Meade LX90GPS Instruction Manual page 2

The Meade Schmidt-Cassegrain Optical System
(2)
(1)
Focal
Plane
In the Schmidt-Cassegrain design of the Meade LX90, 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.
The Meade LX90 Schmidt-Cassegrain includes an oversize primary mirror of an 8.25" diameter, yielding a fully illuminat-
ed field-of-view significantly wider than is possible with a standard-size primary mirror. Note that light ray (2) in the figure
would be lost entirely, except for the oversize primary. It is this phenomenon which results in Meade Schmidt-Cassegrains
having off-axis field illuminations about 10% greater, aperture-for-aperture, than other Schmidt-Cassegrains utilizing stan-
dard-size primary mirrors. Field stops machined into the inside-diameter surface of the primary mirror baffle tube signifi-
cantly increase lunar, planetary, and deep-space image contrast. These field stops effectively block off-axis stray light rays.
(2)
(1)
Ray (2)
Ray (1)
Secondary
Mirror
Primary Baffle Tube
Field Stops
Correcting
Plate
Primary Mirror
(1)
(2)
Secondary
Baffle