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Why are refractors considered to be sharper than reflectors if resolution is a function of the aperture?

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#1 That Dalek

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Posted 02 March 2017 - 07:35 PM

Just a question that came to me. Thanks for any answers!


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#2 BruceJ

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Posted 02 March 2017 - 07:47 PM

Are you assuming that both have the same size primary lens?

 

Any reflector, whether, Newtonian, dob, SCT, etc. loses some light because of the secondary. Larger reflectors generally provide "better views" (however you define that...sharper, more details, brighter, etc.) than smaller refractors. However, when the aperture is the same, some design and operational characteristics (e.g., secondary size, FL and/or coma, mirror parameters, coatings, collimation) can give the nod to refractors (assuming good quality scopes, like APOs).

 

I assume this is a hotly debated topic, as evidenced by comments and discussions. I've seen comments that support both a 5" APO refractor being equivalent to or surpassing an 8" reflector, as well as evidence to the contrary supporting a more balanced aperture equivalency. There is also the quality of the view.

 

There is something about the crispness of the view of even a small APO refractor that sets it apart from a reflector, be it pinpoint stars, darker background and more contrasty views...idk...especially with straight-though views, it somehow looks and feels different.


Edited by BruceJ, 02 March 2017 - 08:36 PM.

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#3 dugpatrick

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Posted 02 March 2017 - 07:53 PM

All good points.  But, yes, resolution is better with larger aperture.  An 8" newt will have better resolution than a 4" APO. And better CA.

 

Doug


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#4 bobzeq25

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Posted 02 March 2017 - 08:01 PM

It's not so much about resolution as it is about contrast (no central obstruction), and fewer aberrations.   That gives refractors a reputation for high contrast planetary views, and sharp stars.  Reflectors, at a given price point, have a significant edge in light gathering for dim DSOs.


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#5 BruceJ

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Posted 02 March 2017 - 08:03 PM

And better CA.

 

Doug

Possibly...though I am assuming a triplet APO. And depending upon FL we might want to add coma into the discussion as well...though easily corrected with Paracorr and/or expensive eyepieces for short FL reflectors (though short FL refractors can suffer from this too).

 

Bottom line, I think they both have their pros and cons (e.g., a large refractor is not a very portable solution). I own both and will probably continue to do so. Some nights I prefer one to the other, but all scopes see time...though not as much as I'd like...lol.


Edited by BruceJ, 02 March 2017 - 08:06 PM.


#6 otocycle

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Posted 02 March 2017 - 09:11 PM

It's not so much about resolution as it is about contrast (no central obstruction), and fewer aberrations.   That gives refractors a reputation for high contrast planetary views, and sharp stars.  Reflectors, at a given price point, have a significant edge in light gathering for dim DSOs.

 

Agreed...as this has been my experience with even the best low obstruction Zambuto optics (<20%) up to 12.5" aperture.   Light scatter contributes to loss of contrast and sharpness.   This is why even straight on viewing with a refractor vs. star diagonal is considered better by some.   Scatter matters....


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#7 Neptune

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Posted 02 March 2017 - 10:00 PM

I have owned SCT's for many years then I got a view through a 140mm APO. The moons of Jupiter were solid disks that could easily be seen. Never really saw that in any SCT I had owned previously. I have been hooked ever since. Faint fuzzies do suffer, but that is the price you pay. Unless you have a really FAT wallet to buy a big APO.


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#8 treadmarks

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Posted 02 March 2017 - 10:44 PM

There are a number of factors working in the refractor's favor. Refractors usually don't have any issues with collimation. While larger aperture does have its obvious advantages, smaller aperture does have ways of getting its revenge. Smaller telescopes are less vulnerable to seeing error, and they cool down quicker. So on a bad night, the small telescope could embarrass the big one. Lastly, refractors, especially expensive apochromats, tend to be better-made.

 

I think all of this means refractors will be the more "consistent" performers. With other, larger telescopes, the user needs to do more to ensure it performs up to its potential, and there will be fewer nights when that is possible. Small refractors will perform to their potential more often, but their potential is lower than that of the large telescope.


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#9 Mitrovarr

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Posted 03 March 2017 - 01:42 AM

A number of factors are working against reflectors:

 

1. Reflectors have central obstructions, which reduce the resolution.There's also a bit of loss to the spider, which creates diffraction spikes.

2. Reflectors tend to have problems with temperature differentials within the tube, which creates air currents that distort the image.

3. Mirrors have more scatter than lenses.

4. Reflectors have a harder time staying in alignment than refractors.

5. Reflectors have coma. Refractors have their own problems (chromatic aberration and spherical aberration) but expensive glasses and lens designs can basically eliminate these.

