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Dob vs Apo

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#1 pao

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Posted 26 September 2020 - 03:14 AM

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

Thanks 

pao



#2 areyoukiddingme

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Posted 26 September 2020 - 03:48 AM

I think it depends on your definition of 'best views'. The apo is always going to give very slightly nicer views without diffraction being an issue. The refractor will resolve less details, but what details it does show should be sharper.

 

But if 'best' is 'what can I see', then there's no contest between 8" and 4". 

 

I'd like to get in some side by side comparisons with my 8" reflector and a top-notch 6" apo. I suspect the 8" will win for resolution of detail on Jupiter.


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#3 Astrojensen

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Posted 26 September 2020 - 04:21 AM

On almost all objects, a premium 8" dob will absolutely TRASH a 4" apo, no matter how good or expensive the latter is. The difference is overwhelming. A premium newtonian can get very close in performance to an apo of the same size.  

 

A real exception is the Sun, where the apo will usually be by far the best choice, mostly due to better thermal behavior.

 

To really shine, the dob will need to have a full tube design, not a truss, to keep the observer's body heat out of the light path. The tube will need to extend at least 16" in front of the focuser, to keep the observer's breath out of the light path. 

 

I have some experience with this. My $200 6" f/8 Sky-Watcher newtonian outperformed a $4000 100/800 TMB apo I had on loan, when it came to details on Jupiter. 

 

 

Clear skies!

Thomas, Denmark


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#4 Avgvstvs

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Posted 26 September 2020 - 06:05 AM

More aperture shows more as well as a larger range of targets.

But a great refractor has qualities all its own

Get both!


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#5 sg6

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Posted 26 September 2020 - 06:08 AM

The premium should win, with one or two small areas it would possibly lose out. However a premium newtonian will cost a lot of money, and most people talk od the kind of standard Synta, Meade, ES items.

 

With a proper "premium" mirror the cost difference is somewhat less. It seems that rarely are 2 items compared that are similar. In general the reflector/refractor comparisons are diameter and quality. Not exactly the same items most of the time.

 

Ultimately it is a case of whatever makes you happy and then leave others with whatever makes them happy. Never put anyone elses scope down. Very often that seems forgotten.


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#6 Rutilus

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Posted 26 September 2020 - 06:44 AM

 

 

 

I have some experience with this. My $200 6" f/8 Sky-Watcher newtonian outperformed a $4000 100/800 TMB apo I had on loan, when it came to details on Jupiter. 

 

 

Clear skies!

Thomas, Denmark

 

Same here.  I've had three 6" f/8 Newtonian reflectors.  All three had the edge over my Takahashi 4 inch apo for

planetary detail.  


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#7 Redbetter

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Posted 26 September 2020 - 07:04 AM

You don't need a premium Dob for such a comparison.  Any better-than-diffraction-limited 8" will do.  

 

With half the resolution and far less overall image brightness the apo will be less sensitive to seeing, but will also show less.  The stars will look tighter on average because it is not as sensitive to the seeing and the central spurious disk is larger and there are no diffraction spikes on bright objects.  The Dob will spill more light into the first diffraction ring...but the pattern is half the diameter.  The spurious disk will have more of a diffraction pattern around it than the smaller apo. 

 

Since the image in the Dob is brighter at the same magnification (3+ times brighter even after subtracting for obstruction and mirror losses), the background sky in the apo will appear ~3x darker at the same magnification.  Aesthetically the apo will look better, except when the seeing is good to very good and the Dob runs significantly past it in terms of optimal magnification and detail.

 

For planets, any time the seeing is good for planetary, the Dob will outperform by a wide margin.  In poor/mediocre conditions the apo will be aesthetically more appealing and show about the same level of detail without all of the "warts" of the seeing showing.  When I want detail on planets and the seeing is decent, I pull out the larger apertures.  They show me things that I don't get from a 110ED and that are not described by owners of 4" premium apo's.

 

4" Apos tend to be short focal length and have high contrast which is great for very large targets.  An 8" Dob will typically not be able to provide anywhere near the same true FOV of a 4" apo so the very largest DSO's, comets and the like are best seen with the smaller/wider FOV scope.  For everything else...which is about everything, the larger Dob will reveal more.  There are an order of magnitude more DSO's in reach.  

 

Part of the fun of planetary observing is watching the moons from one's own yard.  That is a lot less daunting with an 8" scope.  Even my 25 year old 8" SCT with its "Starbright" pre-XLT optics was showing me Uranus' moons Titania and Oberon tonight, in a red zone (18.9 MPSAS at the time.)  I didn't look up positions ahead of time,  instead I noticed a nearly 13th mag star a little bit outboard of where I thought any moons would be, then increased power and studied the field somewhat closer in for a time.  I noticed 13.9 mag Titania in averted vision to the south fairly quickly, but wasn't sure if it might be a star too as it was a little wider than I suspected it would be.  While I was working to confirm it, I picked up 14.2 mag Oberon at 370 or 555x at about the same separation but preceding.  I searched for others, but had no decent hits.  When I pulled up Stellarium I had a perfect match for the two moons and star.  

