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Visual observing: big APO refractor VS. big Dobson

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#1 Urbyz

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Posted 24 September 2020 - 02:06 PM

Hi there,

I’ve been a proud owner of a small 6” dobson and enjoy using it very much. Being a little smaller and looking at other scopes online, naturally the mind starts to wonder what a bigger scope might reveal through the lens. I might’ve caught some aperture envy!

Looking online i started developing an interest for refractors. Not exactly sure why, but something draws me to their form factor and it might be due to the stories about crisp, contrasty views you’ll only get with a high end APO triplet refractor. Visually as wel as for AP.

I’m very aware that larger dobsons deliver the best bang for buck views and aperture possibilities. Their big aperture lends well for viewing both DSO and planets and the dob mount lends itself perfectly for easy visual observing. But when i read high-end APO triplet refractor (~6k $) product descriptions, it tells me they are capable of also showing details in faint DSO’s and sharp images of planets at very high magnification.

Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

I only observe visually so far, but might take up AP in the future.

Best,

Ruben
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#2 Mitrovarr

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Posted 24 September 2020 - 02:30 PM

On DSOs? The big dob will gather a ton more light and enable you to see things that are far fainter than the refractor. You can't cheat physics. A big dob like that is a fantastic visual deep sky telescope, you start to see structure in galaxies and such.

The refractor will show very beautiful images of brighter objects, though. It will also be very worthwhile, just in a different way.

On planets, the refractor will generally always do a good job. The dob will have the potential to exceed it, and by a lot, but you will have to work out all the issues to get it:
1. Great atmospheric seeing.
2. No tube currents, everything cooled down.
3. Perfect collimation.
4. Great optics.
5. Some kind of tracking would help.

If you get all of that the dob will win but seeing is challenging for large scopes.
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#3 gnowellsct

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Posted 24 September 2020 - 02:32 PM

The Dobson will perform better with a few basic details such as setting up an hour or so early and taking the trouble to collimate it.

 

You will get more color saturation and greater resolution--we're assuming that you have a Dob with respectable optics.  I was going to say well get a premium Dob but I hesitate.  A standard diffraction limited .85 Strehl 16" Newtonian from Meade or Celestron will have plenty of resolution and detail to outgun a premium refractor half its diameter.  I'm assuming that the upper end in the refractor is eight inches because costs skyrocket after that.  The dob will typically come with mount included--why it's a dob.  For the Apo double the cost for an appropriate mount.  

 

Since I currently own and operate 4 apos from 81mm thru 130 mm I would also say, "let me say this about that."  You have the wrong idea.  If you want a light bucket get a light bucket.

 

You have to let refractors grow on you.  Get a three to four inch apo (triplet or ED doublet) and work with it for a while and spend the rest of your money on a big dob.  You will find that you this is not an either or situation and you shouldn't expect your refractors to do what your light bucket does and vice versa.

 

If you get a good three or four inch refractor with a nice focuser you will soon enough come to see what the virtues of it are.  And if you don't like it you can sell it in no time they are highly popular.  You don't put much at risk.  

 

As for the big instrument it will be hard to get rid of it once you have used it a while.  There's just too much that it will do.

 

Greg N


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#4 e7FvPDZR

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Posted 24 September 2020 - 02:38 PM

There would be a significant difference in brightness and resolution between 130-140mm and 12-16” (304.8-406.4mm) aperatures.


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#5 gnowellsct

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Posted 24 September 2020 - 02:41 PM

I would point out that for normal middle class budgets an 8 inch apo and mount is a heart-stopping expenditure.    The only reason I brought it up is because you wanted to compare to a big dob.  Well you would need some might fine mighty precise apo aperture to keep up with a 16 inch mirror.

 

You have to adjust your expectations.  In your 16 inch the Perseus double cluster will be a mind blowing shockwave of stars.  In your 4 inch refractor you will get a gorgeous detailed view of the region that will dazzle.  But the clusters themselves will be redueced to one or two dozen granules.

 

When you observe in small apertures, things you take for granted from observing with a big scope just disappear outright, or become very difficult to spot.

 

Generally I like to pair a refractor with another scope.

 

Greg N


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#6 junomike

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Posted 24 September 2020 - 02:43 PM

Along with the stated brightness and resolution, the Dob should have a much larger exit pupil at the same mag.


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#7 Mitrovarr

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Posted 24 September 2020 - 02:44 PM

Yeah, ideally you'd really want both of these scopes. They're kind of orthogonal to each other.

