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Refractor or Reflector for most optical resolution

astrophotography equipment imaging reflector refractor
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#1 Astro_Francis

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Posted 05 September 2018 - 01:07 PM

Hello all,

 

I am going to be working on a project where we will be imaging the full moon with an ultra high resolution camera (more than double that of high end full frame DSLRs). The goal is to utilize the camera's high resolution. Which telescope would provide the sharpest and best image of the moon? A reflector or a refractor? It is said that refractors provide higher resolution at a given aperture, but refractors are generally less than half the diameter of reflectors at similar price points. How do you compare apples to oranges here as far as optical resolution goes? I provided a couple of potential telescopes that we may purchase for the project. I am open to suggestions to other telescopes as well. I imagine we would not want to spend more than $2,500 for the telescope and field flattener/coma corrector. The camera sensor is a super 35, which is a little bigger than a APS-C, cropped DSLR sensor. 

 

 

4" APO Refractor

https://www.highpoin...num-case-s11120

 

vs. 

 

10" Reflector

https://www.highpoin...ctor-ota-s11220

 



#2 mvas

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Posted 05 September 2018 - 01:12 PM

You need high quality surfaces, to make a high quality image.


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#3 rgsalinger

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Posted 05 September 2018 - 01:14 PM

You are likely going to be seeing limited, so the diffraction based resolution limit of the telescope is going to be less important than you seem to think. I'm not sure what you mean by "high resolution camera" but if it means lots of pixels then you will be disappointed. You need to match the size of your pixels to the focal length of the telescope to get the best digital resolution. Read "The Deep Sky Imaging Primer" by Charles Bracken before buying anything.

 

People talk about refractors being better, inch for inch, mostly (IMHO) because refractors have wonderful contrast and that's impossible to match with most reflectors. 

 

The direct answer to your question is that resolution is a function of aperture, nothing else. So the larger aperture always has better resolution. At the same time, (see above) there are other limiting factors - like location - that are usually more important.

 

Rgds-Ross


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#4 Garyth64

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Posted 05 September 2018 - 01:18 PM

A  4" APO does not equal a 10" reflector.  The 10" will have better resolving power.  Obviously more light gathering power, and a shorter exposure time.


Edited by Garyth64, 05 September 2018 - 01:21 PM.

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#5 mashirts

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Posted 05 September 2018 - 01:30 PM

Angular resolution is determined by aperture (more is greater)  Linear resolution is determined by focal ratio (lower f ratio is greater).  



#6 bremms

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Posted 05 September 2018 - 01:30 PM

A good 10" mirror will best a top quality 6" APO. A lot of APO folks may no like it or may disagree, but I've seen it many times. Most reflectors are not really well collimated. Some not well at all some just OK. They are more finicky of collimation and tend to be slapped together on site. No contest unless the seeing is really poor, or the 10 " reflector is just bad or poorly collimated.


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#7 db2005

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Posted 05 September 2018 - 01:31 PM

When comparing visual performance of refractors vs. obstructed apertures I've seen the following simple rule being used: subtract the diameter of the central obstruction from the scope's aperture. I.e. according to that approximation, a 200 mm aperture scope with a 35% obstruction would perform roughly equivalently to a 130 mm refractor in terms of resolution. Of course, this doesn't apply to seeing faint objects. In that case aperture always wins.

 

Also, under my typical night skies I've routinely seen small refractors outperform reflectors with 50% more aperture in terms of resolution and contrast.



#8 Geo31

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Posted 05 September 2018 - 01:54 PM

When comparing visual performance of refractors vs. obstructed apertures I've seen the following simple rule being used: subtract the diameter of the central obstruction from the scope's aperture. I.e. according to that approximation, a 200 mm aperture scope with a 35% obstruction would perform roughly equivalently to a 130 mm refractor in terms of resolution. 

That is NOT what I was taught.  I may be wrong, but the ability to resolve has to do with the diameter of the objective.  Period.  At least that is what I was taught, and why linking radio telescopes that are far from each other will produce much better resolution.

 

If I'm all wet, so be it.  But I think that's wrong.


