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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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#51 csa/montana

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Posted 04 March 2017 - 10:15 AM

"The problem I always had with the Telrad was dewing up."

 

I have a Telrad dew shield on mine, with no problems.  They also sell dew heaters for the Telrad.  I wouldn't be without my Telrad!  I also find pointing my 16" dob is easier than either of my 2 refractors.

 

I


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#52 russell23

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Posted 04 March 2017 - 10:17 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

 

I don't even use a finderscope with my refractor.    The first thing I did when I bought the 120ED was sell the finderscope.     My widest TFOV eyepiece serves as my finderscope.  Sometimes that is the 40mm Pentax XL (2.8 deg TFOV).  Sometimes that is the 32mm plossl, 32mm Brandon or 28mm Pentax XL (1.6 deg TFOV).  Or if I'm feeling really interested in a challenge I might even use the 12mm XF or 9mm Morpheus (0.77 deg TFOV) and go sweeping for the target.    I sight along the tube to locate stars to starhop from or a lot of times I just point the OTA right to the location of the target.   I find it remarkably efficient.  

 

I started observing this way back in the 1980's with a 4" Unitron refractor.  I had a 48mm Brandon, but I hated trying to look into the finderscope.  I found it easier to just align the tube toward the target and then sweep with the 48mm Brandon (1.5 deg TFOV).  I got so good at it most of the the time the object was in the field of view just from sighting along the tube.  

 

I remember one night Don Yeier (who I bought the scope from) stopped by to check it out.  I started showing him a bunch of deep sky objects just sighting along the tube.  About the 5th object I found that way he said "That's remarkable.  You just keep nailing these objects without the finderscope?"   

 

I think most people could learn to observe this way.  But in this age of GOTO technology and with the red dot finders I suppose it is unlikely that most people would see the point.  For me it is just what I have always done ... well since I got my first refractor.


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#53 russell23

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Posted 04 March 2017 - 10:24 AM

"The problem I always had with the Telrad was dewing up."

 

I have a Telrad dew shield on mine, with no problems.  They also sell dew heaters for the Telrad.  I wouldn't be without my Telrad!  I also find pointing my 16" dob is easier than either of my 2 refractors.

 

I

Back in the 80's I'm not sure if they had dew heaters for the Telrad.   If I had a dob I would want a Telrad - great little invention.  

 

I will say that pointing a refractor like I do is aided by having it elevated high enough with your mount and if it is a longer tube that also makes it easier.  

 

But even an 80mm f/7.5 APO was very easy to point and target.   People often ask what finder they should get for scopes like that and my first thought is to give your low power eyepiece a try.  Even a 32mm plossl will give at least a 2.5 deg TFOV.  A 32mm 70 deg eyepiece will give 3.7 deg TFOV.    To me a 2.5 deg FOV is gulping up sky for finding objects.  


Edited by russell23, 04 March 2017 - 10:25 AM.


#54 treadmarks

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Posted 04 March 2017 - 10:59 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.   

 

Well I'm not a big fan of Newtonians but I don't find the Dobsonian mount difficult to use. In fact I think it's pretty good. The appeal of small telescopes, for me, is the grab-n-go factor. There's less worrying about the "3 C's", you can take them out in one trip, you don't knock into every door on the way out, etc.

 

Personally, I find the whole "aperture fever" thing a bit greedy when you consider the limiting factor of the atmosphere. I would like my purchases to live up to their potential. It's kind of like buying a Porsche 911 and being stuck in traffic all the time - frustrating. I bought an 8" SCT a couple weeks ago and the seeing here has been nothing but bad/poor per CDS so far (if it's not cloudy).



#55 Jon Isaacs

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Posted 04 March 2017 - 11:09 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.   

 

Well I'm not a big fan of Newtonians but I don't find the Dobsonian mount difficult to use. In fact I think it's pretty good. The appeal of small telescopes, for me, is the grab-n-go factor. There's less worrying about the "3 C's", you can take them out in one trip, you don't knock into every door on the way out, etc.

 

Personally, I find the whole "aperture fever" thing a bit greedy when you consider the limiting factor of the atmosphere. I would like my purchases to live up to their potential. It's kind of like buying a Porsche 911 and being stuck in traffic all the time - frustrating. I bought an 8" SCT a couple weeks ago and the seeing here has been nothing but bad/poor per CDS so far (if it's not cloudy).

 

 

The resolution of a large aperture scope is not always and advantage but for DSOs,  the combination of light gathering and resolution nearly always is,  as long as the object fits in the field of view..   If one enjoys hunting down small objects,  faint small objects,  in seeing details in such objects,  larger apertures are an advantage..  

