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Other Telescope Designs
While the Schmidt-Cassegrain, Newtonian, and refractor are the most common
optical designs used for CCD imaging and observing in general, other types are
becoming increasingly popular.
Classical Cassegrain
Above: Optical layout of a Classical Cassegrain telescope
The Classical Cassegrain is actually a fairly uncommon type of telescope, but
we discuss it since it is the basis for a variety of other designs. A
Cassegrain telescope consists of a paraboloidal primary mirror (just like a
Newtonian), with a convex secondary mirror (specifically a hyperboloidal shape). The secondary reflects light
back through a hole in the center of the primary mirror and out to the eyepiece
or camera. The secondary is held in place by a spider (as in a
Newtonian). Focusing is usually achieved by an external rack-and-pinion
type system similar to what a refractor would have. The Classical
Cassegrain is generally designed to have a long focal ratio (f/12-f/20 is
common), and is intended most often as a planetary telescope. Because of
its slow focal ratio, the Classical Cassegrain in not usually employed for
deep-sky CCD imaging, although it is possible to design a Cassegrain with a
faster focal ratio (f/8 or so). Cassegrains tend to be much longer
than other folded optical designs since their primary mirrors often have focal
ratios of around f/4 (whereas an SCT is usually about f/2). Another
disadvantage is that Classical Cassegrains tend to be much more expensive than
other designs, the better models currently manufactured rivaling apochromatic
refractors in cost (around $1000 per inch of aperture).
PROS
CONS
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Long optical tube relative to other Cassegrain variations
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Generally very expensive
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Slow focal-ratio makes for longer exposure times
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Suffers from coma and field curvature, large CCD would require field
corrector
Ritchey-Chrétien
Above: Optical layout of a Ritchey-Chrétien telescope
This is an interesting variation on the Cassegrain and a very popular system
for advanced CCD imagers. The Ritchey-Chrétien (RC) telescope uses two
hyperboloidal mirrors. This design allows the system to be free of the aberration known as coma, unlike
the Classical Cassegrain or SCT. The
advantage of this design, for professional astronomers especially, is that the
stars all appear round, even at the very edges of the field (unlike the
comet-shaped stars in a system suffering from coma). This means extremely precise astrometric (positional) measurements are possible. Most professional
telescopes, from the Keck to the Hubble Space Telescope, are Ritchey-Chrétien
designs. This also means they are very expensive, as these aspherical
mirrors are very difficult to make. Also, RCs still suffer from
astigmatism and thus do not give perfectly round stars at the edge of the field.
For professionals measuring stellar positions, this is preferable to coma, but
for imaging is not necessarily a huge improvement. Many RCs employ field
correctors that eliminate the residual astigmatism.
Above: Three stars as they would appear at the edge of a
field of view. On the left, a scope suffering from coma (center of the
field is down) such as a Classical Cassegrain. Middle star is from a scope
with astigmatism, such as an RC. Right star is unaberrated, as might be
the case in an RC with a field corrector or other sophisticated design.
There are several manufacturers of amateur RC telescopes, and these
instruments have gained in popularity in recent years. RCs have the same
advantages as the Schmidt-Cassegrain systems: compact design, relatively
long-focal length (provides good image scale for small targets), and a
relatively fast focal ratios (at least compared to Classical-Cassegrains and
Maksutov-Cassegrains). RCs rival apochromatic refractors in price (about
$1000 per inch of aperture) and since they are rarely built smaller than 10" in
size, it is unlikely that one could be had for less than a 5-digit price.
Still, for the advanced amateur looking to image smaller deep-sky targets like
galaxies, it is hard to choose a better system.
PROS
CONS
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Expensive
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Long focal length limits wide-field imaging
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Suffers from field curvature and astigmatism, requires a field corrector
for large CCD chips
Meade RCX400 and
LX200R Meade introduced a new type of telescope
called an "advanced Ritchey-Chrétien" with its RCX400 series. The company
then modified its popular LX200 series to use the same basic optical
configuration. The RCX400 and LX200R models are essentially modified
Schmidt-Cassegrain telescopes. Commercial SCTs use spherical mirrors and a
corrector plate to eliminate the spherical aberration that such mirrors would
normally produce. This configuration produces off-axis coma, degrading
star images at the edges of an image, especially with large-format cameras.
The RCX400 and LX200R models incorporate an aspherical secondary mirror to
eliminate coma, leading to improved star images at the edges of the field.
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Understanding Mirror Shapes
A variety of shapes
are used in manufacturing various telescopes. Below are the different
possible shapes for mirrors and the telescopes that use certain combinations
of mirrors and the reasons for those mirror choices in a design.
Spheroidal -- The simplest curved mirror shape is that of a
sphere. Simply grinding two pieces of glass together will yield a
spheroidal shape.
Paraboloidal -- A parabola is a slightly more complex shape than a
sphere, but is not too much more difficult to make. Simplest of the
aspheric (non-spherical) shapes.
Hyperboloidal -- A hyperbola has a more complex shape than a parabola
and is thus very difficult to manufacture. Hyperbolas are larger than
parabolas (see below).
Ellipsoidal -- Ellipses are smaller than parabolas but are still
difficult to manufacture.
The image below shows the mathematical shapes behind each mirror design.
Above: The black line represents a flat mirror.
Only two shapes have exact specifications: the sphere and the parabola.
The orange line represents a sphere, which is defined by only one
mathematical equation. The green line represents a parabola, also
defined by a specific equation. Ellipses and hyperbolas, on the other
hand, have infinite variety. The blue line represents a specific
hyperbola, but any curve in the blue section of the graph would be a
hyperbola. Likewise the yellow curve represents a prolate ellipse (in
which the foci are parallel to the optical axis, in other words, the
ellipse's long axis runs along the optical axis), but all the curves within
the yellow area would be prolate ellipses. The same is true for the oblate
ellipse family, which inhabits the red part of the graph. Oblate
ellipses have their long axes perpendicular to the optical axis.
Newtonian -- Paraboloidal primary mirror, flat secondary.
Simple design, suffers from coma.
Classical Cassegrain -- Paraboloidal primary, hyperboloidal
secondary. More complex design, suffers from coma.
Ritchey-Chrétien -- Hyperboloidal primary and secondary.
Complex design, suffers from astigmatism but not coma.
Dall-Kirkham -- Ellipsoidal primary, spheroidal secondary.
Spherical secondary simplifies design, suffers from coma.
Pressman-Camichel -- Spheroidal primary, ellipsoidal secondary.
Spheroidal primary greatly simplifies design, suffers extreme coma.
Schmidt-Cassegrain/Maksutov Newtonian -- Spheroidal primary and
secondary (in most commercial designs). Simple design, requires
corrector lens to eliminate spherical aberration, suffers from coma, using
an aspheric secondary eliminates coma.
Schmidt-Newtonian/Maksutov-Newtonian -- Spheroidal primary, flat
secondary. Requires corrector lens to eliminate spherical aberration,
less coma than standard Newtonian. |
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