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Difference between Refractors and Reflectors Regarding Image Quality

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#1 Rince

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Posted 11 October 2020 - 01:43 PM

Hi all, I was wondering about this question for some time now but couldn’t find a satisfying answer: what are the fundamental differences between reflector (let’s say a Newtonian) and refractor telescopes regarding image quality? For instance, if I were to take two scopes (one of each type) with matching specs as much as possible (same focal length, similar focal ratio and aperture, same camera/filters attached) and then took images of the same object with the same integration time, what would then be the main differences between the resulting images (other than the obvious diffraction spikes on the Newtonian)?


Edited by Rince, 11 October 2020 - 02:40 PM.


#2 sg6

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Posted 11 October 2020 - 01:53 PM

Image on sensor the 8" would be bigger, with same focal ratio the same brightness.

Expect the reflector to be a little "softer" owing to the secondary assembly and spikes on stars.

 

Visual the 8" would be brighter at same magnification.

 

But you are comparing 2 different things, even if they have the same name/title. Bit like comparing a diesel people carrier to a petrol sports. Both are cars but different, even if they had the same size engine.

 

In a way these comparison never help. Always leads to some form of disagreement. Say by post 6 to 8.


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#3 gatsbyiv

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Posted 11 October 2020 - 02:08 PM

... matching specs as much as possible (same focal length, similar focal ratio, same camera/filters attached) with the main difference being the aperture (let’s say 8 inches versus 72 millimeters)...

If you have the same focal length and same focal ratio, your aperture is by definition the same.  smile.gif



#4 MarkGregory

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Posted 11 October 2020 - 02:41 PM

Image on sensor the 8" would be bigger, with same focal ratio the same brightness.

Expect the reflector to be a little "softer" owing to the secondary assembly and spikes on stars.

 

Visual the 8" would be brighter at same magnification.

 

But you are comparing 2 different things, even if they have the same name/title. Bit like comparing a diesel people carrier to a petrol sports. Both are cars but different, even if they had the same size engine.

 

In a way these comparison never help. Always leads to some form of disagreement. Say by post 6 to 8.

I agree 100%. Comparisons sometimes create more confusion and more questions. The best thing to do is try several types, if possible, and buy the one that best meets your particular needs. For me, I have found that a decent small aperture refractor meets my desire to observe the Moon, Sun and planets. 



#5 Rince

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Posted 11 October 2020 - 03:09 PM

 

...you have the same focal length and same focal ratio, your aperture is by definition the same.

You are right of course; I’ve modified the original question.


 


 

In a way these comparison never help.

 

I am not asking which one is better. But, at the end of the day, they are both used for the same purpose: to capture images. Therefore, I don’t feel it is totally unreasonable to ask what are the possible differences in the resulting images. I am not trying to create a fight; I am genuinely curious if there are any difference.
 

 

 

The best thing to do is try several types, if possible, and buy the one that best meets your particular needs.

That would be quite hard even in normal, none-pandemic times. Outside of a few star parties, I cannot even think of a way where you could do that. I think that for the vast majority of people, internet research is all that is available.
 


Edited by Rince, 11 October 2020 - 03:18 PM.


#6 drd715

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Posted 11 October 2020 - 04:30 PM

In general but with some variation you will find the refractors will produce more contrast (especially in longer focal ratios) and pinpoint stars compared to the larger reflectors. However aperture will win out in providing more resolution of details if you were comparing a smaller refractor to a larger reflector.

Aperture is a real plus in .avi type imaging of planets both for resolution and having long focal lengths. Aperature is welcome for low light subjects in a short F-ratio format to collect photons faster and you get the finer resolution - but the reflectors can be a bit finicky with columation. A big reflector definitely has a place in the telescope stable.

There is something special about a refractor - it just seems like a real telescope. Excellent contrasty sharp images are produced with a good quality optic.