 

Jon


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#56 Jon Isaacs

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Posted 04 March 2017 - 11:18 AM

I don't even use a finderscope with my refractor.    The first thing I did when I bought the 120ED was sell the finderscope.     My widest TFOV eyepiece serves as my finderscope.  Sometimes that is the 40mm Pentax XL (2.8 deg TFOV).  Sometimes that is the 32mm plossl, 32mm Brandon or 28mm Pentax XL (1.6 deg TFOV).  Or if I'm feeling really interested in a challenge I might even use the 12mm XF or 9mm Morpheus (0.77 deg TFOV) and go sweeping for the target.    I sight along the tube to locate stars to starhop from or a lot of times I just point the OTA right to the location of the target.   I find it remarkably efficient.

 

Like I said,  I can make it work..  You talk about spending more time observing the object,  working a list of double stars at 60 degree elevation with a 50 mm RACI finder is much more efficient than awkwardly sighting along the tube,  and then using a wide field eyepiece to locate the object.. 

 

With my short focal length refractors,  I generally just shoot from the hip..  But there is no doubt,  the Dob  with the Telrad and RACI finder is much better for easily finding more challenging objects. 

 

Jon



#57 Richard Whalen

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Posted 04 March 2017 - 12:14 PM

I find what you should own depends on what you like to observe and where you live and how you observe. If your into observing faint dso's and globular's from a decent location it's hard to beat a well made Newtonian dollar for dollar. 

 

If you've been there, done that, and prefer open clusters, Planets, brighter DSO objects or the moon in high contrast the refractor can be the best choice. 

 

For refractor like views with more aperture its hard to beat a well made MCT in the 8" to 12" range.

 

After more than 50 years observing, I find the aesthetics of the view more important than the brightness. Also part of the experience for me is also sitting out under the stars on a perfect night and seeing the silhouette of that long white tube against the background of a sky full of stars. Somehow it's how it should be, and all is right in my world.

 

 

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#58 Edd Weninger

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Posted 04 March 2017 - 12:38 PM

When I've got my Traveler out on a Gibraltar and tripod, I simply put my green laser dot on the target.  Don't squint through anything.  ;-)  Otherwise, I just press a few buttons.

 

 

 

.  

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

 



#59 FirstSight

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Posted 04 March 2017 - 12:56 PM

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


Consider that this same principle would apply between two refractors of different apertures, but otherwise of equal optical quality.  The refractor with the larger aperture will be capable of resolving finer-grained detail in objects that aren't discretely separable in the refractor with smaller aperture - but with the unavoidable corollary that this finer-grained detail can include atmospheric turbulence (and its visible blurring of detail on objects) which are unresolvable in the refractor with smaller aperture.

 

Another way to put it is to consider why the more beer you drink, the better-looking the women at the bar become to your eyes. As your judgment aperture declines, so does your ability to accurately discern imperfections in the details, and your seeing bothers you less. grin.gif


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#60 Zapp Brannigan

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Posted 04 March 2017 - 12:58 PM

I love the convenience of CD's but you just don't get that same sound from them as you do from vinyl records.  Oh, sorry, wrong never ending debate.  :D


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#61 russell23

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Posted 04 March 2017 - 12:59 PM

 

I don't even use a finderscope with my refractor.    The first thing I did when I bought the 120ED was sell the finderscope.     My widest TFOV eyepiece serves as my finderscope.  Sometimes that is the 40mm Pentax XL (2.8 deg TFOV).  Sometimes that is the 32mm plossl, 32mm Brandon or 28mm Pentax XL (1.6 deg TFOV).  Or if I'm feeling really interested in a challenge I might even use the 12mm XF or 9mm Morpheus (0.77 deg TFOV) and go sweeping for the target.    I sight along the tube to locate stars to starhop from or a lot of times I just point the OTA right to the location of the target.   I find it remarkably efficient.

 

Like I said,  I can make it work..  You talk about spending more time observing the object,  working a list of double stars at 60 degree elevation with a 50 mm RACI finder is much more efficient than awkwardly sighting along the tube,  and then using a wide field eyepiece to locate the object.. 

 

With my short focal length refractors,  I generally just shoot from the hip..  But there is no doubt,  the Dob  with the Telrad and RACI finder is much better for easily finding more challenging objects. 

 

Jon

 

Well with the dob you are also working with narrower TFOV.   Sighting along  the tube is not something I would do with a dob.  Like I said - with a dob I would want a Telrad.  

 

With a refractor, sighting along the tube isn't all that awkward once you've done it enough.  But again I have a set of tripod legs that allow me to have the OTA elevated enough that the zenith is comfortable.    I've generally found that with 1.5 deg TFOV or larger I'm good to quickly locate targets I'm familiar with.  If it is a new target it might take a little longer but that is part of the fun at that point.   I've been doing it this way for ~30 years so it is natural. 



