Galaxy Rig - advantage of a C11 over a 10 or 12" RC? - Forums

Topic starter

Apr 20, 2022

Hi,

so this seems like an odd question maybe, since I am aware of the basic differences of the SC vs RC designs.

But the C11 (EdgeHD) seems to be very popular still - and I do wonder why that is. On paper, a similarily sized RC system should have an edge (haha..^^) over the SC design. No Schmidt Plate, open design and consequently faster cooling/less turbulence. And the reducers are generally a lot cheaper than for the C11 (which costs around 1000$?). And the scope itself would also be quite a bit cheaper overall.

So what am I missing? Any reason, apart from the Hyperstar option, that would make me choose a C11 over an RC?

Thanks for your thoughts!

/M

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Björn Arnold

Apr 20, 2022

Hi Marc,

A few points to each setup that came up:
EdgeHD:
- Spherical mirrors (easier to produce and cheaper compared to same size hyperbolic mirror)
- Due to the above: optical alignment is easier
- Suitable for visual observation as the central obstruction isn’t a problem. On an RC this is likely to bother.
- Well corrected field but requires more and also refractive optical elements.
- Schmidt plate is a protective element for the interior elements (a nice side effect IMHO)

RC:
- Optical design uses only reflective elements. So, no chromatic aberrations.
- Well corrected field with minimal amount of optical elements.
- More difficult to align but nothing infeasible if one is willing to work on that skill.
- No moving mirrors. Therefore no mirror flop issues.

Note: I wouldn’t use a reducer unless the FOV at native FL is an issue. To get the scope „faster“, bin the pixels if your image scale is oversampled.

CS,
Björn

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andrea tasselli

Frequent contributor

Apr 20, 2022

I want to dispel some misconception about the Celestron SCT optical design and inherent limits. 1st, while it's a catadioptric design, its corrector plate (the Schmidt plate) has zero optical power meaning it can't produce chromatic shift (because of it and because it defines the inlet pupil of the system). Classical SCTs are intrinsically perfectly achromatic in that regard and also do not have sphero-chromatism. Secondly, depending on the specific maker, one of the mirror must be an aspheric. My guess is that in the case of Celestron's it is a prolate spheroid. The flat fielding required at f/10 (or f/11) is achieved by a lens system (an air spaced doublet) inside the baffle, which might or might not cause secondary spectrum because of its distance from the focal point.

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Björn Arnold

Apr 20, 2022

andrea tasselli:
I want to dispel some misconception about the Celestron SCT optical design and inherent limits. 1st, while it's a catadioptric design, its corrector plate (the Schmidt plate) has zero optical power meaning it can't produce chromatic shift (because of it and because it defines the inlet pupil of the system). Classical SCTs are intrinsically perfectly achromatic in that regard and also do not have sphero-chromatism. Secondly, depending on the specific maker, one of the mirror must be an aspheric. My guess is that in the case of Celestron's it is a prolate spheroid. The flat fielding required at f/10 (or f/11) is achieved by a lens system (an air spaced doublet) inside the baffle, which might or might not cause secondary spectrum because of its distance from the focal point.

The EdgeHD is the same optical design as the classical SCT with the addition of a corrector element located in the baffle. The Schmidt-plate is polynomial and both mirrors are spheres. Here’s the Celestron white paper of the EdgeHD design:
https://s3.amazonaws.com/celestron-site-support-files/support_files/edgehd_whitepaper_final.pdf
(see page 4 for the optical design)

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rhedden

Apr 20, 2022 · Edited: Apr 20, 2022

The RCs and Newts are superior for galaxy imaging, IHMO. I own an 1100 EdgeHD, though. Why? I lived in west Texas when I bought it, and the airborne dust and dirt made it next to impossible to work with an open-tube design. Dust, pollen, and grit can settle on the mirrors with an open tube. I had to clean the SCT corrector plate after virtually every imaging session, as it was filthy… but it acted as a "windshield" that kept the filth off the mirrors. I met a guy who was using a 12" Dob, and when I looked into the end of his scope, I couldn't see my reflection in the mirror. He needed a shovel, not a dust cloth, to get dirt off the primary.

Now that I live in upstate NY, water condensation is the main problem, not dirt, and the corrector plate is a liability. I'd buy an f/4 imaging Newt instead of an SCT if I wanted a new galaxy scope for this location.

I'd like to add something to Andrea's comments about the optics of the SCTs. Many people plan to use the 0.7x reducer with their SCT for DSO imaging so that they don't have to deal with f/10. With the reducer in place, chromatic aberrations are a fact of life, especially with APS-C or larger sensors, where the optical aberrations are quite bad at the edges (at least with my setup).

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