6. Refractors are usually higher end than reflectors (so, they tend to be higher quality).

 

However, you can usually resolve these:

 

1, 3. Reflectors scale up far better than refractors, so they can have more aperture, which helps compensate for these problems. Obstruction sizes can be minimized, curved spiders will spread the diffraction spikes around and make them less apparent.

2. Intelligent fan usage can do a lot for air current formation. Good telescope design can keep cool-down times reasonable and mostly eliminate this issue in use.

4. It's pretty easy to get good at reflector collimation. Just keep it collimated.

5. Coma can be mostly eliminated through use of a paracorr. Or, you can use a longer focal ratio.

6. There are premium mirror-makers who produce mirrors up to the quality of the best lenses.

 

If you resolve these issues, reflectors still do not perform up to the standard of a refractor of the same aperture - but will perform as well as a refractor that is slightly smaller. However, you can get a reflector that is far larger than any refractor you can get. It's reasonably feasible to get a 12-16" dobsonian with premium optics and good thermal management, and that will (under good conditions) walk all over any refractor anyone with a normal income will ever be able to afford.


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#10 Whichwayisnorth

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Posted 03 March 2017 - 01:49 AM

Refractors typically do not suffer from thermals, are typically in excellent collimation, are baffled better, and don't have a center obstruction.

 

The number of reflectors that are miscollimated is astronomical.  So overall I think you have a better chance of having a excellent experience with a large APO refractor.  BUT, find a 10" or bigger 1/6th wave or better, perfectly collimated reflector and it will knock your socks off.


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#11 tag1260

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Posted 03 March 2017 - 05:43 AM

Plus diffraction spikes in the reflector that you don't have in the refractor.



#12 caveman_astronomer

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Posted 03 March 2017 - 06:46 AM

Just a question that came to me. Thanks for any answers!

Refractors often have better definition, which is the ability to show fine, low-contrast detail.  A reflector solves that problem by being larger, gathering more light and having higher resolution.

 

A old rule of thumb is that a 6-inch Newtonian, properly designed and built, will beat a 4-inch refractor.


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#13 Jon Isaacs

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Posted 03 March 2017 - 10:09 AM

I will simply say that what we perceive as "sharpness" is not resolution..  A few comments, experiences, thoughts.

 

- If I look at 52 Orionis, a 1 arcsecond double star in my 120 mm Orion Eon.  It is very close to the Dawes limit so on a perfect night, the disks are overlapping and its difficult split at best. If I point my 10 inch F/5 Dob at 52 Orionis on that same night, and the scope is cooled and of course collimated, 52 Orionis is split wide open. Much smaller disks widely separated.

 

In this case, I see 52 Orionis as much sharper in the 10 inch.. But most often, I think the comparisons of both contrast and resolution are made in relative terms, at a 0.5 mm what do I see? 

 

- Looking at the Globular M79 in Lepus is a 6 inch refractor versus my 22 inch Dob, few would perceive that the refractor was sharper.. M79 in the 22 inch looks about like M13 in a 10 inch. M79 in a 6 inch looks, well we know what it looks like..

 

- Reflectors are fininky to the uniniated.. They require care and attention..  Collimation and thermal management are important..   

 

It always seems there comparisons are made between some sort of ideal refractor and the average faster Newt. An 120 mm F/5 achromats versus a 130 F/5 Newtonian.. I think most would rind the Newtonian sharper...

 

Jon


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#14 mogur

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Posted 03 March 2017 - 08:19 PM

All good points.  But, yes, resolution is better with larger aperture.  An 8" newt will have better resolution than a 4" APO. And better CA.

 

Doug

Only if it's PERFECTLY collimated! (a rare find) And I'll take a little CA over loss of contrast because of a spider vane and secondary obstruction.



#15 MalVeauX

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Posted 03 March 2017 - 08:49 PM

Just a question that came to me. Thanks for any answers!

Sharpness is a function of contrast.

 

Very best,


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#16 precaud

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Posted 03 March 2017 - 09:24 PM

It always seems there comparisons are made between some sort of ideal refractor and the average faster Newt. An 120 mm F/5 achromats versus a 130 F/5 Newtonian.. I think most would rind the Newtonian sharper...

I agree. It's typically 120mm APO's vs your basic 6 to 8" dob. The 3X price difference is put aside.


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#17 grif 678

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Posted 03 March 2017 - 09:43 PM

In all my old books, way back before APO's and SCT's. the rule of thumb seemed to be, in all instances, that a 3 inch refractor was about equal to a 6 inch reflector. I often wondered why, since a 6 inch mirror had so much more area than a 3 inch lens, but I guess the focal length and secondary obstruction had something to do with it.