#62 EJN

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Posted 04 March 2017 - 01:11 PM

The answer is simple, refractors have superior fahrvergnugen.



#63 russell23

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Posted 04 March 2017 - 01:25 PM

Here Are pictures of my setup.  Inside because it is freezing and windy out today.  The first picture is vertical next to the sofa which is at most 3" lower than my bench I sit on.  I can raise the legs another 16" if I want.  The second is low to the horizon and I just rotate the diagonal and use some squares of 2x12 boards to adjust height as needed.

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#64 nashvillebill

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Posted 04 March 2017 - 01:27 PM

I have indeed sighted along the tube of a refractor and found my object with low-power eyepieces, but to me it's a lot more convenient to just use a red dot finder, especially if the scope is near the zenith.  Also, if I have a medium or higher power eyepiece already in the scope, then changing back and forth to a lower-pwer eyepiece to locatr an object by sighting along the tube is cumbersome, whereas with the RDF the object is likely to be in the FOV of a medium-power eyepiece.

 

I prefer my ancient Telrad to a regular RDF becaue the Telrad has a circle that does not obscure a faint object like the red dot.  On the flip side, though, my ancient Telrad is far too large to mount on a 70 or 80 mm refractor tube (works great on the 10" Dob though).


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#65 Rollo

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Posted 04 March 2017 - 02:43 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....

 

You are right,  light scatter reduces contrast and sharpness.  Also, slow scopes tend to have less light scatter than fast scopes.  Focal ratio is a factor also.



#66 bobzeq25

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Posted 04 March 2017 - 03:07 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 6 or 8 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.

Making something like a triplet refractor with 6 high precision optical surfaces is always going to be far more expensive.  Not to mention that the glass in the middle of the refractor lens elements must be absolutely flawless.  Compared to that, making a one element mirror that can have internal flaws is a piece of cake.   Even an excellent one.

 

Reflectors will always offer more bang for the buck.  There's a place, and a market, for both.


Edited by bobzeq25, 04 March 2017 - 03:08 PM.

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#67 Redbetter

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Posted 05 March 2017 - 06:04 AM

The answer is simple, refractors have superior fahrvergnugen.

Ok, but what does cheating on emissions have to do with the view through the eyepiece?  lol.gif


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#68 Redbetter

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Posted 05 March 2017 - 06:24 AM

"The problem I always had with the Telrad was dewing up."

 

I have a Telrad dew shield on mine, with no problems.  They also sell dew heaters for the Telrad.  I wouldn't be without my Telrad!  I also find pointing my 16" dob is easier than either of my 2 refractors.

 

I

 

Depends on the local climate and time of year.  I've always had dew shields on my Telrads and while this worked pretty well in most places through out the year, it doesn't work particularly well here this time of the year.  Sometimes it will dew even with the cover over it in between targets.  I've recently gotten accustomed to looking through the dew/frost as best I can and dead reckoning more with the Telrad using both eyes.  I use the RACI finder to compensate for the loss of accuracy of the dewed/frosted over Telrad...and have to keep the finder objective and eyepiece covered most of the time to avoid the same fate...plus the finder eyepiece sometimes has to be swapped out to warm and defog as do all of the eyepieces in the main scope.

 

The little RDF's also have issues with their windows dewing...of course for a few dollars I can swap out with a spare while the other dries in a warming box.  Actually picked up another one recently to make this possible.  And since the finder on the big Dob is an ST80 I have the option of using this same RDF in the finder shoe when the Telrad is hard to see through.  Redundancy can be useful.

 

I don't find initial pointing much easier or more difficult with either refractors, an SCT, or Dobs although the process and orientations are different.  There are situations where any of them are at a disadvantage.  Some of this relates more to equatorial or alt-az orientations and specific viewing positions.



#69 Tony Flanders

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Posted 05 March 2017 - 08:43 AM

Planets, brighter DSO objects or the moon in high contrast the refractor can be the best choice. 

...
 
After more than 50 years observing, I find the aesthetics of the view more important than the brightness. Also part of the experience for me is also sitting out under the stars on a perfect night and seeing the silhouette of that long white tube against the background of a sky full of stars. Somehow it's how it should be, and all is right in my world.

I know what you mean; there's something about those grand old 6-inch achromats on their massive German equatorial mounts that sends a chill down the spine. The views are incredibly clean, and the scopes are big enough to yield some very detailed views of the planets -- but just barely big enough.

 

The fact remains that a 12-inch Dob is far cheaper and more portable than a long-focus 6-inch achromat. And while its aesthetics may be lacking, on a good night it can deliver far more planetary detail than said achromat.


