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[OM] Re: 50/3.5 odd sighting

Subject: [OM] Re: 50/3.5 odd sighting
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
Date: Mon, 19 Mar 2007 06:48:06 -0400
Coma is an abberation that can afflict all lenses.  It is worse at 
larger apertures and also gets worse the further away from the optical 
axis of the lens.  It's not limited to astrophotograpy; it's just that 
star images can make it painfully evident that the optical system 
doesn't handle coma well.  Instead of nice round dots the stars may look 
like mini comets and grow to larger comets the closer they are to the 
edge of the field of view.

I will disagree a bit here with Moose when he says you needn't worry 
about it until you get a nice long lens.  In fact, the nice long lens 
will not likely be a fast lens.  So a 500mm f/8 will not likely exhibit 
a great deal of coma but a 50mm f/1.4 might have significant coma when 
wide open.  But the coma should be significantly reduced by closing down 
a couple of stops.

The typical 500mm f/8 mirror lenses (those with a steeply, inwardly 
curved front element) use what is called a Maksutov corrector lens.  The 
primary mirror is typically a spherical mirror which would suffer badly 
from coma but the Maksutov corrector front element is specifically 
designed to correct coma.  If your long lens is a catadioptric system 
(both lenses and mirrors) and employs either a Maksutov or Schmidt 
corrector as the front element the optical system should be essentially 
free of coma.  Coma free Schmidt designs have been constructed down to 
at least f/2 and possibly faster.

Chuck Norcutt

Moose wrote:
> Scott Peden wrote:
>> I gotta ask, what's a coma? 
> http://en.wikipedia.org/wiki/Coma_%28optics%29
>> I assume this is a term that has to do only with 
>> astro photography and won't interfere with me shooting the moon?
>>   
> Coma is only an off-axis phenomenon, so anything in the central portion 
> of the frame is not affected. So until you get a really long lens that 
> can fill the frame with the moon, you needn't worry.
> 
> It is also reduced by smaller apertures and the moon is bright enough 
> that you needn't be using your lens wide open. Remember to spot meter on 
> the moon itself or stop down 3-4 stops from an average reading.
> 
> Not a problem for the Moon.
> 
> Moose
> 
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