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▪ ** **CONTENTS
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10.2.2.2. Wright, Baker camera, Hyperstar
▐
10.2.2.4. Schmidt-Cassegrain telescope
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#
**10.2.2.3. Schmidt-Newton telescope**
The only
difference in the optical layout of the Schmidt-Newtonian vs. Schmidt camera
is in the position of corrector lens. In the telescope, the corrector is
typically positioned at a distance somewhat inside the focus of the spherical primary (**FIG.
173**).
The reduced mirror-to-corrector distance has practically no effect on
the
**FIGURE 173**:
Schmidt-Newtonian telescope optical elements. Schmidt corrector cancels
spherical aberration of the spherical primary. It also usually supports
the diagonal mount, which eliminates spider diffraction effect. Due to
displaced stop (normally coinciding with the corrector), coma,
astigmatism and field curvature are all lower than in the comparable
Newtonian with paraboloidal primary with the stop at the surface
(while the benefit of lower astigmatism or field curvature can be
achieved by manipulating stop position of a paraboloid as well, its
coma error is independent of stop position). While the corrector is
very forgiving to miscollimation, it does complicate adjusting the
flat.
needed corrector power to cancel spherical
aberration of the mirror. Needed corrector shape is determined from
Eq. 101 or
Eq. 101.1 (higher-order
aberration is negligible). However, as a consequence of the stop now
being closer to the primary, portions of mirror's coma and astigmatism
are reintroduced, which is evident on the
ray spot plot. The P-V wavefront error of
lower-order coma in the
Schmidt-Newtonian for object at infinity is given by:
Wc** **= (1-**σ**)αD/48F**2**
(110)
with **σ**
being the corrector-to-primary separation in units of the primary radius
of curvature (**σ**
is numerically positive),
**α**
the field angle in radians, **D** the aperture diameter and **F** the
focal ratio. With α=h/ƒ,
**h** being the linear height in the image plane, and **ƒ** the mirror
focal length, it can be also expressed as Wc=(1-σ)h/48F3.
It makes coma in the Schmidt-Newtonian lower by a factor of
(1-σ)
vs. paraboloidal Newtonian. The
lower-order astigmatism P-V wavefront error is given by:
Wa** ** = (1-**σ**)**2**Dα**2**/8F (111)
or, alternatively, Wa=-(1-σ)2h2/8DF3.
In other words, it is by a factor (1-σ)2
lower than for a mirror with the stop at the surface. Change in
astigmatism also changes the median (best) image surface. It is
now given by:
with the sagittal surface
curvature radius 1/Rs=(2/R)-[2(1-σ)2/R],
and the tangential surface curvature given by
1/Rt=(2/R)-[6(1-σ)2/R],
**R** being the mirror radius of curvature. It makes median field
curvature lower by a factor of [1-2(1-σ)2]
than for a paraboloid with the stop at the surface.
With the usual value
for **σ** of ~0.45, the Schmidt-Newtonian coma is lower by a factor of
~0.55, astigmatism by a factor of ~0.3, and median field curvature by a
factor of ~0.4 vs. comparable paraboloid with the stop at the surface.
Of course, similar
gains in the reduction of
astigmatism and
field curvature can be just as well
obtained with a paraboloid, by moving the stop away from the surface.
Elements alignment in the
Schmidt-Newtonian is somewhat more complicated than in the
all-reflecting arrangement. The two mirrors and the focuser have to be aligned
as in all-reflecting system, but all three also need to be aligned with the corrector. Decentered corrector will
induce coma, as given by Eq. 109,
while corrector tilt will induce tilted image surface. With the
corrector commonly supporting diagonal mount, the limit to collimation
accuracy is set by the accuracy of the corrector/diagonal alignment
(unless correction is made at the focuser).
On the other hand, better coma
correction of Schmidt-Newtonian makes the tolerance for mirrors/focuser
misalignment nearly twice more forgiving than in a comparable
all-reflecting Newtonian.
◄
10.2.2.2. Wright, Baker camera, Hyperstar
▐
10.2.2.4. Schmidt-Cassegrain telescope
►
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