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How hard is it to make a reflector as good as mid-price ED or Apo refractor?

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#26 Starman1

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Posted 15 April 2018 - 09:48 AM

Well, you have to compare apples with apples.

It isn't fair to compare a $500 10" newtonian with a $10K 127mm triplet apo.

Maybe a comparison should be made between the $500 dob and the 70mm Walmart refractor for fairness.

And you aren't going to find anyone with a 10" triplet apo.

The problem usually is that you aren't going to find equivalent sizes because few people have triplets larger than 5", or newtonians smaller than

8".  So the larger scope will have a thermal disadvantage right out of the gate.

 

The thermal problems can be overcome, but I viewed with someone the other night with a 14" scope with no fans.

Needless to say, his star images showed a lot of heat-related issues.

 

Then there's collimation, and that is a hard one to overcome--the refractor may need it once in a blue moon, while the reflector needs it every time the scope is set up.

Without that, the star image quality is compromised.

 

Then there is the nature of the atmosphere, which affects larger scopes more than smaller scopes.  What is good seeing for a 4" is not the

same as good seeing for a 14".  Larger scopes need steadier air.  The 12" Zeiss refractor here in LA at the Griffith Planetarium usually has really

poor image quality because of the air.  In perfect conditions, it's magnificent.  That doesn't occur often.

 

And a newtonian will typically need a coma corrector to even get its star images in the ballpark of the refractor.

 

Then there is optimizing the scope for contrast--something almost always done in the refractor with dewshield, light baffles and star diagonal.

Reflectors can be equally well-designed for contrast, but seldom are.

 

When everything comes together for either type of scope, the results can be amazing.  But the reflector, being larger, will always have the advantage of resolution.

It is possible to have a larger reflector that yields refractor-like images.  It's just that most people don't want to pay for that.


Edited by Starman1, 15 April 2018 - 09:52 AM.

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#27 starcanoe

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Posted 15 April 2018 - 11:24 AM

I think it is kind of funny that people will make a point of how something like a 10 inch F8 reflector is some kind of unworkable and awkward beast that can be equaled by something like an 8 inch F8  refractor (though it will probably be more like f 10 to be really good)...yet the awkwardness of the refractor will be unmentioned.

 

Not to mention the cost. A determined ATM with a bit of skill can make a world class 10 F8  newt/dob scope. Or for that matter they can buy most of the components and do the easier stuff themselves for a still reasonable cost. There is nothing reasonable about the cost of an 8 inch apo (and we won't even consider the mount) and the number of ATM's who have made large APOs is probably way less than the number of people who have been to space...heck it might be less than the number of people that have been to the moon.
 


Edited by starcanoe, 15 April 2018 - 11:26 AM.

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#28 tommm

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Posted 15 April 2018 - 11:44 AM

 

 

It’s become a sort of quest for Holy Grail for me to find this balace of quality of aperture within the bounds of normal affordability - ideally I’d like an excellent quality reflector (both optically & mechanically) in the  10-12” range for the price of of 130mm Apo with the ssme image quality. Does it exist?

I think most of the considerations have been covered in this thread and should show there is not an unqualified answer to this question (as with many questions in ATM).

 

Yes, central obstruction in the Newt will degrade contrast somewhat. But as some here, and Harold Suiter points out in his book "Star Testing Astronomical Telescopes", this is easily overcome with an increase in aperture of the Newt over the refractor.  For example, Suiter says: "Hence if figure 15.1 depicts the response of a 400mm (16 inch) reflector [25% central obstruction], we can see that it delivers contrast about as well as a perfect [italics his] unobstructed aperture of one-third to half its size. That means it's behaving about as well as the finest 6-inch unobstructed telescope." He adds the footnote: "Differences caused by the atmospheric turbulence scale, changes in eyepiece performance at lower focal ratios, and brighter images [with larger aperature] mean that performance won't be precisely duplicated." Comments in [ ] are mine.  Those effects have also been pointed out by people in this thread.

