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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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#26 caveman_astronomer

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Posted 04 March 2017 - 06:57 AM

I want to see detail when I observe and no practical refractor will do that better than a Newtonian. 

 

 



#27 Cpk133

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Posted 04 March 2017 - 07:25 AM

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  


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#28 caveman_astronomer

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Posted 04 March 2017 - 07:40 AM

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  

What kind of refractor should I buy that would compete with a 12-inch Newtonian?



#29 Cpk133

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Posted 04 March 2017 - 07:45 AM

 

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  

What kind of refractor should I buy that would compete with a 12-inch Newtonian?

 

 

You can't see anything through a Newtonian without refractive optics. lol.gif


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#30 precaud

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Posted 04 March 2017 - 08:04 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.gif

 

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

 

Less than $300, too.

 

- Jim

 

No question, that's a better compare. But even that doesn't include the shipping from Germany, or a mount. And qualitatively I would put my bets on the dob in that shootout.


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#31 Jon Isaacs

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Posted 04 March 2017 - 08:23 AM

 

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.

 

I think one only has to setup and RV-6 alongside a 3 inch F/16 achromat to see that even 50 years ago,  a 6 inch Newtonian was far more capable than a 3 inch refractor... 

 

Been there,  done that,  know the result,  don't need to do it again.. 

 

Jon


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#32 caveman_astronomer

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Posted 04 March 2017 - 08:35 AM

We can confidently say that a well-made 4-inch refractor can do better than a well-made 4-inch reflector, but the issue gets a little murkier when we start looking for a refractor that is a serious competitor for a well-made 12-inch Newtonian, for example, or even for a well-made 8-inch Newtonian.

 

 

 

 

 

 

 

 


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#33 Jon Isaacs

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Posted 04 March 2017 - 08:36 AM

 

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 

 

 

So there I am with my 120 mm F/7.5 Orion Eon with the FLP-53 doublet that cost me $1200 used and next to it is a 10 inch F/5 Dob that cost me $240 used. 

 

Splitting doubles,  the 10 inch does the number on the refractor,  viewing Mars,  the 10 inch does the number on the refractor. This should be no surprise.  This does require an operator who knows how to clean a mirror,  the collimate a scope,  to cool a scope.. And it does require decent seeing.. 

 

Inch for inch, there is nothing as potent as a small refractor..  Dollar for dollar,  pound for pound,  reflectors offer more planetary contrast,  will split tighter doubles.. 

 

Jon


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#34 caveman_astronomer

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Posted 04 March 2017 - 08:37 AM

 

 

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.

 

I think one only has to setup and RV-6 alongside a 3 inch F/16 achromat to see that even 50 years ago,  a 6 inch Newtonian was far more capable than a 3 inch refractor... 

 

Been there,  done that,  know the result,  don't need to do it again.. 

 

Jon

 

Refractors are great.  Too bad they are all so small in aperture.wink.gif


Edited by caveman_astronomer, 04 March 2017 - 08:38 AM.

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#35 russell23

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Posted 04 March 2017 - 08:44 AM

I want to see detail when I observe and no practical refractor will do that better than a Newtonian. 

This is true.  I'm by no means "anti-Newtonian".   But I also have no interest in owning a Newtonian again.  That is a road I've been down and I am quite happy with my 120mm ED.  As I said - I prefer the aesthetics of a refractor view.  For me it is not about hunting down the faintest objects or seeing the most details.   For me it is about an opportunity to have a relaxing couple hours in my busy life.  For that purpose my low maintenance 120ED sitting on a Vixen SP set into alt-azimuth mode meets my needs.  


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#36 caveman_astronomer

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Posted 04 March 2017 - 08:53 AM

 

I want to see detail when I observe and no practical refractor will do that better than a Newtonian. 

This is true.  I'm by no means "anti-Newtonian".   But I also have no interest in owning a Newtonian again.  That is a road I've been down and I am quite happy with my 120mm ED.  As I said - I prefer the aesthetics of a refractor view.  For me it is not about hunting down the faintest objects or seeing the most details.   For me it is about an opportunity to have a relaxing couple hours in my busy life.  For that purpose my low maintenance 120ED sitting on a Vixen SP set into alt-azimuth mode meets my needs.  

 

I keep a small refractor around.



#37 Jon Isaacs

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Posted 04 March 2017 - 08:55 AM

Refractors are great.  Too bad they are all so small in aperture.

 

I love my refractors..  I love my reflectors.. 

 

To directly answer the original question:

 

I think of high quality refractors as providing sharp views because within the limits of their aperture,  they are able to work closer to that limit more often than other designs. I also think of them as sharp because the only off-axis aberration they typically exhibit is field curvature so the views can be very clean. 