It's kind of like asking whether a sports car is better than a heavy truck.
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#8 Sky King

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Posted 24 September 2020 - 02:47 PM

Welcome to Cloudy Nights! Are you in dark skies? A light bucket will also scoop up light pollution. If you are a urban viewer, it limits what you can see...


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#9 t.r.

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Posted 24 September 2020 - 02:49 PM

I couldn’t agree more with what is said above. Nirvana is achieved with a fine example of each!

Edited by t.r., 24 September 2020 - 02:50 PM.

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#10 mccarthymark

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Posted 24 September 2020 - 02:53 PM

Get the best of both worlds.  Use the 16-inch dob for deep sky, then make a 5-inch diameter (or whatever will fit) off-axis mask at the front of the UTA to use for double stars etc., or for when seeing doesn't support the larger aperture.  


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#11 Rutilus

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Posted 24 September 2020 - 02:53 PM

My 14 inch Dob showed colors and detail on the planets that were far better than any apo scope of 6 inches and below

that I had or used. The Dob scope would show structure in DSO's, that the refractor could never match.

Sadly, I can no longer use a Newtonian reflector, and certainly not a Dobson type mount with my medical condition..    


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#12 Garyth64

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Posted 24 September 2020 - 02:56 PM

I've gotten some very pretty crisp views with my 6" f/10 newt, and still do.  My 10" f/7 also gave very nice crisp views too.

 

I have a 130 APO and it is very good, with nice crisp views.  It would not compare to my 10" f/7, and the 6" f/10 gives the 130 a run for its money.


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#13 sg6

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Posted 24 September 2020 - 02:59 PM

Add a location for yourself, it really helps.

A big newtonian will gather more light, but a good refractor will deliver sharper images.

It is all a balance and very much personal.

 

The two are different instruments and are maybe oddly "measured" by different qualities often.



#14 turtle86

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Posted 24 September 2020 - 02:59 PM

Hi there,

I’ve been a proud owner of a small 6” dobson and enjoy using it very much. Being a little smaller and looking at other scopes online, naturally the mind starts to wonder what a bigger scope might reveal through the lens. I might’ve caught some aperture envy!

Looking online i started developing an interest for refractors. Not exactly sure why, but something draws me to their form factor and it might be due to the stories about crisp, contrasty views you’ll only get with a high end APO triplet refractor. Visually as wel as for AP.

I’m very aware that larger dobsons deliver the best bang for buck views and aperture possibilities. Their big aperture lends well for viewing both DSO and planets and the dob mount lends itself perfectly for easy visual observing. But when i read high-end APO triplet refractor (~6k $) product descriptions, it tells me they are capable of also showing details in faint DSO’s and sharp images of planets at very high magnification.

Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

I only observe visually so far, but might take up AP in the future.

Best,

Ruben

 

Something like a 4-5” APO and a 12-16” Dob really complement one another.  They offer different viewing experiences, and each excel at different things.  For example, my NP101 is better for the Pleiades, but my 18” Dob is better for most galaxies.


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#15 Alan French

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Posted 24 September 2020 - 03:01 PM

Get the best of both worlds.  Use the 16-inch dob for deep sky, then make a 5-inch diameter (or whatever will fit) off-axis mask at the front of the UTA to use for double stars etc., or for when seeing doesn't support the larger aperture.  

It's not the same. A 4 or 5-inch refractor will give you a wider true field. 

 

I played with a 6-inch off-axis mask with my wife's 14.5-inch equatorial Newtonian on several nights when Saturn and Jupiter were well placed and tossed the mask in the trash. While the full aperture was not reaching its full potential because of seeing, it did reveal more detail and the larger aperture and more vivid colors were greatly appreciate.

 

Clear skies, Alan


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#16 gnowellsct

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Posted 24 September 2020 - 03:07 PM

I had my 128mm FS128 set up next to my C14.  At 300x it was excellent.  A very good night too.  I thought: this is the night I find out that apos can blast the socks off a c14.  I went over to the C14 and....got lost in the view.  The Tak wasn't even close.  

 

This is all the more surprising given the mean things people like to say about SCTs.  But no matter.  It's fun to observe in a 5 inch apo.  

 

One of the things you have to understand is that exit pupil is a direct function of aperture.  A 1mm exit pupil in the FS128 is 128x.  A 1mm exit pupil in the C14 is 356x.  On this very good night, I happened to be able to get the scopes up to 300x.  

