Hey, great feedback on the 200/4 as not suffering from coma.
"Coma" is correct. The word actually refers to the nebulous cloud
around the head of a comet. As an optical abberation the same word is
used since it causes a star (which should appear as a pinpoint) to be
imaged more like a comet. Bright point at the location of the star and
fuzzy cloud extending off to one side.
Coma is an abberation that grows worse with small focal ratios. In the
astronomical telescope world it is corrected in reflecting telescopes by
the use of front correcting plates of either the Schmidt or Maksutov
design. The Maksutov corrector plate is very thick and steeply concave
and is generally used on small diameter mirror lenses such as the
ubiquitous 500mm f/8's and Questar telescopes up to about 7" diameter.
Above 5-7" diamter the Maksutov corrector becomes very heavy and
difficult to manufacture. This is where Schmidt correctors take over as
they are very thin (about 6-10mm), nearly flat plates. They're typicall
used on 5" and larger scopes made by Celestron and Meade. One side of
the plate is not flat, however, and contains a curve that looks a bit
like a sine wave with the peak of one wave in the center, the trough
about 2/3 (?) of the radius from the center and then rising back up
again to the edge of the plate. The depth of the curve is very shallow
and would likely require a straight edge to detect it. The Schmidt
design typically uses a spherical primary mirror and I think (but don't
recall for sure) that the Maksutov does as well.
Both designs mount a secondary negative FL mirror in the center of the
corrector plate to magnify the image from the primary mirror which is
typically very fast at about f/2. The Maksutov can be (cleverly)
designed such that the secondary negative FL mirror has exactly the same
curvature as the back side of the corrector. The secondary mirror
therefore need be nothing but an aluminzed spot on the corrector plate.
So, from coma to mirror lens/telescope tutorial. Wonder how I got here.
Chuck Norcutt
Fernando Gonzalez Gentile wrote:
> I have talked with Carlos Santisteban about this issue. I will answer this
> for him, until he hangs over here and make the corrections my answer may
> deserve.
>
> Carlos judges the 200/4 very up to astrophotography requirements mainly
> because of its like of coma (comma - sp?) aberration.
> Stars will show as a bright point instead of as a bright triangle, like when
> using the 300/4.5
>
> HTH,
>
> Fernando.
>
>
>
> on 16/09/2005 11:30, Chuck Norcutt at chucknorcutt@xxxxxxxxxxx, wrote:
>
>
>>The only other
>>question is whether the B/B- grades that Gary assigned to the 200/4 when
>>wide open is good enough for the the demands of star images. I don't
>>know the answer.
>
>
> Jim Day wrote:
>
>>>Hi Chuck,
>>>I'm wondering what your opinion is of the zuiko 200/4 lens in general, and
>>>for astrophotography. I'm thinking about purchasing one. On the lens test
>>>site, it looks like it is ok but not necessarily anything to write home
>>>about. I would appreciate your thoughts. Jim
>
>
>
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