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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