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Schmidt-Cassegrain Telescopes
Probably the most popular telescope design is the Schmidt-Cassegrain
telescope (SCT). The SCT is a versatile design, good for both imaging and
visual observation. It is compact for portability and ease of use, and is
relatively inexpensive compared to other systems. For example, other
common designs for CCD imaging include apochromatic refractors and
Ritchey-Chrétiens. Each of these systems costs around $1000 per inch of
aperture (the largest costing nearly twice that). This price is for the
optical tube only and does not include any type of mount. SCTs typically
run about $300-$400 per inch of aperture including a computer-controlled
mount. The optical tubes by themselves run about $100-$200 an inch.
While there are obviously differences between these various types of telescopes,
it is clear that for the average amateur CCD imager, an SCT provides a lot of
versatility for the price.
Above: The optical layout of a Schmidt-Cassegrain telescope
In a Schmidt-Cassegrain, the light passes through a corrector lens to the
primary mirror. The corrector lens is required to minimize the
spherical
aberration which arises from using spheroidal mirrors. (A true
Classical-Cassegrain telescope uses a paraboloidal primary and does not require a
corrector plate. However, this is a much more expensive design than the
SCT due to the difficulties in making the aspheric mirrors.) The light is reflected from the primary mirror to a convex secondary
mirror. This mirror then sends the light to a focus position behind the
primary mirror for easy access with an eyepiece or camera. The convex
secondary mirror amplifies the focal ratio of the telescope, making the
short-tubed SCT act like an f/10 system while being as short as an f/2 system!
While f/10 is advantageous for visual observations and large-image-scale CCD
imaging, what about wide field imaging? Focal reducer lenses can be
attached to the rear of an SCT to decrease the focal length of the telescope.
This provides a wider field of view as well as shorter exposure times. The
most common focal reducer changes as f/10 system to f/6.3, making exposures 2.5
times faster. Recently, f/3.3 reducers have been employed for CCD imaging,
decreasing exposure times by a factor of 9.
Above: Image of NGC253 taken with a 14" f/7 SCT. The
telescope is inherently f/11 but was made f/7 with the use of a focal reducer.
The most extreme example of focal reduction is the Celestron Fastar system.
The primary mirror of a typical SCT has a focal ratio of f/2. The
secondary mirror has a 5x amplification factor. By removing the secondary
mirror and placing the CCD camera at the front of the telescope (with an
appropriate correcting lens), imaging can be
done at f/2, making exposures 25 times faster than at f/10, and nearly 3 times
shorter than at f/3.3. This super-fast imaging system has made CCD imaging
possible for the average amateur who does not want to spend all night taking
pictures, but would rather see results right away!
Above: M17 with the same 14" telescope, but using a HyperStar
lens to operate at f/1.9, yielding a larger field of view and faster imaging
system.
There are advantages and disadvantages to every telescope system, and every
combination of scope and CCD camera. But the SCT provides the most
flexible system, capable of both high-resolution imaging and wide-field, fast
imaging. The combination of portability, versatility and price make the
SCT by far the most popular imaging and observing system.
Maksutov-Cassegrain
A related design is the Maksutov-Cassegrain. The Maksutov-Cassegrain
uses a different corrector lens in place of the Schmidt corrector. The
Maksutov corrector lens is easier to manufacture than the Schmidt corrector, but
is much thicker and therefore requires a longer cool-down time, especially in
larger sizes (7" and larger). Also, Maksutov-Cassegrains normally have
slower focal ratios (f/12 to f/15), making them less well suited to deep-sky
imaging.
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Full-Aperture Corrector Lenses
Both
Schmidt-Cassegrains and Maksutov-Cassegrains use corrector lenses.
These are full-aperture corrector lenses because they cover the entire
aperture of the telescope (as opposed to a sub-aperture corrector such as a
coma corrector for a Newtonian which is inserted along the light path just
ahead of the focal plane).
A true Cassegrain telescope uses a paraboloidal primary mirror and a
hyperboloidal secondary mirror. These aspheric mirrors are difficult
and expensive to manufacture, but they are necessary to eliminate spherical
aberration from a two-mirror optical design. However, by employing a
refractive element (lens) in the design, it is possible to remove spherical
aberration while still using easy-to-manufacture spheroidal mirrors.
Two independent solutions were derived for such a telescope in the early
20th Century.
Bernhard Schmidt in Germany solved the problem by using an aspheric
corrector plate, now called a Schmidt corrector. The Schmidt corrector
is a thin plate of glass which has a very slight aspheric curve ground into
the surface. This curve is so minute that a Schmidt corrector looks
flat. But the curve is sufficient to eliminate spherical aberration
and allow a Cassegrain to be built with spheroidal mirrors for a much less
money than a classical Cassegrain would cost. However, Schmidt
correctors are somewhat difficult to manufacture (although Schmidt not only
invented the corrector but also an ingenious method for making it).
Dmitri Maksutov in Russia also devised a similar solution. The
Maksutov corrector is easier to manufacture because it is an all-spherical
design. It is a highly curved meniscus lens, much thicker than the
Schmidt corrector. This has the advantage of easy manufacture, but the
disadvantage of inducing chromatic aberration into the system. With
the right design, this can be minimized. Schmidt correctors suffer
from chromatic aberration as well, but since they have very little optical
power it is much less than that of a similar Maksutov. |
Above: On the left is a Schmidt aspheric corrector lens
(greatly exaggerated for clarity), on the right a Maksutov meniscus corrector
lens.
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