 

Yes, there is a greater need to collimate the Newt. It's performance will suffer if it is not well collimated, so it needs to be checked each time it is set up.  Refractors can suffer from pinched optics, but a high quality one will (by definition) not have such effects.  I think it is fair to say that a refractor can be set up and used with less care (tinkering, as John I. put it) than a Newt.

 

Yes, a lower f number Newt, depending on magnification use, how good your eyes and the seeing are - maybe f# < 6 - needs a coma corrector for best imaging, and realistically this should then be included in the price of the telescope as in this case it is and integral part.

 

Yes, a Newt either has diffraction spikes from the spider vanes, or it uses curved vanes to smear out the diffracted light, either way it degrades the image a bit.  

 

An "excellent quality reflector" means one with excellent optics, which means one with a primary mirror made by one of the well-know mirror makers with excellent reputation and something like an Antares 1/20 wave (on the surface) secondary.  Some people, like Mike Lockwood, use large aperture, carefully collimated, low f# scopes with excellent optics, Televue eyepieces, and Televue Paracorr 2 coma corrector. Such people will have a different opinion on the quality of images of a Newt than those who use less expensive equipment.  Differences in opinion arise from the above considerations and others.

 

I don't have near the observing experience of Don P. or John I. so I can't speak to the specific comparison of an excellent 10" to 12" Newt and a high quality 130mm (5") APO.  I'm guessing that if you don't mind the diffraction spikes, the answer is yes, the larger Newt will perform as well IFF it has excellent optics including eyepieces and cc if lower f#, is well collimated, etc, etc. and probably better for things like DSOs due to its greater light gathering.  Daniel M. has a thread under the sticky "Best of reflectors" in this forum where he and others compared performance of large reflectors with Newts.  That is one good source of info on comparison (The somewhat larger aperture Newts did better in his opinion) Edit to add link:

https://www.cloudyni...or/#entry305201

Better yet would be to do the comparison yourself at star parties a number of times under different seeing conditions because it will depend on your eyes and what you want to look at.


Edited by tommm, 15 April 2018 - 02:17 PM.


#29 ed_turco

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Posted 15 April 2018 - 01:56 PM

I wrote an article just for you or so it seems.  The link is enclosed in my signature down below.

 

In it, I think you'll find the answer you're looking for.  Also, you will read of my increasing bouts of Rheumatoid Arthritis which at a later date, put me out of business.  Once again, take a peek within my signature.

 

One thing I got out of doing my project was this.  Seeing that I completed the project under extreme duress, only I can say,  "Telescope making is so easy, even a cripple can do it!"

 

In passing, my invitation to apo users to write a semi-scholarly rebuttal to my article.  Three years running and not a peep out of those people.

 

Go ahead and make yourself a telescope!

 

 

ed


Edited by ed_turco, 15 April 2018 - 01:59 PM.

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#30 starman876

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Posted 15 April 2018 - 02:27 PM

Bottom line is this.

 

It all depends on what you want to lug around every time you want to observe.  The bigger the scope the better the resolution.   That small 4" APO will provide a wealth of detail even on nights of not great seeing. However, that large reflector on nights of good seeing will do even better. It boils down to how many nights in your area do you have really good seeing.   Also, can you afford to have two scopes.   This question comes up on a regular basis.  Good APO versus large Newtonian. There is never a clear answer.   However, the bottom line it we always want more aperture.


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#31 barbie

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Posted 15 April 2018 - 03:06 PM

After having large aperture Newtonians and Refractors, I would say yes, it's possible to make a Newtonian as good as a a Mid priced apo.  My current(and final) scope, a 6"F8 newtonian provides refractor like images of the planets and double stars.  Everything snaps into focus and looks as good as in my former 4 and 5 inch apos.  At this point in my life, an arthritic back and knees prevent me from owning anything larger and I like the convenience of a dobsonian mounting.

This 6" f8 dob gave me my best view ever of M42 last winter and I'm looking forward to using it on other DSO's in the future.smile.gif

I've always said that if I could have only one scope, it would be a 6"f8 reflector.  It's an outstanding performer and an excellent compromise between aperture and focal length!!