 

Saturn's fainter moons are more readily seen with an 8".  Neptune's moon Triton is easy to see with an 8".  Neptune itself is more readily seen as a sharper disk of characteristic blue color with an 8".  In dark sky Pluto is not hard with an 8".  It requires dark and steady sky and some experience to track Pluto down with a 4" class scope (I have done it a few times with the 110ED, no luck with an 80ED so far.)  


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#8 CHASLX200

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Posted 26 September 2020 - 08:08 AM

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

Thanks 

pao

The slower 8" Newt will win everytime for high power planet work while the APO will win with wider FOV's and a tad sharper image. But both scopes at 400x on Jupiter and the Newt has a much brighter image vs the smaller 4" APO.


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#9 Galicapernistein

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Posted 26 September 2020 - 08:18 AM

I sold my 8” F6 Discovery dob after comparing the planetary images to my 100mm ED F11. The refractor provided better images from the start. It was an education in how thermal issues destroy fine detail in reflectors. It also shows to me the advantages of slow refractors for planetary viewing. I notice that many of the other refractors people have used are relatively fast. Increasing the F-ratio goes a long way in compensating for reduced aperture. Even my 8” F9 “ planet killer” has never provided me with the views of my F11, again, because of thermal issues. If you live in a better climate, your experience may differ. But for me, a refractor is just so much more convenient for planetary work, my 8” is now my deep sky scope.



#10 CHASLX200

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Posted 26 September 2020 - 08:23 AM

I have world class seeing so the bigger Newts just kills the smaller APO for the high powers i like using.


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#11 junomike

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Posted 26 September 2020 - 08:25 AM

I'd take the Dob for Planetary/Lunar + most DSO's accept the Widest one's that would better fit in a fast 4" Apo.

 

Also a 4" Doublet should handle the cooler temps better and acclimate faster.

 

IMO these are more complimentary than competitive.


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#12 junomike

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Posted 26 September 2020 - 08:26 AM

I have world class seeing so the bigger Newts just kills the smaller APO for the high powers i like using.

One doesn't even require world class seeing for the Newt to out-perform the Apo in most circumstances.


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#13 Jon Isaacs

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Posted 26 September 2020 - 08:29 AM

I think it depends on your definition of 'best views'. The apo is always going to give very slightly nicer views without diffraction being an issue. The refractor will resolve less details, but what details it does show should be sharper.

 

 

Something to think about:

 

- Diffraction is always an issue.  It's diffraction that determines the diameter of the Airy disk. In terms of resolution and contrast, the diffraction effects of the 4 inch aperture are the governing factor for the apo... 

 

- I am currently going through a similar issue.  I have a 120mm Eon which is a 120mm F/7.5 FPL-53 Doublet,  No complaints about the performance, it does what a 120 mm refractor can do.  

 

But when faced with the choice between the 120mm Eon and my 10 inch F/5 GSO Dob, the Dob provides better planetary images and splits much closer doubles besides of course going deeper.  I am just not hooking up with the refractor in a satisfying way.  This is my second 120mm Eon, I sold the first one for pretty much this same reason, I had some regrets and bought another one.  Seems like I might sell this one too but I am giving myself more time.  

 

Jon


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#14 sw196060

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Posted 26 September 2020 - 08:51 AM

I have a decent, non "premium", 8" f/6 dob and am quite happy with it.  I have gone out star gazing with a friend who has a Tak FS102.

The views are different to be sure but I am happy with my dob.  At first glance I find the apo views of planets more pleasing and "clean".

However, after a bit of time I prefer the view in my 8" newt ... more detail and brighter.  

With my old eyes I find 4" aperture runs out of light at high mags.

The are complementary scopes for sure and, if I had the $$, I would like to have both.


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#15 ismosi

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Posted 26 September 2020 - 09:45 AM

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

Thanks 

pao

I don't own a telescope in those particular sizes, but, in my opinion, my refractors always give a better view .. that is in terms of image fidelity. My 18-inch Dob however, always shows me more (excluding planets perhaps) - by a huge margin. The other night I deployed my 92mm along side the big Dob and pointed them both at M13. In the 92mm it was a nice cotton ball, but in the 18-inch it was an 'explosion' of suns.

 

I have yet to see better view of the planets than in my 6-inch refractor. However, those of us living under the nearly-permanent Northeast Nebula (US) rarely get better than average seeing.

 

If you have to pick one, get the 8-inch Dob.


