The Meade Schmidt-Cassegrain Optical System
(1) | | (2) |
| Ray (2) | 1/2° |
Ray (1) | | |
8.218" | | |
(2) | | |
8.016" | | 8.0" |
(1) | Secondary |
8.0" | |
Mirror | |
Focal | | |
Plane | | Secondary |
Primary Baffle Tube | | Baffle |
| |
Field Stops | Correcting | |
| |
Primary Mirror | Plate | |
| |
| | (8" model depicted in diagram. Not to scale.) |
In the Schmidt-Cassegrain design of the Meade 8", 10" and 12" LX200GPS models, 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 8", 10" and 12" Schmidt-Cassegrain models include an oversize primary mirror, yielding a fully illuminated 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 standard-size primary mirrors. Field stops machined into the inside-diameter surface of the primary mirror baffle tube significantly increase lunar, planetary, and deep-space image contrast. These field stops effectively block off-axis stray light rays.
The Meade Maksutov-Cassegrain Optical System
The Meade 7" (178mm) Maksutov-Cassegrain Optical System
Field Stops | |
8.25" | 7" |
Focal | Meniscus |
Plane | Lens |
Primary Baffle Tube | |
| Secondary Baffle |
Primary Mirror (f/2.5) | (Diagram not to scale) |
The Meade 7" Maksutov-Cassegrain design optimizes imaging performance by utilizing a combination of a two-sided spherical meniscus lens (right), a strongly aspheric f/2.5 primary mirror, and a spherical secondary mirror. The convex sec- ondary mirror multiplies the effective focal length of the primary by a factor of six, resulting in an overall a focal length of 2670mm and a focal ratio of f/15 system at the Cassegrain focus.
The oversize 8.25" primary mirror results in a fully-illuminated (unvignetted) field of view significantly wider than can be obtained with Maksutov optics incorporating primary mirrors of the same aperture as their meniscus correcting lenses. Computer-optimized primary and secondary mirror baffles, as well as a sequence of field stops internal to the primary mir- ror baffle, yield lunar, planetary, stellar, and deep-space images of uncommonly high contrast and resolution.