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#9 photoracer18

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Posted 05 September 2018 - 01:59 PM

I hope you plan on using filters because the full moon glare will reduce contrast compared to quarter or half-moon images so details will be less. You also need to match the camera sensor size with the telescope's focal length to get the entire moon on the sensor. Also if your sensor is MF or larger (Pentax 645Z 50 MP size or larger) you are going to need a larger than 2" focuser and/or camera adapter on the optical tube if you plan on maximizing the size of the image on the large sensor. There is a reason AP used to supply a 4" focuser for those that wanted to image with the Pentax 6x7 MF film camera although the 2.7" focuser would work for smaller objects. And seeing will effect everything also as mentioned.
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#10 fcathell

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Posted 05 September 2018 - 02:08 PM

I think db2005's statement about obstructed optics does not apply to resolving power. As George stated, aperture diameter is the primary parameter determining resolution. An obstruction may reduce detail or contrast on an extended object like a planet or the moon, but it won't impact actual resolution for the most part.  Some people who are hard core double star observers actually prefer a central obstruction because it will actually increase resolution when looking for the separation between the two components of a close double.  This is because the central obstruction actually puts more light in the 1st diffraction ring at the expense of light in the airy disk, so the airy disc looks smaller which helps resolving close doubles.

 

Frank


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#11 db2005

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Posted 05 September 2018 - 02:09 PM

That is NOT what I was taught.  I may be wrong, but the ability to resolve has to do with the diameter of the objective.  Period.  At least that is what I was taught, and why linking radio telescopes that are far from each other will produce much better resolution.

 

If I'm all wet, so be it.  But I think that's wrong.

The rule I'm referring to is simply a heuristic I've seen used for visual observation. But, if the resolving power was only dependent on the diameter of the objective, try considering an imagined 200 mm scope with a 199 mm obstruction... Obviously the central obstruction will affect visual performance. Radio astronomy is different from visual astronomy because the wavelengths are much longer than visible light waves, and so are not affected as much by diffraction. Diffraction around spider vanes and central obstruction cause image degradation in Newtonians and CATs, but they tend to make up for much of this by means of sheer aperture.


Edited by db2005, 05 September 2018 - 02:12 PM.


#12 vtornado

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Posted 05 September 2018 - 02:28 PM

I own both a 4 inch  f/5 ED refractor, and a 10 inch dob.

I don't have lunar notes on performance, but I can see a little more in the 10 than the 4.

I only do visual, and quick pics of the moon, nothing serious.

If I remember correctly 1000mm of focal length gives pretty much a full frame moon image on

an aps-c sensor.

 

However there are a lot of BUT factors.

A 10 inch reflector takes about an hour to warm/cool 20 degrees.

Which is critical for high mag use.

 

For peak performance the dob must be collimated well.

 

In planetary resolution, I usually am limited to about 225x because of my atmosphere.  So the higher theoretical resolution of the 10 may be over-come with atmospheric turbulence.

 

Once again, I do visual, and I like to observe at a 1mm exit pupil.  Which means 100x on the

refractor.   Ergo viewing at 200x,  The resolution, light gathering of my eye enters into the

equation.  It would not on a camera.

 

A refractor can be easily set to track.  A 10 inch reflector can be too, but at much larger expense, because it will require a much larger mount.  I have an 8 inch f/5 newt and

a CG5 class mount "barely" holds it.  A breeze might make it too wobbly for a photo.

 

The camera changes the game in that it stores light over time.  So the light gathering prowess of the reflector is not needed. 

 

Maybe you should consider a 120mm ED.

 

For visual an ED doublet is fine.  I'm not sure if your application might require you going to a more

corrected ED triplet?

 

I don't know too much about this, but I assume comma is an issue on a 10 inch f/4 newt. 

You will not be able to get sharp focus across the image without a coma corrector.


Edited by vtornado, 05 September 2018 - 02:44 PM.


#13 tim53

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Posted 05 September 2018 - 02:33 PM

No, the central obstruction produces diffraction, which lowers contrast.  It doesn't decrease resolution.  So yes, a 200mm telescope with a 199mm obstruction will have the same resolution as an unobstructed 200mm telescope, but it will have a helluva lot less contrast.


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#14 photoracer18

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Posted 05 September 2018 - 02:42 PM

That is NOT what I was taught.  I may be wrong, but the ability to resolve has to do with the diameter of the objective.  Period.  At least that is what I was taught, and why linking radio telescopes that are far from each other will produce much better resolution.
 