 

On the FS128 that is 0.4 exit pupil.  On the C14 300x is 13 or 14mm ocular with an exit pupil ~2.9 times the size of the exit pupil in the refractor.  That is a LOT more exit pupil.  That means the view is more relaxed, defects in the eye are less likely to interfere in the views.  You are more likely to settle in and spend a long time looking.   While refractor buffs like to vaunt the optical defects of scope designs that have a central obstruction, they conveniently forget that exit pupil and the organic structure of the eye are a powerful limitation imposed by the small scale and high price of their instruments.  The size of the aperture is as much "a feature of the optic" as the presence or absence of central obstruction.  Moreover small apertures really can't provide the same color saturation that you get out of larger ones.  

 

To put it more concretely, I was out last summer, maybe two summers ago at a local event that our club provided some scopes to.  I took my 5" apo.

Behind me I had a 10" Newtonian and a 12" Newtonian, both by Skywatcher, and both lovingly tended to by their respective owners.  And the two of them together not worth even a quarter what I paid for the apo.  The views their scopes were providing blew mine away.  There was just no way it could keep up.  It was very had to say that it was good value for the money as the mythical "planetary scope" that so often gets posted about in this forum.  

 

If you're going to get a pricey refractor you might want to make sure that you have other reasons besides planet viewing.  A very fast small refractor (like the Skywatcher Esprit 80mm) is an excellent learner's scope because a fast focal ratio makes for a quicker shot.  

 

You should get refractors because you really like refractors.  I don't think it's a bad thing to do.  And unlike many people handing out advice in the beginner's section I do think there are some arguments for an 80mm refractor as a starter scope, rather than an 8 inch Dob.  

 

Greg N


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#17 Jeffmar

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Posted 24 September 2020 - 03:15 PM

It is hard to beat a large aperture scope for planetary and deep space views. A 14 inch+ Dob will pretty much blow just about any refractor out of the water for details, color, and yes, even contrast. If you could find a nice 10 inch apo refractor, that might be different. 


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#18 Urbyz

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Posted 24 September 2020 - 03:22 PM

On planets, the refractor will generally always do a good job. The dob will have the potential to exceed it, and by a lot, but you will have to work out all the issues to get it:
1. Great atmospheric seeing.
2. No tube currents, everything cooled down.
3. Perfect collimation.
4. Great optics.
5. Some kind of tracking would help.

If you get all of that the dob will win but seeing is challenging for large scopes.


Very interesting and informative! Thanks for listing those. It makes comparing the two more insightful for sure.

#19 NYJohn S

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Posted 24 September 2020 - 03:25 PM

I was out last night observing with some friends. I had My AT102ED the others had an Altair Starwave 152 and a 14" dob. We viewed M15 in all 3 scopes. I thought my AT102 did well. It resolved quite a few pinpoint stars on the edges of the cluster and some outliers. The Starwave 152 resolved some more stars and the image was brighter. A noticeable difference. In the 14" dob M15 looked like a completely different object. The bright core actually looked smaller because it resolved so many stars towards the center of the cluster. The number of stars spilling out of the globular was breathtaking. The big dob was just a better scope for that type of object. 

 

On this night the planets actually looked better in the AT102ED. The seeing wasn't good enough for the big dob to gain an advantage. At 200x the views in the AT102 were stable and sharp with nice contrast. At 250x they were swimming in the dob. 

 

Each of the scopes did different things well. I enjoy being out with with my dob and having one of my small refractors with me. Some night's I just l go out with a refractor some nights binoculars. It's nice to have different instruments to be out under the sky with.


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#20 Ihtegla Sar

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Posted 24 September 2020 - 03:28 PM

 

What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

Well, I don't have a premium 130-140mm  refractor or a 16" dob, but I do have a couple premium 4" refractors (FC100DL and FC100DF) and a premium 20" dob (Obsession Classic with an Ostahowski mirror).

 

From my experience is that the dob gives much better views of the planets but only when the scope is precisely collimated, well cooled and the seeing is good.  The advantage of the smaller refractor is that you can just plop it down on any given night and fifteen minutes later it is ready for high magnification viewing (small doublets cool very fast), there is no need to collimate and it performs reasonably well even when the seeing isn't good.  The 20" Obsession is capable of showing much more planetary detail but  only after it has been properly collimated and cooled (cooling can be a challenge with a big dob) and it is much more sensitive to bad seeing (common where I am).

 

On DSOs the big dob gives a much brighter view but a small refractor has a much wider view, so you can see large objects (Veil nebula, California nebula, M31 to name a few examples) in their entirety, something you just cannot do with a big dob.  You can only see small portions of such objects in a big dob, but those portions will be much brighter and more detailed than what you will see in a small refractor.

 

So there are tradeoffs and many people like me choose to own both and often use both at the same time, switching back and forth during a session.