Edited by barbie, 15 April 2018 - 03:11 PM.

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#32 Starman1

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Posted 15 April 2018 - 03:44 PM

I think if you could have only one scope, it might be:

a 4" apo (superb images all the time)

an 8" SCT (a lifetime's worth of deep sky objects to view and immensely protable)

a 12.5" dob (the biggest small scope or largest big scope--you won't live long enough to see everything it's capable of)

a 6-7" Maksutov (the star images of a large apo and superb high power viewing with small size)

 

Physical abilities may limit the sizes, but any of the above could be lifetime scopes in dark skies.

Lunar and planetary specialists may have different preferences, but the above would make a powerful arsenal of scopes to own.

 

And a note to Barbie: I agree with you in one way: I'd rather have a superb 6" f/8 reflector than a 6" f/8 apo refractor.


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#33 izar187

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Posted 15 April 2018 - 04:23 PM

"It’s become a sort of quest for Holy Grail for me to find this balace of quality of aperture within the bounds of normal affordability - ideally I’d like an excellent quality reflector (both optically & mechanically) in the  10-12” range for the price of of 130mm Apo with the ssme image quality. Does it exist?"

 

Edited by johngwheeler, Yesterday, 09:50 AM.

 

From the slightly silly side of things, buy that 12" of choice.

Then hang a 130mm aperture mask on it.

There you are.

 

Lowest power wide field will not be the same at all, betwixt a 130 apo and 12 newt.  Nope.

But at equal magnifications...

 

Do store the 12 at ambient, if at all possible.

Roll it out to observe, rather than carry it.


Edited by izar187, 15 April 2018 - 04:27 PM.

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#34 Richard Whalen

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Posted 15 April 2018 - 04:54 PM

I would say look at high quality MNT's. No diffraction spikes, well corrected pinpoint stars over the entire fov, superb contrast, modest size, easy to mount. Most are around f6, though a few are faster.



#35 dgoldb

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Posted 15 April 2018 - 05:07 PM

That’s a fair point, but imagine one of the advantages of the reflector is that even a top of the line mirror should be much cheaper than the a good doublet or triplet lens of the same aperture.

 

I’ve never had the opportunity to look through a premium Newt/Dob, but I have heard good things about Teeter. Probably hard to get hold of in Australia though, without a nervewracking shipping option!

The advantage of a reflector is that it scales to a larger size much more affordably; at small sizes that advantage is much less.  That is why you see small refractors and large reflectors.  The notion that a 4-5" reflector should be the same price as a 10-12" reflector, though, is an unrealistic expectation.  Next to a 12" dob, a 127mm refractor looks like a kids toy... they are totally different leagues.  Make no doubt about it, a 12" premium dob will blow a 127mm refractor out of the water in every category except wide field views and ease of use. 


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#36 Mike W.

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Posted 15 April 2018 - 05:37 PM

How  well would a ES AR152mm compare in this discussion.

I'm absolutely new to refractors, I have an opportunity to acquire one, needs work,,,, but the optics are undamaged.



#37 George N

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Posted 15 April 2018 - 06:20 PM

The advantage of a reflector is that it scales to a larger size much more affordably; at small sizes that advantage is much less.  That is why you see small refractors and large reflectors.  The notion that a 4-5" reflector should be the same price as a 10-12" reflector, though, is an unrealistic expectation.  Next to a 12" dob, a 127mm refractor looks like a kids toy... they are totally different leagues.  Make no doubt about it, a 12" premium dob will blow a 127mm refractor out of the water in every category except wide field views and ease of use. 

Even with 'ease of use' the 12" premium Dob might win. I would think that most 12" Dob owners are already observing while 5-inch APO owners are still fooling around with polar alignment and pointing models.

 

Scaling: Right again! I can store my 20" F/5 Dob in my garden shed - fully assembled and ready to roll out 10 feet to observe. I'm not quite sure how I'd deal with a 20-inch F/15 refractor. That would be a pretty big dome!



