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INDEX
OPTICAL SYSTEMS
49
ACF AND SC OPTICAL SYSTEMS
APPENDIX D:
Unlike Schmidt-Cassegrain systems, Meade’s
Advanced Coma-Free (ACF) optical system provides
razor-sharp pin-point star images all the way to the very
edge of the fi eld of view. This aplanatic (coma-free)
optical system brings the highest level of performance
to the amateur astronomer.
In the ACF design of the Meade LT Series, light enters
from the right, passes through a thin lens with 2-sided
aspheric correction (“correcting plate”), proceeds to
a spherical primary mirror, and then to a hyperbolic
secondary mirror. The hyperbolic secondary mirror
multiplies the effective focal length of the primary
mirror and results in a focus at the focal plane, with light
passing through a central perforation in the primary
mirror.
All models of the Meade LT includes oversize primary
mirrors, yielding a fully illuminated fi eld-of-view
signifi cantly wider than is possible with a standard-size
primary mirror. Note that light ray (2) in the fi gure would
be lost entirely, except for the oversize primary. It is
this phenomenon which results in Meade ACF having
off-axis fi eld illuminations about 10% greater, aperture-
for-aperture, than other systems utilizing standard-size
primary mirrors. Field stops machined into the inside-
diameter surface of the primary mirror baffl e tube
signifi cantly increase lunar, planetary, and deep-space
image contrast. These fi eld stops effectively block off-
axis stray light rays.
In the Schmidt-Cassegrain design of the Meade LT,
light enters from the right, passes through a thin lens
with 2-sided aspheric correction (“correcting plate”),
proceeds to a spherical primary mirror, and then to
a convex secondary mirror. The convex secondary
mirror multiplies the effective focal length of the
primary mirror and results in a focus at the focal plane,
with light passing through a central perforation in the
primary mirror.
All models of the Meade LT Schmidt-Cassegrain
includes oversize primary mirrors, yielding a fully
illuminated fi eld-of-view signifi cantly wider than is
possible with a standard-size primary mirror. Note that
light ray (2) in the fi gure would be lost entirely, except
for the oversize primary. It is this phenomenon which
results in Meade Schmidt-Cassegrains having off-axis
fi eld illuminations about 10% greater, aperture-for-
aperture, than other Schmidt-Cassegrains utilizing
standard-size primary mirrors. Field stops machined
into the inside-diameter surface of the primary mirror
baffl e tube signifi cantly increase lunar, planetary,
and deep-space image contrast. These fi eld stops
effectively block off-axis stray light rays.
Primary Baffle Tube
Field Stops
Primary Mirror
Ray (1)
Ray (2)
(2)
(1)
(2)
(1)
Secondary
Mirror
Correcting
Plate
Secondary
Baffle
(2)
(1)
Focal
Plane
Primary Baffle Tube
Field Stops
Primary Mirror
Ray (1)
Ray (2)
(2)
(1)
(2)
(1)
Secondary
Mirror
Correcting
Plate
Secondary
Baffle
(2)
(1)
Focal
Plane
The Meade Advanced Coma-Free Optical System
The Meade Schmidt-Cassegrain Optical System