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#21 Urbyz

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Posted 24 September 2020 - 03:28 PM

You have to let refractors grow on you. Get a three to four inch apo (triplet or ED doublet) and work with it for a while and spend the rest of your money on a big dob. You will find that you this is not an either or situation and you shouldn't expect your refractors to do what your light bucket does and vice versa.

If you get a good three or four inch refractor with a nice focuser you will soon enough come to see what the virtues of it are.


This seems to be the consensus. I’m starting to realize that one cannot replace the other.
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#22 Bomber Bob

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Posted 24 September 2020 - 03:31 PM

Welcome to Cloudy Nights! Are you in dark skies? A light bucket will also scoop up light pollution. If you are a urban viewer, it limits what you can see...

Yep.  IF you have to drive to dark sites, how transportable is your Big Dob?  I can rest my 5" F5 frac on the passenger seat of my Mustang, put the VersaGo in the trunk, and loading / unloading is no sweat.  And, since it's a Mustang, I can get to the dark site... faster...


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#23 Eddgie

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Posted 24 September 2020 - 03:34 PM

Hi there,

I’ve been a proud owner of a small 6” dobson and enjoy using it very much. Being a little smaller and looking at other scopes online, naturally the mind starts to wonder what a bigger scope might reveal through the lens. I might’ve caught some aperture envy!

Looking online i started developing an interest for refractors. Not exactly sure why, but something draws me to their form factor and it might be due to the stories about crisp, contrasty views you’ll only get with a high end APO triplet refractor. Visually as wel as for AP.

I’m very aware that larger dobsons deliver the best bang for buck views and aperture possibilities. Their big aperture lends well for viewing both DSO and planets and the dob mount lends itself perfectly for easy visual observing. But when i read high-end APO triplet refractor (~6k $) product descriptions, it tells me they are capable of also showing details in faint DSO’s and sharp images of planets at very high magnification.

Now my question is: (And forget about the cost for a minute. I realize it can costs thousands on top of the scope for a big refractor) Purely visually. What will be the difference looking at planets and dso’s through a 6k, let’s say 130-140mm, APO refractor compared to a good 12-16” dobson?

I only observe visually so far, but might take up AP in the future.

Best,

Ruben

I have owned six 152mm refractors, two f/8 achromats (not recommended for planetary), a Meade 152ED f/9, and an Astro Physics 6" f/8.

 

None of them were nearly as competent on any class of object as my 12" dob.  Under my light polluted skies, the 6" refractors lacked the limiting magnitude to do very well on Globular Clusters and open clusters (Caroline's Rose is far better in a 12" than a 6" anything) and had far to small of an image scale (at the same exit pupil) as the 12". 

 

On Planetary Nebula, once again, to get the image scale, the image gets very dim in a 6" anything.  Subjects like the Eskimo or the Saturn nebula are much more gratifying in a 12".

 

You can see small, low surface brigntess galaxies in a 6" Apo, but once again, if you use enough power to make them big, the larger scope has a major advantage and in most cases, galaxies will show more extension and more structure in a 12" than in a 6" Apo.

 

Planets are far better in a 12" when seeing permits which is not a rare thing, though it is not a common thing either. I have had the best planetary results in my 35 years of observing with a 12", and I have owned scopes up to a C14.

 

When you factor in the mount required to hold these big, long telescopes steady at high power, the total effort to use one can be as much as that required to run a C14 (and I owned both of these scopes at the same time, so made numerous comparisons and in the end concluded that the 6" Apo was too much trouble for the aperture it provided.)

 

This being the refractor forum and everything, I don't plan on getting into any battles that can't be won, but my experience is that even a 10" reflector with good mirrors is a better visual scope than a 6" Apo.  You just see more on just about every kind of object with it....


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#24 Urbyz

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Posted 24 September 2020 - 03:37 PM

Yep. IF you have to drive to dark sites, how transportable is your Big Dob? I can rest my 5" F5 frac on the passenger seat of my Mustang, put the VersaGo in the trunk, and loading / unloading is no sweat. And, since it's a Mustang, I can get to the dark site... faster...


Haha if that isn’t a flex I don’t know what is ;) We do live in very polluted skies actually. Looking at the LP world map it almost looks like one of the worst in the world.. Mid-west of the Netherlands. People are making me believe a big dob wouldn’t fare very well over here.

#25 Mitrovarr

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Posted 24 September 2020 - 03:38 PM

With dob, up to 10" isn't bad to transport. At 12" you start to need a truss tube, which is more of a hassle. 16" and above gets heavy and hard to fit in vehicles.
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