Now you should also take "seeing" into the equation as it is usually the aperture limiting factor. There is a general old time idea that a 6 inch refractor was the sweet spot to reach seeing limitations, the idea being that a column of air less than 6 inches would show less "turbulence" than a larger diameter column of air - not so sure this is the current philosophy. But anyway there are a lot of factors that come into play in comparisons of this type.

Personally I think that a refractor should be at least 100mm for visual use and long focal length for planets and lunar viewing. Refractors over 6 inches become real beasts (or longer than 1200mm). 5 inches is much more transportable.

The smaller short focal length refractors are excellent for wide view imaging - a particular nitch. The RASA scopes are fitting into this space also, but with more aperture.

Pick your tool for yhe job you are trying to accomplish. But buy quality, it is worth the difference. Quality can be had at good value, but avoid the junk.

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#7 DeanD

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Posted 12 October 2020 - 01:04 AM

If you are using a top quality refractor and a top quality reflector of the same FL and aperture there will be two main differences in the images produced: the refractor will show slightly higher contrast because of the unobstructed aperture (and a very marginally brighter image), and of course you will have diffracton spikes in the reflector.

 

My understanding is that field curvature for a refractor is higher than for a reflector, but this will depend on the f-ratio. Field curvature will affect the size of the in-focus image at the detecter, and most scopes require an additional field flattener to eliminate edge-of-field problems. Perhaps the imaging purists will weigh in on this...


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#8 Rince

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Posted 12 October 2020 - 04:16 AM

Thank you guys, very informative answers!

 

So, generally speaking, refractors produce a more contrasty image with more pinpoint stars, but because of their typically smaller size and aperture, they lack slightly in detail. Reflectors, on the other hand, produce "softer" images, but because they are usually bigger, there are more details (and diffraction spikes of course). Would this be a more or less accurate summary?

 

Also, if I understand this correctly, aside from the potential to gather more light, aperture is also responsible for the detail in our images. So, it is kind of like resolution in computer science; the bigger the aperture, the more "resolution" it will have (in a grossly generalized way of course). Am I understanding this correctly?



#9 drd715

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Posted 12 October 2020 - 04:07 PM

Thank you guys, very informative answers!

So, generally speaking, refractors produce a more contrasty image with more pinpoint stars, but because of their typically smaller size and aperture, they lack slightly in detail. Reflectors, on the other hand, produce "softer" images, but because they are usually bigger, there are more details (and diffraction spikes of course). Would this be a more or less accurate summary?

Also, if I understand this correctly, aside from the potential to gather more light, aperture is also responsible for the detail in our images. So, it is kind of like resolution in computer science; the bigger the aperture, the more "resolution" it will have (in a grossly generalized way of course). Am I understanding this correctly?

"Generally " - check out "Dawes limit " and you will see how separation in detail increases with aperture.
- Which resolves finer details.

Keep in mind that the quality of the optics whether refractor or reflector is the ultimate factor in getting the most resolution out of a given scope. In addition to the objective the focuser will have a big effect upon usability of the scope. I refer to the stability, ease of focus and pleasurable use of a fine zero backlash assembly. Larger diameter focusers are nice.

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#10 Jon Isaacs

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Posted 12 October 2020 - 04:19 PM

Hi all, I was wondering about this question for some time now but couldn’t find a satisfying answer: what are the fundamental differences between reflector (let’s say a Newtonian) and refractor telescopes regarding image quality? For instance, if I were to take two scopes (one of each type) with matching specs as much as possible (same focal length, similar focal ratio and aperture, same camera/filters attached) and then took images of the same object with the same integration time, what would then be the main differences between the resulting images (other than the obvious diffraction spikes on the Newtonian)?

 

You've asked this question in terms of imaging.

 

When you compare refractors and reflectors of the same aperture and focal ratio and stipulate that the refractor will be of high quality, (no achromats), then the images in the refractor will be brighter and maybe more contrasty.

 

But realistically, refractors are limited to small apertures and slower focal ratios, particularly in the apertures that are large, for a refractor.  No one is imaging with a 10 inch F/3 refractor.

