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Went on with a wide range of telescopes and its mechanics. To my understanding, the only change in the optical function in Schmidt Cassegrain telescope is a Schmidt's corrector plate in front of an Newtonian telescope. And the secondary mirror placed exactly opposite to the primary mirror with a hole with diameter < of the diameter of secondary mirror to let the light reach the eyepiece placed at the back of the telescope, Where in Newtonian the secondary mirror is inclined 45 degrees to make the light erect so that it reaches the eyepiece are some of the mechanical and physical changes. So, Is my understanding correct ? If We add a Schmidt corrector plate in front of a Newtonian telescope, does this change make it a Cassegrain one ?. And why the light is sent back of the telescope by the secondary mirror in Cassegrain and not erecting it to the eyepiece like it is in Newtonian ?

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Schmidt corrector plate on a Newtonian = Schmidt Cassegrain?

No.

And why the light is sent back of the telescope by the secondary mirror in Cassegrain and not erecting it to the eyepiece like it is in Newtonian ?

A Cassegrain telescope has a hyperbolic secondary mirror which a certain position and focal length to minimize off-axis aberrations. This makes the diffraction-limited field of view much wider than for a single-mirrored Newtonian.

For more on that see

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  • $\begingroup$ Thank you uhoh !!. $\endgroup$
    – Ishwaran
    Aug 10 at 13:12
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    $\begingroup$ That's fine mate !!. You already cleared my pestering mind :) $\endgroup$
    – Ishwaran
    Aug 10 at 13:15
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    $\begingroup$ @Ishwaran now you've got me thinking! Why are satellite ground station and deep space communications dishes with secondary mirrors usually of Cassegrain design? $\endgroup$
    – uhoh
    Aug 10 at 14:34
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    $\begingroup$ Note that the Schmidt Cassegrain is slightly different than the Classical Cassegrain (and there are variations of both). I think the Classical usually has a parabolic primary mirror & a hyperbolic secondary, while the Schmidt usually has a spherical primary & a specially-shaped secondary. $\endgroup$
    – D. Halsey
    Aug 14 at 22:22
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    $\begingroup$ Correction to my earlier comment: Many Schmidt designs have spherical primary, spherical secondary, and specially-shaped corrector plates. $\endgroup$
    – D. Halsey
    Aug 15 at 0:20

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