Re: MAK-CAS
- From: "CLT" <chucktaylorDelete@xxxxxxxxxxxxx>
- Date: Mon, 15 Aug 2005 12:35:22 -0700
Hi Paul,
> >For starters, commercial SCTs do not use hyperboloidal secondaries. They
are
> >also figured to a lower standard than the good mak-newts out there.
>
> A Schmidt telescope is supposed to use a spherical main mirror, and
> the correction glass in front of it is supposed to supply the needed
> corrections. So I suppose a SCT works fine with spherical main and
> secondary mirrors, with the corrector glass supplying the needed
> corrections.
A Schmidt is different from an SCT. The term SCT (like most telescope names)
covers some wide ground. It encompasses anything with a Schmidt -style
corrector placed before the primary with a cassegrain configuration (concave
primary mirror with convex secondary).
Straight (Non-Schmidt) Cassegrains include classical cass (Paraboloid
primary and hyperboloid secondary), Dall-Kirkham (prolate ellipsoid primary
and spheroidal secondary), Ritchey Chretien (hyperboloid primary and
secondary), Pressman-Carmichel (Spheroid primary and oblate ellipsoid
secondary)
So an SCT can have almost any shape of primary and secondary when combined
with a Schmidt-style corrector. Giving one shape to the primary is going to
largely determine the shape of your secondary.
> OTOH a genuine Schmidt telescope shall have its corrector glass at
> twice the focal length from the main mirror (i.e. at the center of
> curvature of the spherical mirror), with the focus of the main mirrir
> appearing in the middle of the tube. So today's SCT's aren't really
> Schmidt telescopes.
A "genuine" Schmidt telescope is a camera. The film is in the tube and the
field is very curved --- the film holder holds the film to that curve. All
SCTs are derived from that, but are somewhat different. The corrector is at
twice the focal length so that by placing the aperture stop at the radius of
curvature for the primary, it eliminates all coma. As far as the primary is
concerned, all portions of the field are on-axis (draw it out and you'll see
what I mean). For the SCT, the corrector was moved in to give the compact
tube. This reintroduces coma as the optical path now differs for on and off
axis.
> A pure Cassegrain telescope must have a parabolic primary and a
> hyperbolic secondary.
Yes, a "classical" Cass will have those. However "Cassegrain" also includes
RC, D-K and P-C with the mirror combinations listed above.
> Supposedly a SCT is some compromise between a Schmidt and a
> Casseggrain, having mirrors which are neither spherical nor
> parabolic/hyperbolic.
There are a wide range of mirror combinations available for an SCT. Most of
the commercial SCTs are spherical, simply because this is cheapest.
Tradition has it that Meade makes primaries and secondaries and then swaps
them in and out to find a set of corrector, primary and secondary that give
acceptable correction. Supposedly Celestron makes them but then has the
secondaries figured on a spindle at the end to match the corrector and
primary. This would give Meades a smoother surface but give Celestron the
better correction. I've had people who worked for M or C tell me this is how
it works at their plant, but have never seen anything official from either
company.
Clear Skies
Chuck Taylor
Do you observe the moon?
Try http://groups.yahoo.com/group/lunar-observing/
To reply, remove Delete and change period com to period net
**************************************************************
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- References:
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- Re: MAK-CAS
- From: Bob May
- Re: MAK-CAS
- From: szaki
- Re: MAK-CAS
- From: CLT
- Re: MAK-CAS
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