Above: Optical layout of a typical Ritchey-Chrétien,
with CCD camera attached
Ritchey-Chrétiens have become popular in
recent years for deep-sky imaging. They are intended primarily as
photographic instruments and are not as well-suited to visual observing, but for
imaging smaller deep-sky objects such as galaxies and small
nebulae, they are
hard to beat. Ritchey-Chrétien
is commonly abbreviated RC.
RCs are a type of Cassegrain telescope, meaning they use a folded optical
design like the more familiar Schmidt-Cassegrain telescopes (SCTs). However, they are an
all-reflective system, using only mirrors and no lenses. Instead of the
mirror being supported on the corrector lens as it
is in a catadioptric design like an SCT, the mirror
is supported by a spider consisting of four metal vanes. These produce the
common diffraction spikes seen as crosses on bright stars in many astrophotos.
Because of the folded optical design, RCs provide a long
focal length in a short package. This high
magnification makes them ideal for imaging small targets.
Aberrations in Ritchey-Chrétiens
In a classical Cassegrain telescope (another all-reflective design like the
RC), the primary mirror at the back of the
telescope has a parabolic shape, just like the
primary mirror in a Newtonian.
And like a Newtonian, classical Cassegrains suffer from off-axis
coma, which prevents the stars at the edge of the
field from being sharp.
Above: Shape of an off-axis star affected by coma. The
center of the field is down in this diagram.
For professional astronomers, this is a problem because they need to be able
to accurately measure the position of stars across the entire field of view.
Coma is an asymmetrical aberration, and displaces the star images from their
actual positions. The RC design trades off coma for astigmatism.
The RC uses hyperbolic primary and secondary
mirrors to eliminate coma and spherical aberration. However, the stars are
still elongated at the edge of the field, but they are no longer asymmetrical as
they would be with coma. This is critical for making astrometric
measurements, and many professional telescopes, from Hubble to Keck, are
Above: Shape of an off-axis star affected by astigmatism
For amateur astronomers who are interested only in pretty pictures, whether
the telescope suffers from coma or astigmatism is less important. For
advanced amateurs engaged in scientific study such as searching for or studying
asteroids, an RC would be preferable. But for imaging deep-sky objects,
coma versus astigmatism is more of a toss-up. A classical Cassegrain gives
star images only about 10% larger than those of an RC at the edge of the field.
So why are RCs so popular while classical Cassegrains tend to be rarer?
One reason is that these are expensive telescopes, normally used by advanced
amateurs, many of whom are doing scientific studies with them. But more
often, the users are astrophotographers who are taking beautiful images of the
night sky. Most commercial classical Cassegrains are designed as planetary
telescopes and have very slow focal ratios (around
f/15 or f/20). This makes them poorly suited to deep-sky imaging because
of the longer time required to capture the light from a faint object. RCs
on the other hand, have focal ratios in the range of f/7 to f/9, making them
preferable for deep-sky photography. There is no reason an f/7 classical
Cassegrain could not be built (and certainly some have been), but very few such
commercial models exist, so RCs have become more popular.
Ritchey-Chrétiens also suffer from
field curvature. Field curvature distorts the star images across the
field of view. This is because field
curvature produces a curved
focal plane. The outer parts of the focal
plane are focused closer to the telescope than the inner parts (in an RC or
other Cassegrain). But the detector used for imaging (whether a CCD or
film) is flat, meaning the entire field cannot be in focus at once. RCs
and classical Cassegrains have very comparable amounts of field curvature.
But the curvature is almost 7 times greater than in a comparable Newtonian and
about twice that of a comparable SCT. To use a very large imaging detector
such as a 35mm format CCD, a field flattener lens is required. This lens
provides a flatter field and can also minimize astigmatism to produce excellent
star images across the entire field of view. These are common accessories
Above: A curved focal plane cannot focus
sharp star images across the full field of a flat CCD chip
Meade Advanced Ritchey-Chrétiens
Meade has introduced a line of telescopes using an "Advanced
Ritchey-Chrétien" design. This design still incorporates a corrector lens
like an SCT, and therefore is not a true RC. It is really an
aspheric Schmidt-Cassegrain (using an aspheric
secondary mirror in place of the normally spherical mirror). It is
essentially an SCT with no coma, and while this is an improvement over the
standard SCT, it is not the same as a true RC design. It has much more in
common with the standard SCTs, so for more details on this design, see the
RCs are considerably more expensive than the common Schmidt-Cassegrain
telescopes, but their performance is much higher. Part of the reason for
the high price or RCs is, of course, the hyperbolic optics, which are very
difficult to manufacture. But also, since RCs are intended as high-end
instruments for advanced astronomers, they are also typically built to better
specifications than most mass-produced commercial instruments. They often
include advanced features such as carbon fiber optical tubes (or truss tubes),
high-precision mirrors, automatic control for focusing, etc. RCs start at
about 10" in aperture. These smaller instruments cost about $1000 per inch
of aperture, while larger sizes cost about $2000 per inch. The most common
large sizes are about 20-25". These scopes can cost over $50,000 with a
suitable heavy-duty mount. 32" and larger RCs are also available, although
for obvious reasons (not the least of which is the quarter-million-dollar price
tag for the scope, mount, and observatory) these are pretty rare.
Is a Ritchey-Chrétien
Best for Me?
Odds are, if it is, you'll already know, since these scopes are intended for
advanced observers who have decided that high-resolution CCD imaging is their
primary goal. But, RCs are not the only choice for advanced CCD imagers,
so how do you know if you should get an RC versus a large apochromatic refractor
or some sort of high-speed astrograph? It depends primarily on what you
want to image. RCs are designed for high-magnification imaging of small
targets. Most imagers who own an RC also own a small apo refractor for
wide-field imaging. These two types of telescopes perfectly compliment
each other. A more specialized telescope like a wide-field astrograph
might be good if you have a preference for imaging only large objects such as
nebulas and big galaxies. But an RC has a bit more versatility. An
RC can be used with a focal reducer to allow a wider field of view, and with new
large-format CCD cameras an RC can be used for small targets as well as all but
the very largest objects.