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11.7.
Maksutov-Newtonian
Telescopes with meniscus-type full
aperture corrector - usually referred to as Maksutov corrector - are
respected among amateurs almost as no other telescope type. Many are
inclined to believe that the meniscus gives them some extra optical
quality, not achievable with other telescope types. Or, at least, that
this kind of telescopes, generally featuring spherical surfaces, is
easier to make to higher optical standards. Neither is factual -
Maksutov corrector is not perfect optically, and is all but easy to make
- and if there is a particular reason that it performs better, the
reason is to be sought in above average fabrication quality.
Similarly to Schmidt-Newtonian, the only
difference between the camera and telescope arrangement with Maksutov
corrector in the Newtonian configuration comes from the
corrector position. In the Maksutov-Newtonian, the meniscus is closer to the
spherical primary, commonly inside its focal point. Corrector position usually
nearly coincides with the aperture stop, so that reduction in mirror's off-axis
aberrations is very similar to that in the Schmidt-Newtonian. However,
unlike the Schmidt corrector, the meniscus induces some coma and astigmatism
of its own, which makes the final system properties somewhat different.
System properties in
the Newtonian configuration
with spherical primary (FIG. 94) can be quite well approximated
based on the mirror/meniscus combination described in the previous
section.
FIGURE 94: Illustration of the
Maksutov-Newtonian telescope configuration. The primary is spherical, and the
corrector lens is at ~0.8 of the primary's focal length in front of it. Aperture
stop at the corrector reduces mirror coma by
~40%. About half of the remaining mirror coma is
cancelled by the offset with the
opposite coma of the corrector.
That puts the coma of a typical MNT at ~30% of that of a
comparable Newtonian. Astigmatism and field curvature in
Maksutov-Newtonian are also lower, mostly due to the effect of
displaced aperture stop.
With σ~0.43, according
to Eq. 129, coma is reduced to ~30% of that in a comparable paraboloid.
Somewhat lower astigmatism - mostly due to the
aperture stop being
displaced from the spherical primary, and the rest due to the offset by
the opposite astigmatism of the corrector - results in less than half as strongly curved median
image surface. Residual spherical
aberration is still considerably higher than in the comparable Schmidt
corrector, but the correction level is very good at ~ƒ/4 and smaller relative
apertures, and the
chromatism is, for all practical purposes, non-existent (FIG. 95).
As a consequence of the
corrector being closer to the mirror, its effective power for the mirror
is slightly higher. In other words, the corrector's spherical aberration
contribution is slightly higher. Since the mirror contribution is
unchanged, in order to strike the optimum balance between the two, radii
of the corrector need to be slightly more relaxed than in the camera
arrangement. Reflecting this, the first corrector radius is better
approximated by R1~(1-2τ-F/100)R"1,
than as given with
Eq. 128.
FIGURE
95: Ray spot diagrams for 200mm ƒ/4 MN and SN, for (from left) the violet (436nm), blue (486nm), green
(546nm) and red (654nm) spectral lines. Seemingly greater axial
chromatism in the MN is actually its residual (not
correctable) higher-order spherical
aberration (~1/8 wave P-V level). Closer look reveals that the
chromatic error is actually greater in the SN. However, in either
system the chromatism is, for all practical purposes, non-existent. Both
systems have inferior field performance to that of
Houghton-Newtonian varieties (FIG.107
a/b).
SPEC'S:
MN
SN
Similarly
to a Schmidt-Newtonian, alignment of the Maksutov-Newtonian is more complex
than that of all-reflecting Newtonian, due to the lens corrector added
to the system. Ray tracing indicates that sensitivity to decenter of the
Maksutov corrector is similar to that of a Schmidt; however, its sensitivity
to tilt is several times higher.
◄
11.6.1. Approximating
corrector radii
▐
11.8. Maksutov-Cassegrain
►
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