If I'm all wet, so be it.  But I think that's wrong.

Resolution is a combination of sharpness and contrast same as with cameras and their lenses. The central obstruction on mirror scopes reduces contrast and therefore reduces perceived resolution and the MTF function of splitting parallel lines. As for combing telescopes to produce an interferometer, radio or optical, that requires software to produce aperture synthesis and take advantage of certain wave properties of light, easier in long wavelength radio, but harder in short wavelength light (but doable). The resulting image is also adversely affected by the minimum distance between elements, which is why the VLA was a bunch of dishes and not just 2 at the extreme ends of the tracks.
Larger optical objectives tend to be more adversely affected by seeing which is why most large professional scopes use adaptive optics to improve resolution by counteracting the atmospheric turbulence. Ordinary telescope resolution is also affected by how much of the light actually reaches the eye or sensor. Things like reflectivity of mirrors and light loss from the coatings of air-to-glass surfaces reduces the amount of light that gets thru. As do mirror surface smoothness which drives up costs.

#15 db2005

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Posted 05 September 2018 - 02:53 PM

No, the central obstruction produces diffraction, which lowers contrast.  It doesn't decrease resolution.  So yes, a 200mm telescope with a 199mm obstruction will have the same resolution as an unobstructed 200mm telescope, but it will have a helluva lot less contrast.

Thanks for the clarifications. Being a visual observer as I am, it's easy mix up the causes for image degradation.



#16 DAVIDG

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Posted 05 September 2018 - 03:14 PM

 One also needs to understand that refractors even APO have chromatic aberration that lowers resolution. The eye and brain are  very good at focusing on the image in  a refractor that is in focus while ignoring the out of focus image. A camera see all so you have a sharp image swimming inside a blurry one. Ask any planetary imager about trying to take high resolution images of the planets with a refractor and they will tell you that you need to take an Red, Green and Blue images thru filters were the focus is adjusted for each color and then combine them. If not the image is soft because the camera sees the out of focus wavelengths from the chromatic aberration of the lens. That is why modern DSLR CCD cameras have IR blocking filters  built in since camera lens are not corrector for IR and if not block the CCD seeing this out of focus image. 

 Aperture determines resolution, the bigger the aperture the better the resolution. The issue is that when people start comparing refractors to reflectors they aren't taking into account the actual quality of the optics of those two exact telescopes being judged.  One needs to first look at what the theoretical resolution of any optical system could be and then actually bench test that system to see how well it was made.  A poorly made reflector will easily be beaten  by a well made refractor but that doesn't mean one type is better then another. 

  So for this application one needs to do some homework and match focal length to pixel size. Then you need to look at off axis performance since  the Moon is 1/2 a  degree in diameter and you need  to be sure that the field size of your image is well corrected. You also need to look at the spectral response of the camera's detector to see how that matches the optical system your using. 

 

          - Dave 


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#17 bremms

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Posted 05 September 2018 - 03:15 PM

Yes the central obstruction changes the point spread function. The central peak has less intensity and the light is is distributed to the rings. Below about 20% diameter it's almost unnoticeable. Many optics have more wavefront / zonal errors than what is contributed by that 20%. Even very good  APO's have some not insignificant higher order spectrum and higher order SA. David G most likely has a more in depth knowledge of these things.



#18 bremms

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Posted 05 September 2018 - 03:15 PM

 One also needs to understand that refractors even APO have chromatic aberration that lowers resolution. The eye and brain are  very good at focusing on the image in  a refractor that is in focus while ignoring the out of focus image. A camera see all so you have a sharp image swimming inside a blurry one. Ask any planetary imager about trying to take high resolution images of the planets with a refractor and they will tell you that you need to take an Red, Green and Blue images thru filters were the focus is adjusted for each color and then combine them. If not the image is soft because the camera sees the out of focus wavelengths from the chromatic aberration of the lens. That is why modern DSLR CCD cameras have IR blocking filters  built in since camera lens are not corrector for IR and if not block the CCD seeing this out of focus image. 