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#16 gwlee

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Posted 27 September 2020 - 01:13 PM

Which one gives you the best views in your opinion: a good 8" dobsonian with premium optics or a 4" apo refractor? I never owned a newtonian so, to me, the answer is not so obvious.

Thanks 

pao

My 8” Dob clearly goes deeper and resolves smaller details. My almost-4” APO can gives wider views, and it’s a touch sharper at the same magnification. One is not “better” than the other, just different. 


Edited by gwlee, 27 September 2020 - 03:24 PM.

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#17 cuzimthedad

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Posted 27 September 2020 - 01:50 PM

...One is not “better” than the other, just different. 

Today's pearl of wisdom. Well said and in less than 10 words.


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#18 Eddgie

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Posted 27 September 2020 - 04:00 PM

8" dob has better contrast transfer and better limiting magnitude than a 4" Apo. Also, at the same exit pupil (brightness) it will show the object at greater scale.  An 8" f/6 dob with excellent mirrors is a far better observing instrument than a 4" Apo. You can get bigger, brighter galaxies, brighter nebula, more stars in clusters, and more detail in Planetary Nebula. 

 

These differences are not at all difficult to see.  I would expect that any person off the street could easily see these differences.  As telescope things go, the difference in not at all subtle.  For example, M82 can be made bigger and still be brighter in an 8" dob and will reveal more structure.  Same with the M57, the ring nebula, and Globular Clusters like M22 will be much richer and better resolved in an 8" than a 4" anything. 

 

At 250x in a 4" Apo, the cones in the retina are starving for photons.  In an 8" dob, 250x is a practical power for most people and if the mirrors are truly excellent, this kind of scope will rival or even surpass a 6" Apo, (if seeing allows).  (And yes, I have owned all of these scope types and this is both what the physics says should happen, and what I have actually observed in person) 

 

Again, none of these are particularly subtle difference (as is the case between something like the difference that 25% more aperture makes). I am quite confident that I could pick out the 8" dob from the 4" Apo on a good example of every one of these class objects. There are a lot of things in astronomy that are often discussed at length where difference are often so subtle that it would be hard to see, but this is not one of those cases.  

 

A good 4" Apo has a wider field. Typically they will also have a much larger fully illuminated circle, which is not important at all for visual observing, but when combined with the wide field of view, make them excellent choices for wide field imaging with large sensors.


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#19 Redbetter

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Posted 28 September 2020 - 01:19 AM

 

A good 4" Apo has a wider field. Typically they will also have a much larger fully illuminated circle, which is not important at all for visual observing, but when combined with the wide field of view, make them excellent choices for wide field imaging with large sensors.

 

I was with you up until the final paragraph, or more precisely, the way the final sentence is stated.   I don't know that it is what you intended, but It makes it sound as if the various advantages of an apo do not apply to visual observing, and that would be incorrect. Wider field of view is indeed quite useful for visual.  The wide field views that are available make such scopes better for certain kinds of observing, including sweeping along the Milky Way and also looking for more diffuse wider scale nebula structure, as well as viewing the occasional large comet.  In addition the greater illumination (surface brightness) at a given exit pupil, along with associated greater contrast, are worth keeping in mind.

 

The maximum available surface brightness of the apo at a given exit pupil will be significantly greater--which applies to the largest targets where the refractor has more of a niche.  There will also be less scatter and the result will be increased contrast in the refractor. 

 

Consider the brightness of a given exit pupil for an apo with a 2" dielectric diagonal looking at wide field views (e.g. the Veil).  The refractor's transmission (ignoring the eyepiece) will be something like 98% for the objective and 99% for the diagonal for 97% overall.  The Newtonian will be more in line with 92 or 95% max transmission per surface when new, typically something like 90% per mirror is assumed.  Even if one assumes a 95% secondary and 90% primary the net is 85.5% transmission.  If the scope has 25% obstruction then the overall transmission will be 80.2%. 

 

Therefore the net maximum ratio of surface brightness at a given exit pupil is 0.97/0.802 ~ 1.21.  The refractor can present a roughly 20% brighter image.   That is useful for low surface brightness targets, even though the background is dimmed by 20% as well, because we are starved for photons on these objects.  This doesn't directly improve contrast, but using fewer mirrored surfaces does because of reduced scatter--I am not sure how to quantify that.)  There is also some level of reduction of "encircled energy" lying outside of the 4th diffraction ring, which is in effect brightening of the background/reducing local contrast on an already somewhat dimmed image. 

