Are there real differences between sct’s,cdk, classic cas etc

[Robindonne]

I still haven't taken a good dive into astrophotography.  sold almost all my 8 inches of the above models with the idea of buying a good 11+ inch.  but I catch myself that I actually have no idea which 11+ is the most appreciated type right now.  In short, where do the models differ from each other?  I understand that there are differences in height, weight and construction style.  but are the final results really recognizable?  can someone use a final photo to see with which type of mirror telescope the photos were taken?  Does a cdk not have the same result as a coma corrected newtonian or a corrected sct (edge hd) of the same diameter.  For example, I don't understand why someone would prefer a Sharpstar sca260 with a gigantic 5" central obstruction over another 10" or even 11" build that can be made to the same speed but does not have that gigantic central obstruction.  Where are the differences that make one reflector more suitable than another?  is it the sensor size that can be exposed or is it more emotion and feeling that makes someone choose a certain type?

 

For refractors its made very clear   A minimum of 3 elements of very good quality glass which is machined in a very fine quality will give a satisfying instrument.

Or even better, is there a place on internet where the biggest differences are explained?

Edited December 5, 2021 by Robindonne

[Nik271]

There are differences in the native aberrations from each design. For example RCs have no coma but suffer from astigmatism. SCTs and Newtonians have  coma, Maks have higher order spherical aberrations. With correctors you can probably get them all to produce a flat undistorted image circle. Where they differ most from eash other is the difficulty of manufacturing the optical surfaces (which becomes closely related to cost), the need and ease of regular collimation, and of course the practicality and convenience factor ( weight/size ). I think a large fast Newtonian with coma corrector is probably the most bang for bucks equipment you can get but at 10-11 inches aperture they become hard to mount even in an observatory. And you have to live with diffraction spikes.  Note that for imaging the size of the central abstruction is not so important.  Unless you are doing planetary for everything else your mount will probably be the biggest limiting factor with these large focal distance scopes.

[vlaiv]

Each telescope design will have different spot diagram over its illuminated and corrected field.

For example - Newtonian without coma corrector is next to useless for AP. It suffers from serious coma as soon as you move off axis. If you want to use one without coma corrector (for some reason) - you need to use very slow version - so you'll have extremely long tube with very small field of view.

Add coma corrector - and you might get system that is no longer diffraction limited / suffers from too much spherical aberration. In order to avoid that - you need to choose your CC carefully and get good design - like 3 or 4 element one over simpler 2 element designs. This introduces constraints on your imaging train as you need to put gear at certain distance from CC for it to operate the best.

Similar thing happens with EdgeHD or other designs that have corrective lens close to focal plane.

Other than that - there are differences in F/ratio of telescope. SCTs are usually at F/10, RCs are at F/8 and DKs and Modified DKs are at F/6.8. That means different focal lengths / FOVs for given aperture and different choice of camera (pixel size and/or binning) for optimum sampling rate.

Some designs have better baffling of stray light than others - for example Newtonian needs much longer tube than it usually has to properly baffle it. My RC has plenty of knife edge baffles in tube as well as primary and secondary baffles.

1 hour ago, Robindonne said:

 is it the sensor size that can be exposed or is it more emotion and feeling that makes someone choose a certain type?

I'm choosing things purely on technical bases - yes, optical characteristics come first - but mechanical are also important - you'll need to mount that scope and scope needs to be rigid as well. Some designs fare better in this regard.

[Robindonne]

5 hours ago, Nik271 said:

With correctors you can probably get them all to produce a flat undistorted image circle

Yes thats what makes me wonder why and for what reason one prefers a model over the other, while someone else with the same goal prefers the other model.  Except for the large newtonians.  They are the easy choice for best bang for your bucks but impractical.  

[Robindonne]

4 hours ago, vlaiv said:

I'm choosing things purely on technical bases - yes, optical characteristics come first - but mechanical are also important - you'll need to mount that scope and scope needs to be rigid

Yes.  So every serious journey in an astrophotographers life will end in a reverse engineered choice.  Which part of the sky do you prefer, which camera is going to be used to achieve that and what is your mount capable of moving around in a steady way.   
 

something keeps attracting me to a c11/edge hd.  Perfect aperture, perfect length and weight.  Easily transformable from f/10 to f/7 or f/2 Well known and probably free of design failures etc.  Both visual and Ap friendly.   
 

isnt this close to the perfect instrument?

[vlaiv]

1 hour ago, Robindonne said:

isnt this close to the perfect instrument?

I probably should not comment on that since I have certain disliking for SCTs that I can't explain.

Never used one, but as a design - I don't really like them for some reason.

[Robindonne]

42 minutes ago, vlaiv said:

I probably should not comment on that since I have certain disliking for SCTs that I can't explain.

Never used one, but as a design - I don't really like them for some reason.

Ha you see, the emotion can play a role