 Aperture determines resolution, the bigger the aperture the better the resolution. The issue is that when people start comparing refractors to reflectors they aren't taking into account the actual quality of the optics of those two exact telescopes being judged.  One needs to first look at what the theoretical resolution of any optical system could be and then actually bench test that system to see how well it was made.  A poorly made reflector will easily be beaten  by a well made refractor but that doesn't mean one type is better then another. 

  So for this application one needs to do some homework and match focal length to pixel size. Then you need to look at off axis performance since  the Moon is 1/2 a  degree in diameter and you need  to be sure that the field size of your image is well corrected. You also need to look at the spectral response of the camera's detector to see how that matches the optical system your using. 

 

          - Dave 

Dave, I hoped you would chime in.



#19 Garyth64

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Posted 05 September 2018 - 04:14 PM

When comparing visual performance of refractors vs. obstructed apertures I've seen the following simple rule being used: subtract the diameter of the central obstruction from the scope's aperture. I.e. according to that approximation, a 200 mm aperture scope with a 35% obstruction would perform roughly equivalently to a 130 mm refractor in terms of resolution. Of course, this doesn't apply to seeing faint objects. In that case aperture always wins.

 

Also, under my typical night skies I've routinely seen small refractors outperform reflectors with 50% more aperture in terms of resolution and contrast.

I'm pretty sure neither of these statements is correct.



#20 RedLionNJ

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Posted 05 September 2018 - 04:24 PM

Hello all,

 

I am going to be working on a project where we will be imaging the full moon with an ultra high resolution camera (more than double that of high end full frame DSLRs). The goal is to utilize the camera's high resolution. Which telescope would provide the sharpest and best image of the moon? A reflector or a refractor? It is said that refractors provide higher resolution at a given aperture, but refractors are generally less than half the diameter of reflectors at similar price points. How do you compare apples to oranges here as far as optical resolution goes? I provided a couple of potential telescopes that we may purchase for the project. I am open to suggestions to other telescopes as well. I imagine we would not want to spend more than $2,500 for the telescope and field flattener/coma corrector. The camera sensor is a super 35, which is a little bigger than a APS-C, cropped DSLR sensor. 

 

 

4" APO Refractor

https://www.highpoin...num-case-s11120

 

vs. 

 

10" Reflector

https://www.highpoin...ctor-ota-s11220

Sensor size and pixel pitch are everything here - these may well be your limiting factors.

 

Assess each candidate scope for sensor coverage - can you fit the full moon on the sensor?

Once you've achieved that, how much can you multiply (note: not reduce) the f-ratio to try to get as close as possible to five times the pixel pitch, without the image overflowing the sensor?

 

If you image at anything less than an f-ratio of five times the pixel pitch, you're potentially tossing away resolution.

If you're really trying for high-resolution imaging, you will likely be seeing-limited, anyway.  So the resolution won't be anywhere near what you hoped for.


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#21 Tom Glenn

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Posted 05 September 2018 - 05:09 PM

Optical resolution depends on the aperture and nothing else.  Assuming a quality made scope, the potential resolution will always be larger with a larger aperture scope.  It's a mathematical certainty.  Seeing conditions will dictate whether or not the theoretical resolution is ever achieved (it may never be).  You do not subtract the diameter of the secondary to determine the resolution.  

 

What are the goals of this moon imaging project?  The potential resolution of the image comes from the scope, not the camera.  If the goal is absolute resolution of the lunar surface, then a larger aperture will always win, at least up until the atmospheric seeing limits are reached.  As RedLion says above though, in order to obtain this full resolution of the scope, you have to match the focal ratio to approximately 5x the pixel diameter of the camera sensor.  So if the goal of the project is to fit the entire surface of the moon into one shot, then you will likely be limited by your focal length and will be "wasting" resolution.  This is not a bad thing if you must acquire the entire moon in one shot, and it could save you some money because a larger scope would not be needed.

 

I happen to have an example of lunar images, from near the same illumination phase, that illustrate the difference in resolution between two different aperture scopes and the same camera.  In fact, the apertures in question are near to those listed in your initial inquiry, although in this case they are both reflectors.  The first image is taken with a 4.5 inch reflector and an ASI183mm camera.  The focal length is 910mm, and the entire moon fit into one shot.  The second image is taken with a C9.25 Edge HD and the same ASI183mm.  The focal length is 2350mm, and this shot required a mosaic of 4 panels.

