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11.8.
Maksutov-Cassegrain telescope (MCT)
Full-aperture meniscus corrector can be also used in various
arrangements with two-mirror systems. Usually, it is used in the
Cassegrain configuration (FIG.
96), hence these systems are known as Maksutov-Cassegrain
telescopes (MCT). The benefit of such an arrangement is - similarly to that in
combination with the Schmidt corrector - greater flexibility in
correcting primary aberrations than in an all-reflecting system. As it
will be illustrated in this text, an MCT with separate secondary can be
made corrected for all aberrations (correcting field curvature requires
large secondary mirror) in an all-spherical arrangement. In addition,
corrector's chromatism can be made nearly non-existent.
However, in compact systems with fast
primary mirrors, strongly curved surfaces of Maksutov corrector quickly
begin to generate higher-order spherical aberration, which changes in
proportion to the aperture and, approximately, with the fourth power of
primary's relative aperture. It can only be corrected by applying
aspheric surface term to either mirrors or the corrector - something too
complex to be viable for amateur telescopes. While it varies somewhat
with the specifics of corrector, acceptable optical quality in this
respect doesn't extend significantly beyond a 6"
ƒ/2.5 primary level.
Another option is to have the primary aspherized, which allows for
weaker corrector, with reduced higher-order spherical aberration.
FIGURE 96: Maksutov-Cassegrain
telescope optical elements. Meniscus corrector, which induces not
only spherical, but also off-axis aberrations, offers greater
flexibility in correcting system aberrations. As a result,
Maksutov-Cassegrain can be made free from coma and astigmatism in a
compact all-spherical arrangement. The weak point is strong
higher-order spherical aberration with fast primary mirrors that
require strongly curved corrector surfaces. It can not be corrected
in an all-spherical arrangement. Secondary mirror can be either an
aluminized spot on the rear of corrector, or a separate surface,
which generally allows for better field quality.
First published telescope arrangements that followed the
1941 introduction of full-aperture meniscus corrector for spherical
mirror was a pair of two-mirror Cassegrain systems -
ƒ/15 and
ƒ/23 - and
Maksutov-type corrector by John Gregory in 1957. In it, the secondary
was an aluminized spot at the back surface of the corrector. At the
time, Lawrence Braymer was already producing his
famous-to-be Questar Maksutov-Cassegrain, a design very similar to
Gregory's (in order to avoid patent infringement, Questars had - for
about a decade - the aluminized spot placed at the front meniscus
surface). As a later development, an arrangement with separate secondary
was introduced. Nowadays, both Gregory-type and the arrangement with
separated secondary are being used in various forms. In general,
separate secondary is preferred, since it gives additional degrees of
freedom for correction of
aberrations (FIG. 97).
FIGURE 97: Two basic
Maksutov-Cassegrain arrangements: (a) all-spherical with an
aluminized spot on the back of corrector for the secondary
(Gregory-style), and (b) with separated secondary. Due to
design limitations imposed by the two surfaces of identical radius
of curvature, the former has noticeably inferior off-axis
performance: at 0.5° off-axis the wavefront error (best surface) is
0.24 wave RMS, mostly due to the coma, but also astigmatism. The
design with separated secondary is highly corrected, with less than
0.025 wave RMS wavefront error at 0.5° off-axis. It is also better
corrected axially (1/50 vs. 1/33 wave RMS), with lower field
curvature due to both, lower astigmatism and larger secondary. Some
lateral color (LC) is noticeable. Both systems have very low chromatism:
0.09 and 0.075 wave RMS combined h- and r-line on
axis.
SPEC'S
As described in the
previous section, full-aperture meniscus corrector has properties making
it quite complex optically. Part of it is due to its steeply curved
surfaces, generating relatively significant amount of higher-order
aberrations. It is a thick lens, requiring more complex expressions for
accurate assessment. Also, its relatively strong power makes system
properties - including spherical aberration level - dependant on its
location relative to the mirror. In two-mirror systems, this only
becomes more complicated with the secondary mirror added. As a result,
the path to defining a working system of this kind is fairly
complicated, and can not be expressed with reasonably small set of
equations.
With the arrangement
with an aluminized spot for the secondary, needed secondary curvature
for desired focus location determines back radius of the corrector, thus
the only variable is corrector thickness and, to that extent, the front
radius. The secondary curvature itself is determined by the properties
of primary, which in turn are known only with the corrector properties
specified. However, there is a typical level of the effect of the
corrector on the primary, which can be used to obtain better initial
approximation of needed system properties (FIG.
98).
FIGURE 98:
Effect of the
meniscus corrector in a two-mirror system. Being thicker toward the
edges, meniscus delays outer portions of the flat incoming wavefront
more, changing its form into convex toward the primary. As a result,
the outer rays diverge, as if coming from an object placed at
meniscus' focal point of (in terms of lens power, the meniscus is a
negative lens). Due to the aperture stop being displaced from the
primary, ray height at the primary is greater than at the first
corrector surface (aperture opening), making the primary effectively
larger. It focuses (F'1)
farther away than without corrector in place (F1),
however, if effective focal length (f'1)
is smaller, and its relative aperture is larger. Ray height on the
secondary - and its magnification - also increase, resulting in the
final focus F' shifted farther away. Changes in the
ray/wavefront geometry change aberration contributions of the two
mirrors. Typical ray height increase on the primary due to
corrector's power is 3%-4%, and its effective focal ratio number
F is reduced to ~(F-0.1). These quantities help make a closer
estimate of needed properties of the secondary - and the corrector -
in the initial stage of designing.
Two-mirror
Maksutov system aberrations are addressed in more details in the
following chapter. While fourth-order coefficients - in particular
for spherical aberration - are not sufficiently accurate for determining
the final system properties, they are necessary for understanding
system's properties.
◄
11.7. Maksutov-Newtonian
▐
11.8.1. Maksutov-Cassegrain aberrations
►
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