Yep, Chromatic Aberration I know a little bit about - from my astronomy
days. Newton thought CA couldnt be cured so he invented the reflecting
telescope - but with refractor telescopes, CA is very largely compensated
for (even if not completely elminated) by using a compound objective lens
made up of a convex AND a concave lens cemented together (usually, the
objective lens is a compound lens composed of several lenses).
Having used both refractors and reflectors, you can definitely tell the
difference between a cheap n nasty refractor and an (usually) expensive and
properly CA corrected one - but reflectors are still better (at least CA
wise) - which is one of the reason why the vast majority of large diameter
telescopes in the world use reflectors.
The other important factors regarding telescopes are:
the diameter of the object lens (or mirror), which determines the light
gathering power of the telescope and its optical resolution (the ability of
the telescope to resolve details - e.g. to clearly split and distinguish
very fine parallel lines in very close proximity to each other) - the bigger
the object lens the better.
The focal length of the object lens - which directly affects the
magnification. The magnification is given by dividing the focal length of
the object lens (which is fixed) by the focal length of the eyepiece - so
the longer the focal length of the object lens the greater the
magnification. Though in astronomy, high magnification is rarely on the list
of priorities (astronomers will use the lowest magnification that can do the
job - because high magnifications brings in a host of problems - if you
think camera shake is bad with telephoto lenses on a camera, wait until you
try to see details on planets using magnications of 50x or more!)
The focal length of the eyepiece - the smaller the focal length the higher
the magnification - but again, astronomers generally prefer to use longer
focal lengths on the eye-piece which gives them a wider field of view (in
other words astronomers prefer 'wide angle' to ridiculously high
magnification) - and usually a 'brighter view' with wa as well :) The
majority of astronomical objects, stars, galaxies, nebulae and planets,
aren't very bright in the eyepiece - apart from the odd exception like say
the moon, Jupiter and Venus, so it's better to have a bright, steady image
with as much of the [preferably slower moving] image in view as possible -
rather than a very dim, highly magnified subject where you can only see a
tiny bit of it and which is probably shaking and moving so fast that you
can't see any details at all - let alone track the thing. Don't forget, 10x
magnification means 10x as much apparent movement! 100x magnification means
100x as much apparent movement!
In short, the last thing an astronomer needs to add to the list of problems
is CA - so astronomers will spend more money on a telescope to eliminate CA
as much as possible as it really is a worthwhile investment.
Allan
PS No trees were harmed in the sending of this message and a very large
number of electrons were asked their permission to be terribly
inconvenienced. (And threw a party for them afterwards for being really cool
about it).
Disrupting the unnatural balance that you, as a conscious human being and a
confused mass of energy, have created.
-Disturb the mind -
>From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>Reply-To: olympus@xxxxxxxxxx
>To: olympus@xxxxxxxxxx
>Subject: [OM] Re: What is CA?
>Date: Thu, 07 Dec 2006 23:01:33 -0500
>
>Caused by a difference in magnification based on wavelength. The far
>ends of the spectrum (red and purple) get spread apart and (especially
>on high contrast areas) show up as red and purple fringing. It's
>corrected by pulling the red and blue channel pixels closer together by
>a variable distance based on their distance from the center of the image.
>
>Chuck Norcutt
>
>ScottGee1 wrote:
>
> > Chromatic Abberation
> >
> > See:
> >
> >
>http://www.dpreview.com/learn/?/Glossary/Optical/chromatic_aberration_01.htm
> >
> > hth/ScottGee1
> >
> > On 12/7/06, Ali Shah <alizookoman@xxxxxxxxx> wrote:
> >
> >>When people say "many excellent lenses have CA". What
> >>is CA?
>
>
>
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