#38 Jeff Lee

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Posted 15 April 2018 - 07:21 PM

The thing about SCT's is that most folks only seldom keep them properly tuned (and that includes cool down). I have one. The thing about APO's, is that people expect them to cost less than a larger scope. I have one.

 

I had the SCT since 2001 and have had the APO for a short period of time. If I could breed them I'd have the perfect one scope for me. But everyone has a different set of needs based on lots of different criteria. Opinions are just that, scope(s) that satisfy your personal needs is key to your happiness.

 

It good to have lots of data points but you won't really know what you need until you get something and use it. We humans are funny that way, several billion of us each of us has a different set of values and needs. When someone says "its the best" , I always think, "how does one person know what the best is for so many different people. Needs drive wants, things like cost, performance , looks, feel, texture - and many others combine to make the "best" of anything for each and every person.

 

I like what I have and it may not be the best for anyone else but my scopes are the best for me. You can't quantitatively argue this because there are some many  qualitative factors in my decision:)



#39 Starman1

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Posted 15 April 2018 - 08:53 PM

How  well would a ES AR152mm compare in this discussion.

I'm absolutely new to refractors, I have an opportunity to acquire one, needs work,,,, but the optics are undamaged.

It's an achromat, so needs some serious minus violet filtration.  Otherwise, everything has a purple fringe around it.

A 6" scope needs to be f/18 to f/30 to be free from false color (notably, purple), and that scope is much faster.

So, just figure a strong minus violet filter will be required, and you're good to go.

If it's also in bad mechanical condition, it should be very inexpensive.


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#40 Mike W.

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Posted 16 April 2018 - 09:50 AM

What might cause dust inside the tube?

 

these tube vented somehow?

 

It's at a pretty good price, down side just gave away my 3.5 WO,,,

 

Oh well, it was a passing whim on my part, 

thank you for your help Don.



#41 Starman1

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Posted 16 April 2018 - 10:05 AM

Refractors are not air tight.  Fine dust can enter in through the star diagonal end very easily.

Removing the front lens cell to clean the inside lens surface is usually fairly easy.


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#42 Mike W.

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Posted 17 April 2018 - 08:27 AM

Thanks Don, would $500 be considered inexpensive?

 

The ad mentioned no mechanical issues just some surface marring.



#43 Starman1

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Posted 17 April 2018 - 09:32 AM

Thanks Don, would $500 be considered inexpensive?

 

The ad mentioned no mechanical issues just some surface marring.

New, they're $799

They've been on sale in the last couple years for as little as $599.

I'd try for $400.

One caveat: this scope requires a big and heavy mount.  Your typical <$1000 mount isn't going to be adequate, so budget

about $1200-$1500 for a mount--something in the Orion Atlas class or SkyWatcher EQ6 or Celestron CGEM

Anything lighter will be "Shimmy City".



#44 Mike W.

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Posted 17 April 2018 - 07:55 PM

Think I'll invest in a go-to SW 12 instead.or just a go-to base for my 10.

 

Anyway, thanks again for your time Don.



#45 Deep13

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Posted 18 April 2018 - 02:35 AM

The key to image quality for most Newts is collimation and especially thermal control. While top quality optics help (including a really flat secondary) a forgiving focal ratio like an off the shelf 6" f/8 ought to do well.


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#46 gwlee

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Posted 18 April 2018 - 12:30 PM

After having large aperture Newtonians and Refractors, I would say yes, it's possible to make a Newtonian as good as a a Mid priced apo.  My current(and final) scope, a 6"F8 newtonian provides refractor like images of the planets and double stars.  Everything snaps into focus and looks as good as in my former 4 and 5 inch apos.  At this point in my life, an arthritic back and knees prevent me from owning anything larger and I like the convenience of a dobsonian mounting.

This 6" f8 dob gave me my best view ever of M42 last winter and I'm looking forward to using it on other DSO's in the future.smile.gif

I've always said that if I could have only one scope, it would be a 6"f8 reflector.  It's an outstanding performer and an excellent compromise between aperture and focal length!!