 

So it's really about the right tool for the job.. 

 

Jon


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#11 Hermie

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Posted 12 October 2020 - 08:00 PM

My first telescope was an 8” reflector, and then I moved to a 4” refractor. The reflector never performed well for me, mainly because I never got the collimation spot on, but also thermal issues.

The refractor never disappoints. It takes about a half hour to cool down, and then I know the weakest link in the optical train is my eyeball. 

 

My point is that reflectors take a certain level of user skill to reach their potential, and that is as significant as the theoretical limitations of aperture.

 

Hermie


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#12 sydney

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Posted 12 October 2020 - 08:26 PM

If you are looking for a scope to begin as an imager, you will likely get the best results by

 

1. buying the best mount you can afford

2. starting with a small refractor

 

Imaging has a significant learning curve.  A quality mount is crucial to minimizing frustration, maximizing early results, and gaining useful experience.  Small refractors are more forgiving and easier to image with because they have a short focal length, do not need routine collimation, and do not have a mirror that can shift when crossing the meridian.

 

Both refractors and relectors can take great images.  The quality of the mount and the experience and skill of the imager, among other variables, are more relevant to achieving good results.


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#13 Ihtegla Sar

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Posted 12 October 2020 - 08:47 PM

Another difference between reflectors and apo refractors is that reflectors are subject to coma while apo refractors are subject to field curvature. You can correct coma with a coma corrector and field curvature with a flattener. So in the end it may be a distinction without a difference. But since you were asking about differences in the images between reflectors and refractors, I thought I would point it out.

I also agree with Hermie that refractors are eaaier to use and cool faster, but a lot of that is because small refractors are typically compared to big fast reflectors. A fast reflector is harder to collimate than a slow reflector and a big mirror cools slower than a small lens. But if comparing a small refractor to a small reflector of the same size and focal ratio these differences will be slight.
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#14 StarAlert

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Posted 12 October 2020 - 11:53 PM

Here is an image that SkySafari includes in its description of the Double Double. It was taken with a 16” Meade.
 

EA269077 FC12 40A2 B40A 63D30AE213B1
 
If that’s the best a 16” reflector can do on Epsilon Lyra, I’ll keep my 4” Tak. 

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#15 Ihtegla Sar

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Posted 13 October 2020 - 01:07 AM


Here is an image that SkySafari includes in its description of the Double Double. It was taken with a 16” Meade.



If that’s the best a 16” reflector can do on Epsilon Lyra, I’ll keep my 4” Tak.


Unfortunately that's how it looks all too often in my 20" Obsession. First time I looked at epsilon Lyrae in the 20" it wouldn't even split it, it just looked like two boiling goose eggs. Second time I tried on a night of better seeing when it was at the zenith and it looked much like that photo -- it was a clean split but not a pretty split. I too am keeping my 4" Tak. I'm also keeping the Obsession because for certain things (pretty much any DSO, and even planets after 4:00 am) its phonominal and I'm sure it can give good star images, I just haven't quite gotten there yet.
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#16 Jon Isaacs

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Posted 15 October 2020 - 01:20 PM

 

Here is an image that SkySafari includes in its description of the Double Double. It was taken with a 16” Meade.
 

 
 
If that’s the best a 16” reflector can do on Epsilon Lyra, I’ll keep my 4” Tak. 

 

 

The double-double is doable in a 50 mm, easy in a 70 mm.

 

Try STF 2215 in your 4 inch.. I've made the split with my 13.1 inch..I've done tighter with my 16 inch.

 

0.44" at mags 6-6.9

 

https://www.stelledo...?iddoppia=71332

 

This highlights some of the differences between refractors and reflectors. Refractors are easy scopes, they're generally small so seeing isn't an issue. Reflectors take some operator skill and when conditions are right, they go places refractors just don't go.

 

But this thread seems to be about imaging and a first imaging scope. A small refractor is the place to begin.. fewer headaches. Imaging is tough enough, a reflector just complicates things.

 

Jon


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