 

But that sharpness is not resolving power,  it's relative resolving power..  Relative to the aperture..  If you want to resolve M80 to the core,  that takes horsepower, cubic inches.. (Actually inches of diameter, square inches of area.) 

 

Jon


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#38 russell23

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Posted 04 March 2017 - 08:56 AM

Back in the mid-90's I had an experience one night that also changed my view on the benefits of aperture.  I had a 90mm f/11 Meade refractor and I had an 8" f/6 Orion dob.   Both scopes were set up one February night and I was going back and forth between scopes looking at different open clusters in the Canis Major and Monoceros.   At one point I had two different clusters in the two scopes - a brighter open cluster in the 90mm refractor and a fainter open cluster in the 8".  But in the eyepiece the two clusters looked remarkably identical.  But this was only because the larger aperture of the 8" made the fainter cluster look about the same as the brighter cluster looked in the smaller aperture refractor.  

 

So that got me to thinking.  I enjoy the refractor more.  Why was I spending time hunting for fainter objects in the 8", diverting from the eyepiece to the charts etc, to get the same view I already was getting with the smaller refractor looking at the brighter objects?   So aperture fever ended for me that night.   Now when I am out I have my refractor and a box of eyepieces.  No finderscope, no charts.  If there are new objects I am interested in checking out I look them up on the charts before I go out to verify where I should sweep for them.  If I come across an object for which I am uncertain as to its identity I go into the house and pull out my charts there to identify it.

 

I find that I now spend far more time in the eyepiece than I used to and am far more relaxed when I am done observing for the night.  


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#39 PXR-5

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Posted 04 March 2017 - 09:00 AM

Which is better to own? A screw driver or a cresant wrench? I keep going back and forth.

I Think I'm going to buy a hammer and just beat things to death....
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#40 russell23

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Posted 04 March 2017 - 09:02 AM

 

 

I want to see detail when I observe and no practical refractor will do that better than a Newtonian. 

This is true.  I'm by no means "anti-Newtonian".   But I also have no interest in owning a Newtonian again.  That is a road I've been down and I am quite happy with my 120mm ED.  As I said - I prefer the aesthetics of a refractor view.  For me it is not about hunting down the faintest objects or seeing the most details.   For me it is about an opportunity to have a relaxing couple hours in my busy life.  For that purpose my low maintenance 120ED sitting on a Vixen SP set into alt-azimuth mode meets my needs.  

 

I keep a small refractor around.

 

"Small refractor" ... ahh but you repeat yourself ... just "refractor".  :)


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#41 Cotts

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Posted 04 March 2017 - 09:12 AM

 

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  

What kind of refractor should I buy that would compete with a 12-inch Newtonian?

 

This 10" refractor should do the trick.  http://www.cloudynig...nch-tec-at-wsp/

 

$50 000 + $15 000 for the mount and $8 000 for the tripod.

 

Dave


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#42 treadmarks

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Posted 04 March 2017 - 09:14 AM

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.


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#43 russell23

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Posted 04 March 2017 - 09:19 AM

 

 

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  

What kind of refractor should I buy that would compete with a 12-inch Newtonian?

 

This 10" refractor should do the trick.  http://www.cloudynig...nch-tec-at-wsp/

 

$50 000 + $15 000 for the mount and $8 000 for the tripod.

 

Dave

 

Or you could pay your mortgage!   LOL



#44 russell23

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Posted 04 March 2017 - 09:24 AM

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.

That certainly could be part of it.  Another factor for me is the simplicity of the observing.  I am able to sit at the back end of the scope and sight along the tube to locate objects or stars for star hopping.  The viewing is always comfortable like that and sighting along the tube with your eye next to the eyepiece is not as easy with a newt.  

 

Like I said - I'm not ant-Newtonian.  I might even look to pick up a large dob when I retire.  But for now I'm very happy with what I have.   



#45 Jon Isaacs

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Posted 04 March 2017 - 09:33 AM

 

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.

That certainly could be part of it.  Another factor for me is the simplicity of the observing.  I am able to sit at the back end of the scope and sight along the tube to locate objects or stars for star hopping.  The viewing is always comfortable like that and sighting along the tube with your eye next to the eyepiece is not as easy with a newt.  

 

Like I said - I'm not ant-Newtonian.  I might even look to pick up a large dob when I retire.  But for now I'm very happy with what I have.  

