<p>In a schmidt camera, the corrector is placed at the centre of curvature, which is at a distance of twice the focal length from the primary mirror, and minimizes aberrations such as coma and astigmatism.  This makes for relatively long telescopes, particularly with large primary mirrors.</p>

<p>The sct design is similar, in that it uses a corrector plate, but this (along with the secondary mirror) is placed slightly closer to the primary mirror than its focal length.  This design allows much more compact telescopes, but at the expense of more aberrations.  As with most if not all optical designs, it is a compromise.</p>

CC BY-SA 4.0
2018-12-31T18:08:32.643Z
Thanks for the help! I would appreciate it if you may further explain these aberrations. I would also like to know if the corrector plate that is used in the compact design is identical to the one in the Schmidt camera.
Thamer AL Sadoun
CC BY-SA 4.0
2019-01-01T19:31:32.333Z
If you have further follow up questions, it's best to post a new question.
Dr Chuck
CC BY-SA 4.0
2019-01-02T16:55:58.743Z
Got it! A new question was posted.
Thamer AL Sadoun
CC BY-SA 4.0
2018-12-29T19:35:57.950Z
2018-12-29T19:35:57.950Z
Dr Chuck
<p>According to wikipedia, Schmidt corrector plates must be aligned with the center of curvature to eliminate the spherical aberration. However, in compact SCT designs by Celestron and Meade, that is clearly not the case since the optical tube is usually shorter than the radius of curvature of the primary mirror. My question here is <strong><em>WHY?</em></strong></p>

<telescope><amateur-observing><optics><telescope-lens><mirror>
Why aren't corrector plates aligned with the center of curvature in an SCT?
Thamer AL Sadoun
# Why aren't corrector plates aligned with the center of curvature in an SCT?

(Asked by: username_0 on 2018-12-29)

According to wikipedia, Schmidt corrector plates must be aligned with the center of curvature to eliminate the spherical aberration. However, in compact SCT designs by Celestron and Meade, that is clearly not the case since the optical tube is usually shorter than the radius of curvature of the primary mirror. My question here is ***WHY?***

**Tags:** telescope, amateur-observing, optics, telescope-lens, mirror

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## 1 answers

### Answer by username_1 on 2018-12-29. Score: 2 | Accepted answer

In a schmidt camera, the corrector is placed at the centre of curvature, which is at a distance of twice the focal length from the primary mirror, and minimizes aberrations such as coma and astigmatism. This makes for relatively long telescopes, particularly with large primary mirrors.

The sct design is similar, in that it uses a corrector plate, but this (along with the secondary mirror) is placed slightly closer to the primary mirror than its focal length. This design allows much more compact telescopes, but at the expense of more aberrations. As with most if not all optical designs, it is a compromise.

> **Comments:**
> **username_0**: Thanks for the help! I would appreciate it if you may further explain these aberrations. I would also like to know if the corrector plate that is used in the compact design is identical to the one in the Schmidt camera.  
> **username_1**: If you have further follow up questions, it's best to post a new question.  
> **username_0**: Got it! A new question was posted.  

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