<blockquote>
<p>Schmidt corrector plate on a Newtonian = Schmidt Cassegrain?</p>
</blockquote>
<p>No.</p>
<blockquote>
<p>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 ?</p>
</blockquote>
<p>A Cassegrain telescope has a <em><strong>hyperbolic</strong></em> 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.</p>
<p>For more on that see</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/a/43362/7982">this answer to <em>Are X-ray telescopes with glancing angle surfaces basically “funny-looking” Cassegrain telescopes mathematically?</em></a></li>
<li>all answers to <a href="https://astronomy.stackexchange.com/q/21478/7982">In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/45101/7982">What is the difference between a Cassegrain telescope and a Gregorian one? Which one is better?</a> currently unanswered</li>
<li><a href="https://hsm.stackexchange.com/a/2059/3869">this answer to <em>When was the first true Gregorian telescope built?</em></a></li>
<li><a href="https://hsm.stackexchange.com/q/13203/3869">How was the first parabolic telescope mirror made, and how was it used, and for what kind of telescope was this work done?</a> currently unanswered</li>
</ul>

CC BY-SA 4.0
2021-08-10T13:12:46.793Z
Thank you uhoh !!.
Kavin Ishwaran
CC BY-SA 4.0
2021-08-10T13:13:09.823Z
@Ishwaran Thanks for the accept, but your question is only 39 minutes old! You might want to consider un-accepting for now, and waiting a few days to see of additional helpful and informative answers are posted. It's totally up to you of course.
uhoh
CC BY-SA 4.0
2021-08-10T13:15:36.680Z
That's fine mate !!. You already cleared my pestering mind :)
Kavin Ishwaran
CC BY-SA 4.0
2021-08-10T14:34:53.563Z
@Ishwaran  now you've got me thinking! [Why are satellite ground station and deep space communications dishes with secondary mirrors usually of Cassegrain design?](https://space.stackexchange.com/q/54545/12102)
uhoh
CC BY-SA 4.0
2021-08-14T22:22:44.670Z
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.
D. Halsey
CC BY-SA 4.0
2021-08-14T22:33:14.743Z
@D.Halsey I think that's important enough to post as an additional, supplemental answer. Alternatively, please feel free to edit this one. I'm not familiar/confident enough to do it myself. *Thanks!*
uhoh
CC BY-SA 4.0
2021-08-15T00:20:47.593Z
Correction to my earlier comment: Many Schmidt designs have spherical primary, spherical secondary, and specially-shaped corrector plates.
D. Halsey
CC BY-SA 4.0
2021-08-10T13:07:07.713Z
2021-08-10T13:07:07.713Z
uhoh
<p>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 &lt; 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 ?</p>

<telescope><amateur-observing><optics><celestial-mechanics>
Schmidt corrector plate on a Newtonian = Schmidt Cassegrain?
Kavin Ishwaran
# Schmidt corrector plate on a Newtonian = Schmidt Cassegrain?

(Asked by: username_0 on 2021-08-10)

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 ?

**Tags:** telescope, amateur-observing, optics, celestial-mechanics

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## 1 answers

### Answer by username_1 on 2021-08-10. Score: 2 | Accepted answer

> 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

* [this answer to *Are X-ray telescopes with glancing angle surfaces basically “funny-looking” Cassegrain telescopes mathematically?*](https://astronomy.stackexchange.com/a/43362/7982)
* all answers to [In a reflecting telescope (Cassegrain), is there a difference between using a concave or convex secondary mirror?](https://astronomy.stackexchange.com/q/21478/7982)
* [What is the difference between a Cassegrain telescope and a Gregorian one? Which one is better?](https://astronomy.stackexchange.com/q/45101/7982) currently unanswered
* [this answer to *When was the first true Gregorian telescope built?*](https://hsm.stackexchange.com/a/2059/3869)
* [How was the first parabolic telescope mirror made, and how was it used, and for what kind of telescope was this work done?](https://hsm.stackexchange.com/q/13203/3869) currently unanswered

> **Comments:**
> **username_0**: Thank you uhoh !!.  
> **username_1**: @Ishwaran Thanks for the accept, but your question is only 39 minutes old! You might want to consider un-accepting for now, and waiting a few days to see of additional helpful and informative answers are posted. It's totally up to you of course.  
> **username_0**: That's fine mate !!. You already cleared my pestering mind :)  
> **username_1**: @Ishwaran  now you've got me thinking! [Why are satellite ground station and deep space communications dishes with secondary mirrors usually of Cassegrain design?](https://space.stackexchange.com/q/54545/12102)  
> **username_2**: 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.  
> **username_1**: @D.Halsey I think that's important enough to post as an additional, supplemental answer. Alternatively, please feel free to edit this one. I'm not familiar/confident enough to do it myself. *Thanks!*  
> **username_2**: Correction to my earlier comment: Many Schmidt designs have spherical primary, spherical secondary, and specially-shaped corrector plates.  

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