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#18 Mitrovarr

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Posted 03 March 2017 - 10:26 PM

In all my old books, way back before APO's and SCT's. the rule of thumb seemed to be, in all instances, that a 3 inch refractor was about equal to a 6 inch reflector. I often wondered why, since a 6 inch mirror had so much more area than a 3 inch lens, but I guess the focal length and secondary obstruction had something to do with it.


I wonder if that figure was due to worse coatings back in the day. I really wouldn't expect a modern 3" refractor (any kind) to beat a 6" of equivalent quality. Even back in the day, I'm not sure. I have a really good long 3" achromat and a good 6" homemade (not by me) dob, both are at least 30 years old, and the dob totally destroys the refractor on planetary detail.

#19 FirstSight

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Posted 03 March 2017 - 10:36 PM

 

All good points.  But, yes, resolution is better with larger aperture.  An 8" newt will have better resolution than a 4" APO. And better CA.

 

Doug

Only if it's PERFECTLY collimated! (a rare find) And I'll take a little CA over loss of contrast because of a spider vane and secondary obstruction.

 

Perfect collimation of reflectors is not hard to obtain, with the right tools (Glatter laser + TuBlug or Catseye cheshire + autocollimator).   But not every reflector owner is so demanding of collimation, nor willing to spend for the top-level tools that reliably produce perfect collimation.  OTOH, others of us are a bit happily OCD about collimating our reflectors.


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#20 GJJim

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Posted 03 March 2017 - 10:41 PM

Apo refractors exist in a sweet spot where their unobstructed aperture and single-pass light path tends to produce better images than similar aperture reflectors in the same seeing conditions. Most amateurs view with seeing conditions that put anything larger than about ten inches at a disadvantage because the scope resolution is limited by the seeing, not the aperture. With steady seeing and constant temperatures (e.g. Florida) reflectors can do just as well as apo refractors for visual use.


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#21 That Dalek

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Posted 03 March 2017 - 10:52 PM

Thanks for all the responses! It seems like I was wrong in thinking that sharpness is based on resolution, with it being based on contrast instead. Thanks for the help!

The thing I'm wondering now though is why refractors are so much more expensive than reflectors for similar performance. I mean, in this thread, most people are comparing 6 inch newts with 4 or 5 in. refractors. A 6 in dob goes for around 250-300$ while a 120 mm refractor seems to start at around $500 and can increase up to well over $1000 based on the particular model. I understand that a refractor can perform better in some situations and has some benefits, but that is a huge price difference.

Edit: changed "6 or 8 in. refractors" to "4 or 5 in. refractors."

Edited by That Dalek, 04 March 2017 - 07:02 PM.


#22 jrbarnett

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Posted 03 March 2017 - 11:58 PM

Just a question that came to me. Thanks for any answers!

Obstructions, rough, dirty surfaces, low-ish reflectivity, etc., all conspire to render obstructed designs less efficient and lower contrast per inch of aperture.

 

Refractors put most photons collected through the system and in the correct location in the final image.  Reflectors divert a lot of light collected, and place more of the light that makes it to the final image in the wrong place, reducing contrast.

 

Other factors include larger aperture being more affected by poor seeing than small, and larger optics taking longer acclimate.

 

All of these factors contribute to the refractor reputation for sharper visual performance.

 

- Jim 


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#23 jrbarnett

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Posted 04 March 2017 - 12:23 AM

 

It always seems there comparisons are made between some sort of ideal refractor and the average faster Newt. An 120 mm F/5 achromats versus a 130 F/5 Newtonian.. I think most would rind the Newtonian sharper...

I agree. It's typically 120mm APO's vs your basic 6 to 8" dob. The 3X price difference is put aside.

 

So go for a nice, affordable f/9.4 127mm achromat and skip the 130mm and 150mm obstructed, spidered light buckets.  :grin:

 

http://www.teleskop-...e-Assembly.html

 

Less than $300, too.

 

- Jim



#24 Tony Flanders

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Posted 04 March 2017 - 05:19 AM

 

All good points.  But, yes, resolution is better with larger aperture.  An 8" newt will have better resolution than a 4" APO. And better CA.

Only if it's PERFECTLY collimated! (a rare find) And I'll take a little CA over loss of contrast because of a spider vane and secondary obstruction.

 


The difference in inherent resolution between an 8-inch scope and a 4-inch scope is so vast that the Newt would have to have disastrously poor optics or be really badly collimated to flunk this particular test.

Operating at the magnifications useable in a 4-inch APO, the loss of contrast due to the 8-inch Newt's central obstruction is barely detectable.


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#25 russell23

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Posted 04 March 2017 - 06:05 AM

For me the refractor is low maintenance and the view is more aesthetically appealing.  


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