Schmidt-Cassegrains

Schmidt-Cassegrains

Although there are many variations (both mirrors spherical, both mirrors aspherical or one of the two). Can they be divided into two main design forms: compact and non-compact.

In its compact form, the corrector plate is located on or near the focus of the primary mirror. In the non-compact, the corrector plate remains at or near the center of curvature (twice the focal length) of the primary mirror.

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Typical examples of the compact design are Celestron commercial instruments. Which combine a fast primary mirror and a small, highly curved secondary mirror. This results in a very short tube length, at the expense of the field curvature.

Most of Celestron’s compact designs have a primary mirror with a focus ratio of f / 2 and a secondary with a focus ratio of f / 5, resulting in a focal ratio of f / 10.

A notable exception is the Celestron C9.25, which has a primary focus ratio of f / 2.3. And has a secondary focus ratio of f / 4.3. The result is a slightly flatter field and slightly longer tube aspect ratio than most other compact designs.

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