 

I am reproducing the images below, at a reduced size, to demonstrate that if you display the images at a small image scale, there will be no detectable difference in detail between the two scopes (note, the images are not identical in all aspects, but I am only focusing on the resolution of detail here). However, if you follow the links provided to Flickr and then choose to download or view the "Original" size to view the full resolution image, the differences in resolution will be obvious.  Although you really have to view the image at large size to appreciate the difference.  Even the 4.5 inch scope produces an image that looks very decent until you download the original file and zoom in considerably.

 

So, which scope to buy depends on what type of resolution you want, and whether you want to fit the entire moon in one shot or do a mosaic.  But a larger aperture will absolutely produce a higher resolution image if the focal length is matched appropriately and the seeing conditions allow.  

 

First image, taken with 4.5 inch reflector.

 

Flickr link:

https://flic.kr/p/29GU2pA

 

Moon_072218_0437UT_TG_small.jpg

 

Second image, taken with 9.25 inch reflector.

 

Flickr link:

https://flic.kr/p/267xckY

 

Moon_042518_0430UT_TG_small.jpg


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#22 Astro_Francis

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Posted 05 September 2018 - 07:22 PM

Thank you for the responses. Can you explain further what you mean by focal # should be 5x the pixel pitch?

#23 Kokatha man

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Posted 05 September 2018 - 07:35 PM

Hello all,

 

I am going to be working on a project where we will be imaging the full moon with an ultra high resolution camera (more than double that of high end full frame DSLRs).

 

I appreciate a topic like this gives everyone the chance to sound off, but as someone has already intimated - who would want to image a Full Moon..?!?

 

Astro Francis, this posting of yours' (or the entire premise) seems quite silly imho...at that Moon phase one of the most important aspects required - immensely important in just about all photography, is the almost complete lack of shadows..!!!

 

This really does sound like a dream scenario to me: much more like something you have "dreamed up" tbh when you have very little idea about basics...a "Moon mosaic" compiled from areas covering the entire Moon captured at various phases would yield a far superior result, whatever the camera used! lol.gif



#24 Tom Glenn

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Posted 05 September 2018 - 07:37 PM

Thank you for the responses. Can you explain further what you mean by focal # should be 5x the pixel pitch?

Sure.  Take the pixel size of your camera in microns.  Multiply by 5.  This is the F-ratio you should be imaging at in order to capture all information from the scope and transfer it to the sensor.  

 

There is a finite amount of information captured by the telescope itself that exists at the image plane.  The amount of information is controlled by the aperture.  In order to maximize resolution, you need to capture all of this information.  There is a lot of theory involved here, and this has been written about many times before (you can use the search function), but from a practical standpoint all you need to know is that you can approximate the optimal focal ratio by multiplying pixel size by 5x.

 

Example:  My camera has 2.4um pixels.  Therefore, I need to operate at F/12 to capture all available information at the image plane. I actually operate my main scope (C9.25 Edge) at F/10.  So I'm not actually getting all theoretical information, but I am close enough to not want to mess around with a low-powered barlow lens.  But using barlows is how you change the F-ratio if you so choose.  In my 4.5 inch scope at F/8, I am considerably under sampled, but the whole point in using that scope is for quick and easy imaging that fits the entire moon into one shot.  The goal there is not maximum resolution.  This is why I asked you what your goals were for lunar imaging, because the answer will greatly change the equipment decisions, as well as the expense and efforts involved.  Taking overview shots of the moon that look good at low to medium size is relatively easy and cheap.  Taking high resolution shots of the entire lunar surface that can be blown up to very large sizes is not so easy.  


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#25 Tom Glenn

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Posted 05 September 2018 - 07:41 PM

I appreciate a topic like this gives everyone the chance to sound off, but as someone has already intimated - who would want to image a Full Moon..?!?

 

 

I actually ignored that aspect of it and just assumed the poster meant "entire moon".  But yes, full moon is not ideal, although actually, depending on the libration at the time, there is always some relief at either the poles or one limb.  In any event, this post appears as though it was originally posted on another CN forum and then moved here?  I assume the poster has no experience but would like to image the moon.  




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