Meade Instruments LX90-GPS - Manual (Page 59)

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
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.
The Meade Schmidt-Cassegrain Optical System