My most used telescope is an inexpensive, garden variety 6”f8 Dob for its unique combination of portability and optical performance, but finding a mechanically and optically refined premium 6”f8 Dob to purchase today is very difficult, and I don’t want to build one. 

 

On the other hand, a sensibly perfect 4” refractor and mount of equal quality can be delivered to my door by a big brown truck within 72 hours of making the phone call. Scope and mount together will weigh as much as a 6” Dob. From experience with both scopes, I know the expensive refractor won’t equal the the optical performance of the inexpensive Dob, but it will be much more refined though, and it’s available off the shelf, so I am reluctantly considering retiring the 6”f8 Dob. 


Edited by gwlee, 18 April 2018 - 12:45 PM.


#47 gwlee

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Posted 18 April 2018 - 01:08 PM

Even with 'ease of use' the 12" premium Dob might win. I would think that most 12" Dob owners are already observing while 5-inch APO owners are still fooling around with polar alignment and pointing models.

All the APOs that I own or have ever owned were on alt/az mounts, so never required “fooling around with polar alignment or pointing models.” 

 

All the Dobs I own or have ever owned received a primary collimation tweak each use (~3 minutes) and required 30-60 minutes of cooling to give their best images in my climate. My APOs have been a bit more forgiving in this regard. 

 

Comparing like mounts (alt/az to alt/az) and like purpose (visual use to visual use) would be a more valid comparison of setup time between a 5" refractor and a 12" Dob.


Edited by gwlee, 18 April 2018 - 04:04 PM.

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#48 barbie

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Posted 18 April 2018 - 05:09 PM

My most used telescope is an inexpensive, garden variety 6”f8 Dob for its unique combination of portability and optical performance, but finding a mechanically and optically refined premium 6”f8 Dob to purchase today is very difficult, and I don’t want to build one. 

 

On the other hand, a sensibly perfect 4” refractor and mount of equal quality can be delivered to my door by a big brown truck within 72 hours of making the phone call. Scope and mount together will weigh as much as a 6” Dob. From experience with both scopes, I know the expensive refractor won’t equal the the optical performance of the inexpensive Dob, but it will be much more refined though, and it’s available off the shelf, so I am reluctantly considering retiring the 6”f8 Dob. 

lol.gif I used to think the same way till I got my 6"F8 dob with excellent optics.  It absolutely blows away ANY 4 inch scope no matter how well refined it may be.  BTW my 6" dob gives me views that are just as fine as my former 4" and 5" apos and is much easier to set up.  Collimation is also extremely easy as is thermal equillibrium.  No triplet Apo refractor will cool as quickly.  The views in the 6 inch reflector are superior, in every way.  Mechanicals are also quite excellent!

While my former large Apos were nice, they were not convenient to set up which is why I sold them and am no longer a fan of such scopes. YMMV.grin.gif


Edited by barbie, 18 April 2018 - 05:22 PM.


#49 nashvillebill

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Posted 18 April 2018 - 05:22 PM

Vanilla!  No, chocolate!

 

Beatles! No, the Stones!

 

Ginger!  No, Mary Ann!



#50 CHASLX200

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Posted 18 April 2018 - 05:53 PM

I think if you could have only one scope, it might be:

a 4" apo (superb images all the time)

an 8" SCT (a lifetime's worth of deep sky objects to view and immensely protable)

a 12.5" dob (the biggest small scope or largest big scope--you won't live long enough to see everything it's capable of)

a 6-7" Maksutov (the star images of a large apo and superb high power viewing with small size)

 

Physical abilities may limit the sizes, but any of the above could be lifetime scopes in dark skies.

Lunar and planetary specialists may have different preferences, but the above would make a powerful arsenal of scopes to own.

 

And a note to Barbie: I agree with you in one way: I'd rather have a superb 6" f/8 reflector than a 6" f/8 apo refractor.

If i had to pick one scope it would be a 8" F/8 Newt.  Not too big and always gives great views. I don't feel like lifting 10" or bigger OTA's anymore.


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