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#70 rowdy388

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Posted 05 March 2017 - 10:16 AM

 

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


Consider that this same principle would apply between two refractors of different apertures, but otherwise of equal optical quality.  The refractor with the larger aperture will be capable of resolving finer-grained detail in objects that aren't discretely separable in the refractor with smaller aperture - but the unavoidable corollary that this finer-grained detail can include atmospheric turbulence (and its visible blurring of detail on objects) which are unresolvable in the refractor with smaller aperture.

 

Another way to put it is to consider why the more beer you drink, the better-looking the women at the bar become to your eyes. As your judgment aperture declines, so does your ability to accurately discern imperfections in the details, and your seeing bothers you less. grin.gif

 

My wife just asked me what I was laughing at and I told her " Oh nothing , just a funny comment on my silly astronomy forum."



#71 James Stanski

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Posted 17 March 2017 - 09:58 AM

Without getting into any particular design, I can think of a few reasons why aperture won't always deliver on sharpness or contrast.

 

1.  Temperature differentials in a larger piece of glass.

2.  Temperature differential in the surrounding air due to larger glass or external heat source.

3.  Poor collimation, or a scope that won't stay collimated

4.  Poor optical figure

5.  Rough optical surface

6.  Poor seeing

7.  Stray light due to design and surrounding lights.

 

Some designs will be less susceptible than others.



#72 mlbex

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Posted 17 March 2017 - 12:34 PM

When is the last time a major observatory built a refractor? As far as I know, the largest refractor still in use is the 36-incher on Mt Hamilton, built in the 1880s (according to Wikipedia)! It's still a fine telescope, but there's a reason observatories are building reflectors. Perhaps they scale better. That wouldn't really be a problem with everyday astronomers. 

 

An aside... I got to see it once :=) but not look through it :-( .It is truly a sight to behold... well worth the drive up the mountain.

 

When I win the lotto, I'm going to buy one of those triplets to go with my CasMak.



#73 Tony Flanders

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Posted 17 March 2017 - 02:05 PM

When is the last time a major observatory built a refractor? As far as I know, the largest refractor still in use is the 36-incher on Mt Hamilton, built in the 1880s (according to Wikipedia)! It's still a fine telescope, but there's a reason observatories are building reflectors. Perhaps they scale better. That wouldn't really be a problem with everyday astronomers.

Yes, reflectors scale vastly better, for several different reasons. To be precise: false color scales linearly with aperture, large lenses are hard to support, and the glass for a lens has to be perfect throughout its thickness rather than just at the surface. And this is indeed an issue for everyday backyard astronomers.

 

Refractors pretty much rule supreme in apertures smaller than 90 mm. There are some pretty good 76-mm Newtonians on the market, but they're only marginally cheaper than equivalent reflectors, and they have a number of disadvantages. So they appeal mainly to people who are really hard-up for money. There are also a handful of Mak-Cas scopes in apertures of 60 or 70 mm, but since the main benefit of that design is small physical size, and 60- or 70-mm refractors are already quite small, the tiny Mak-Cas's aren't very popular.

 

Refractors are also quite competitive in apertures from 90 to 125 mm. But toward the top of that range, the disadvantages of the design are beginning to kick in big-time. At 125 mm, either you end up with a short-focus achromat with tons of false color, or a long-focus achromat that's really unwieldy and hard to mount, or an apochromat that costs a minor fortune.

 

At 150 mm, refractors are really a stretch. Very few people can afford apochromats in this size, and with achromats you typically end up with both lots of false color and an unwieldy size. There are nonetheless some people who love 150-mm achromats because of their low light scatter, but that's truly the end of the line. Refractors bigger than 150 mm (6 inches) are rare indeed in the amateur world.

 

With reflective designs, by contrast, you're just getting started at 150 mm. That's considered quite small for a Newtonian, and not quite there for an SCT. Eight-inch Newts are really cheap and effective, especially on Dobsonian mounts, and eight inches is the standard size for SCTs.

 

In the modern world of amateur astronomy, where deep-sky objects are the most popular targets, even 8 inches isn't much. That's barely enough to resolve most globular clusters or see the spiral arms of the biggest and brightest galaxies. So while refractors certainly have their place for viewing wide fields, for viewing the planets in less-than-perfect seeing, and above all for photography, the fact that they scale up poorly definitely limits their popularity among amateur astronomers.


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#74 Napersky

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Posted 17 March 2017 - 02:43 PM

I asked a similar question to George Hunter one of the founders of Zygo and the inventor of the first commercial interferometer.

His answer, aberrations.



#75 caveman_astronomer

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Posted 17 March 2017 - 07:48 PM

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

Depends on what you mean by "sharp."  Refractors can produce sharp, low-resolution images.




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