I think a Newtonian is actually easier to point.  Imagine an object 75 degrees elevation.  With a refractor,  it is very awkward to position my head to look along the tube or through a red Dot or Telrad finder.  With a Newtonian,  the focuser and finders are at the sky end of the scope,  I just lean over,  glance through the Telrad,  point the scope, comfortable and effective. 

 

Jon


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#46 caveman_astronomer

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Posted 04 March 2017 - 09:40 AM

 

 

God, or natural selection, depending on your persuasion, seems to favor refractive optics for wide fields, low maintenance, and the sharpest views per mm of aperture.  

What kind of refractor should I buy that would compete with a 12-inch Newtonian?

 

This 10" refractor should do the trick.  http://www.cloudynig...nch-tec-at-wsp/

 

$50 000 + $15 000 for the mount and $8 000 for the tripod.

 

Dave

 

I can drag a 12-inch Newtonian out the back door and be observing in 15 seconds.


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#47 caveman_astronomer

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Posted 04 March 2017 - 09:47 AM

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.

I almost always find that if seeing is bad for a larger Newtonian, then it is as bad for the smaller refractor as well. 


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#48 russell23

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Posted 04 March 2017 - 09:56 AM

 

 

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.

That certainly could be part of it.  Another factor for me is the simplicity of the observing.  I am able to sit at the back end of the scope and sight along the tube to locate objects or stars for star hopping.  The viewing is always comfortable like that and sighting along the tube with your eye next to the eyepiece is not as easy with a newt.  

 

Like I said - I'm not ant-Newtonian.  I might even look to pick up a large dob when I retire.  But for now I'm very happy with what I have.  

I think a Newtonian is actually easier to point.  Imagine an object 75 degrees elevation.  With a refractor,  it is very awkward to position my head to look along the tube or through a red Dot or Telrad finder.  With a Newtonian,  the focuser and finders are at the sky end of the scope,  I just lean over,  glance through the Telrad,  point the scope, comfortable and effective. 

 

Jon

 

It depends on the set-up.  I have my SuperPolaris mount on a beefier set of extendable tripod legs.  If I want to I can extend them to a height that is extremely comfortable for near zenith viewing.

 

But what I have done is adjusted them to a height that works well for near horizon and for zenith observations.  Sometimes I rotate the diagonal near the horizon.  Other times I set a few pieces of 2x12 boards onto the bench I use to raise my height.  I also have a slightly lower bench that I can pull out if the zenith is a problem.  

 

The problem I always had with the Telrad was dewing up.   Great finder though - works really well.  



#49 precaud

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Posted 04 March 2017 - 10:01 AM

Refractors are great.  Too bad they are all so small in aperture.wink.gif

My sentiment exactly.


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#50 Jon Isaacs

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Posted 04 March 2017 - 10:03 AM

 

 

 

People often say refractor images are more "aesthetically pleasing" (sharper?) even if they don't show more detail. Aside from the quality issues mentioned, I'm thinking it's also because smaller telescopes are more resistant to bad seeing. My understanding of the theory is that larger telescopes can have better contrast through brute-force, by having more clear aperture. So it's not the contrast giving refractors more aesthetic images, it's their smallness and the fact that refractors take the most advantage of that smallness.

That certainly could be part of it.  Another factor for me is the simplicity of the observing.  I am able to sit at the back end of the scope and sight along the tube to locate objects or stars for star hopping.  The viewing is always comfortable like that and sighting along the tube with your eye next to the eyepiece is not as easy with a newt.  

 

Like I said - I'm not ant-Newtonian.  I might even look to pick up a large dob when I retire.  But for now I'm very happy with what I have.  

I think a Newtonian is actually easier to point.  Imagine an object 75 degrees elevation.  With a refractor,  it is very awkward to position my head to look along the tube or through a red Dot or Telrad finder.  With a Newtonian,  the focuser and finders are at the sky end of the scope,  I just lean over,  glance through the Telrad,  point the scope, comfortable and effective. 

 

Jon

 

It depends on the set-up.  I have my SuperPolaris mount on a beefier set of extendable tripod legs.  If I want to I can extend them to a height that is extremely comfortable for near zenith viewing.

 

But what I have done is adjusted them to a height that works well for near horizon and for zenith observations.  Sometimes I rotate the diagonal near the horizon.  Other times I set a few pieces of 2x12 boards onto the bench I use to raise my height.  I also have a slightly lower bench that I can pull out if the zenith is a problem.  

 

The problem I always had with the Telrad was dewing up.   Great finder though - works really well.  

 

 

I can make the refractor work but I find that a Dob is just easier to point,  easier to look through the finder.. I like the viewing position of an alt-az mounted refractor but using a finder for pointing can be very awkward.. 

 

Jon




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