 

This latter wider scale contrast effect (due to obstruction) might not seem important to some, but it is not unlike the contrast destroying impact of very poor seeing...  The background adjacent to bright objects always seems brighter when the seeing is poor.  Even in poor seeing it isn't like I have trouble seeing similar objects of the brightness of Sirius B, Phobos, or Deimos if they are by themselves.  But put them close to something tens of thousands or a million times brighter, and they disappear in sufficiently poor seeing.  This is true even when hidden behind an occulting bar, etc.so it isn't merely direct glare that is responsible.  Similarly, one can look at the encircled energy still beyond the 4th ring with 40% obstruction and recognize that there is some wider scale loss of contrast occurring with obstructions, even if it is not as readily seen with obstructions of 20% or less.  



#20 luxo II

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Posted 28 September 2020 - 04:30 AM

Having had a superb 130mm f/7 APO side-by-side with a good 8" f/7 newtonian the newtonian was vastly better on all objects by a wide margin - and it was enough to make me sell the APO and end my dalliance with refractors, and revert to maksutovs.

 

A good 8" newtonian will easily slay a 4" APO on all objects. Not even close. In all conditions bar overcast, rain or snow.

 

The downside is size, weight, and the mount needed.


Edited by luxo II, 28 September 2020 - 04:35 AM.

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#21 Jon Isaacs

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Posted 28 September 2020 - 07:04 AM

Having had a superb 130mm f/7 APO side-by-side with a good 8" f/7 newtonian the newtonian was vastly better on all objects by a wide margin - and it was enough to make me sell the APO and end my dalliance with refractors, and revert to maksutovs.

 

A good 8" newtonian will easily slay a 4" APO on all objects. Not even close. In all conditions bar overcast, rain or snow.

 

The downside is size, weight, and the mount needed.

I would add:

 

All objects that fit in the Newtonians's field of view.  

 

I see a good 4 inch apo and a larger Newtonian as companions, they are different scopes with different advantages and limitations.  A 4 inch apo provides wider fields of view, sometimes much wider.  A 4 inch apo will provide it's best views nearly 100% of the time, no need to set it out an hour before sunset and start the fans running and hope it's cooled enough for viewing the planets when it's time to start. It won't need collimation, at least not very often.  

 

Under dark skies, I almost always setup a larger Dob as well as a shorter focal length 80mm or 4 inch refractor.  The complement each other.  One does deep, one goes wide.

 

Jon


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#22 Galicapernistein

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Posted 28 September 2020 - 10:07 AM

It’s no surprise that people rate their fast refractors as worse than their dobs. Fast refractors are poor visual performers, in comparison to fast dobs, due solely to aperture. The focal length of my 4” F11 is only 3” less than the 8” F6 dob I had, and sold. The 4” is better in every quality but image brightness. Globulars in the F11 look almost identical. Any loss of brightness is more than made up for with improved resolution and contrast. The dark edge of M31 stands out much better in the 4”. The cores of spiral galaxies look better in the 4”. Planets definitely look better in the 4”. I’ve had plenty of dobs and seen plenty of deep sky objects. I guess I’m just at that stage of the hobby where I only want to see the best optical images of things I have seen many, many times.



#23 KI5CAW

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Posted 28 September 2020 - 10:16 AM

My 4" f/15 refractor provides great images and appears to out perform my reflectors early in the evening before the seeing and the reflectors settle down. But after midnight, the reflectors (6, 8 and 12 inches) are way out in front, especially my 8" f/8 planet killer. And with a 60" FL the refractor is not small or easy to mount.


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#24 Jon Isaacs

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Posted 28 September 2020 - 10:18 AM

It’s no surprise that people rate their fast refractors as worse than their dobs. Fast refractors are poor visual performers, in comparison to fast dobs, due solely to aperture. The focal length of my 4” F11 is only 3” less than the 8” F6 dob I had, and sold. The 4” is better in every quality but image brightness. Globulars in the F11 look almost identical. Any loss of brightness is more than made up for with improved resolution and contrast. The dark edge of M31 stands out much better in the 4”. The cores of spiral galaxies look better in the 4”. Planets definitely look better in the 4”. I’ve had plenty of dobs and seen plenty of deep sky objects. I guess I’m just at that stage of the hobby where I only want to see the best optical images of things I have seen many, many times.

 

I think you're on the wrong track.

 

If your 4 inch has better resolution and contrast that the 8 inch Newtonian, either the seeing is very poor or something is optically wrong with the Newtonian.

 

My 10 inch Dob routinely splits doubles far closer than the Dawes limit for a 4 or 5 inch refractor.  That's pure resolution.

 

Jon


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#25 Galicapernistein

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Posted 28 September 2020 - 10:32 AM

I think you're on the wrong track.

 

If your 4 inch has better resolution and contrast that the 8 inch Newtonian, either the seeing is very poor or something is optically wrong with the Newtonian.

 

My 10 inch Dob routinely splits doubles far closer than the Dawes limit for a 4 or 5 inch refractor.  That's pure resolution.

 

Jon

And looking at Jupiter without a massive secondary blurring the features is called contrast.




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