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

[Ken Hunter <atm_ken_hunter@yahoo.com>]

Tim...

Coma increases the more you get away from the
"on-axis" condition. It is increased also as your
focal ratio decreases. Large fast mirrors have more
coma than large slow (long focal length) mirrors.

Note that coma is an OFF-AXIS phenomena. If you cut a
6 inch section out of a 16 inch mirror (or a 40 inch
mirror), you will have the same coma (coming from the
6 inch offset from the axis) as in a 12 inch mirror
system assuming that the 6 inch segment is set up so
the edge is 6 inches from "on-axis" however, since the
number of square inches is less, you will not observe
the coma as well (or anything else). If you were to
set the 6 inch mirror segment up so that the edge of
the mirror were at 10 inches from "on-axis" you should
see coma as if you were using a 20 inch mirror.

 If you are observing planets, it makes very little
difference if the scope has coma as you are observing
mainly ON-AXIS... Lage star fields at the focal plane
and "faint Fuzzy" photography is something else
entirely but here's the rub...

In general for faint fuzzies you want as much light as
possible. Cutting away most of the mirror just to
become unobstructed is trading light loss for whatever
you gain by cutting away the rest of the mirror. Is
the loss of light justified by the loss of difraction
effects? Which is worse in the end?

I kinda like those nice fat spikes on star images
but... Hey, that's just me.

There are systems that minimize coma but usually at
the expense of field curvature or other aberations.
Most systems are tradeoffs.

First thoughts should be to decide what you are really
wanting in capabilities and then search for an optical
system that will give you MOST OF WHAT YOU WANT
because so far, we haven't designed the perfect system
yet.

Ken Hunter


--- "Cross, Timothy" <TCross@NRCan.gc.ca> wrote:

> Ken,
> Would the coma of an off-axis Newt depend upon from
> where the off-axis
> primary was cut on the parent mirror?  I.e. if you
> use a larger than
> necessary primary (e.g. 16") to cut four 6" off-axis
> primaries and thus
> avoid certain zones (e.g. the edge) of the parent
> primary, could you reduce
> coma?  Obviously this wouldn't make full use of the
> parent primary mirror
> though.  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?
> Regards,
> Tim Cross
> 
> 
> ------------------------------
> 
> Message: 24
> Date: Sun, 24 Oct 2004 17:20:30 -0700 (PDT)
> From: Ken Hunter <atm_ken_hunter@yahoo.com>
> Subject: Re: [ATM] Off-centered Mirrors?
> To: DHA352@aol.com, atm@atmlist.net
> Message-ID:
> <20041025002030.96344.qmail@web52703.mail.yahoo.com>
> Content-Type: text/plain; charset=us-ascii
> 
> The coma is contributed by the photons that are
> coming
> off-axis to the Parabola. In the case of a mirror
> like
> you describe, you would get the same coma (but less
> light) than you would from the original mirror.
> 
> Ken Hunter
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



	
		
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