AutoStar #497 HANDBOX
Appendix J
70
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
APPENDIX J:
ADVANCED COMA-FREE OPTICS
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 shown above, 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.
The Meade ACF optical system includes an oversized primary
mirror, 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.
