<p>Astigmatism. No way to eliminate in SCT. Reduce it with proper fixing of every optical element. Astigmatism is only eliminated with 3-mirror scheme.</p>

<p>Curvature of field. No way to eliminate in SCT.
You are forced to use so-called "field flatteners".
If kept in good range, does not hurt, if you are not going to do astrophotography (which with SCT nobody does).</p>

<p>Chromatism. No way to eliminate residual chromatism from corrector.</p>

<p>Corrector blur. No way to eliminate low-quality corrector aberrations.</p>

<p>Overall, SCT is good consumer design (together with MCT), providing nice compromise for all aberrations for astronomical observations (not astrophotography).</p>

CC BY-SA 4.0
2019-01-04T09:04:22.537Z
2019-01-04T09:04:22.537Z
sanaris
<p>The primary aberration present in a <a href="https://en.wikipedia.org/wiki/Schmidt%E2%80%93Cassegrain_telescope" rel="nofollow noreferrer">Schmidt–Cassegrain telescope</a> is spherical, due to the primary mirror being spherical in shape; that causes light at the edges to have a different path length than light at the center, and thus a different focal point.
<a href="https://i.sstatic.net/8EXTp.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8EXTp.png" alt="Exaggerated cross section of a Schmidt corrector plate"></a>
The image above, from Wikipedia's "<a href="https://en.wikipedia.org/wiki/Schmidt_corrector_plate" rel="nofollow noreferrer">Schmidt corrector plate</a>" webpage, shows an exaggerated corrector; they are actually much flatter.</p>

<p>The secondary aberration is coma, though this is present to a much lesser extent. Coma is a spherical aberration from rays that come in off-axis. Coma shows up as little off-axis comet-shaped blobs that point inwards towards the center of the field and get bigger as you look towards the edge of the field of view. </p>

<p>Persons wanting a lengthy technical and mathematical explanation will enjoy reading sections: "<a href="https://www.telescope-optics.net/Schmidt-camera.htm" rel="nofollow noreferrer">10.2.2. Full-aperture Schmidt corrector: Schmidt camera</a>" <strong>and</strong> "<a href="https://www.telescope-optics.net/SCT.htm" rel="nofollow noreferrer">10.2.2.4. Schmidt-Cassegrain telescope (SCT)</a>" of the <a href="https://www.telescope-optics.net/" rel="nofollow noreferrer">https://www.telescope-optics.net/</a> website.</p>

<p>Recent improvements in the design are offered using <a href="https://en.wikipedia.org/wiki/Aplanatic_lens" rel="nofollow noreferrer">aplanatic</a> flat field Schmidt-Cassegrain optics. Here's a screenshot from <a href="https://www.celestron.com/products/edgehd-8-optical-tube-assembly-cg5" rel="nofollow noreferrer">Celestron's video</a> showing the difference between a standard versus aplanatic Schmidt telescope design:</p>

<p><a href="https://i.sstatic.net/ZrFhd.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZrFhd.png" alt="Standard versus aplanatic Schmidt telescope"></a></p>

<p>The competing system from <a href="https://www.meade.com/lx65-acf-6.html" rel="nofollow noreferrer">Meade is called Advanced Coma-Free</a> (ACF™) optical design. Claiming also to offer coma-free pinpoint star images and flatter image fields while reducing astigmatism and eliminating diffraction spikes. </p>

<p>The website forum "<a href="https://www.cloudynights.com/topic/462785-meade-vs-celestron/page-3" rel="nofollow noreferrer">Starry Nights - Cats &amp; Casses</a>" compares Celestron and Meade.</p>

CC BY-SA 4.0
2019-01-03T09:04:54.520Z
2019-01-03T09:04:54.520Z
Rob
<p>The Schmidt-Cassegrain telescope is amazing! It is compact, suitable for astrophotography ,and it requires little to no collimation. However, the SCT has some inherent optical issues. What are they? What are their causes? and how can they be eliminated?</p>

<telescope><optics><telescope-lens><mirror><collimation>
What are the aberrations of an SCT? And how can they be eliminated?
Thamer AL Sadoun
# What are the aberrations of an SCT? And how can they be eliminated?

(Asked by: username_0 on 2019-01-02)

The Schmidt-Cassegrain telescope is amazing! It is compact, suitable for astrophotography ,and it requires little to no collimation. However, the SCT has some inherent optical issues. What are they? What are their causes? and how can they be eliminated?

**Tags:** telescope, optics, telescope-lens, mirror, collimation

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## 2 answers

### Answer by username_1 on 2019-01-04. Score: 1 | Accepted answer

Astigmatism. No way to eliminate in SCT. Reduce it with proper fixing of every optical element. Astigmatism is only eliminated with 3-mirror scheme.

Curvature of field. No way to eliminate in SCT.
You are forced to use so-called "field flatteners".
If kept in good range, does not hurt, if you are not going to do astrophotography (which with SCT nobody does).

Chromatism. No way to eliminate residual chromatism from corrector.

Corrector blur. No way to eliminate low-quality corrector aberrations.

Overall, SCT is good consumer design (together with MCT), providing nice compromise for all aberrations for astronomical observations (not astrophotography).


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### Answer by username_2 on 2019-01-03. Score: 2

The primary aberration present in a [Schmidt–Cassegrain telescope](https://en.wikipedia.org/wiki/Schmidt%E2%80%93Cassegrain_telescope) is spherical, due to the primary mirror being spherical in shape; that causes light at the edges to have a different path length than light at the center, and thus a different focal point.
[![Exaggerated cross section of a Schmidt corrector plate](https://i.sstatic.net/8EXTp.png)](https://i.sstatic.net/8EXTp.png)
The image above, from Wikipedia's "[Schmidt corrector plate](https://en.wikipedia.org/wiki/Schmidt_corrector_plate)" webpage, shows an exaggerated corrector; they are actually much flatter.

The secondary aberration is coma, though this is present to a much lesser extent. Coma is a spherical aberration from rays that come in off-axis. Coma shows up as little off-axis comet-shaped blobs that point inwards towards the center of the field and get bigger as you look towards the edge of the field of view.

Persons wanting a lengthy technical and mathematical explanation will enjoy reading sections: "[10.2.2. Full-aperture Schmidt corrector: Schmidt camera](https://www.telescope-optics.net/Schmidt-camera.htm)" **and** "[10.2.2.4. Schmidt-Cassegrain telescope (SCT)](https://www.telescope-optics.net/SCT.htm)" of the <https://www.telescope-optics.net/> website.

Recent improvements in the design are offered using [aplanatic](https://en.wikipedia.org/wiki/Aplanatic_lens) flat field Schmidt-Cassegrain optics. Here's a screenshot from [Celestron's video](https://www.celestron.com/products/edgehd-8-optical-tube-assembly-cg5) showing the difference between a standard versus aplanatic Schmidt telescope design:

[![Standard versus aplanatic Schmidt telescope](https://i.sstatic.net/ZrFhd.png)](https://i.sstatic.net/ZrFhd.png)

The competing system from [Meade is called Advanced Coma-Free](https://www.meade.com/lx65-acf-6.html) (ACF™) optical design. Claiming also to offer coma-free pinpoint star images and flatter image fields while reducing astigmatism and eliminating diffraction spikes.

The website forum "[Starry Nights - Cats & Casses](https://www.cloudynights.com/topic/462785-meade-vs-celestron/page-3)" compares Celestron and Meade.


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