less
frequently. But in regard to useable field size, OA's have advantage 
over large Newtonians. For instance, 5mm exit pupil requires 80x 
magnification (~1 degree field diameter tops), with a 16", while only 
30x (1.5+ degree field) with a 6". Even if an OA scope would use only
1.25"
low-power ep, it could still show field comparable to that of large
Newtonian.

For any given field angle, an OA section will generate nearly as much
coma (blur length)
as its parent mirror - assuming it's diameter being between 2/3 and 3/4
of the parent 
mirror radius. The main reduction in the physical blur size (mainly blur
width) comes from the 
OA section covering only a small part of the coma producing parent mirror
surface, 
not from its usual off-axis position at that surface. The blur is there,
but it doesn't have 
comatic shape; even if OA owners would look for coma, they wouldn't be
able to see 
its usual telltale signs. It would be probably helpful to call the OA
coma differently
(segmentary coma, or alike): it has different shape, position and effects
vs. ordinary coma.

Vlad

 
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