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Visual observing: big APO refractor VS. big Dobson

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#101 gwlee

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Posted 27 September 2020 - 06:40 PM

To be truly honest. The thread has turned out to be exactly what i believe confuses beginners looking to find what scope suits them best. But this time around it was slightly different, not as confusing, since i saw the whole discussion unfold.

The last couple of posts is where it gets confusing. Different experienced users claiming opposite to each other. If it weren’t for the start of this topic, where posts felt more balanced and objective, i still would’ve been left with confusion. The take-away for me is, even though some might back their claims with research, that the choice between a big dob or refractor depends greatly on seeing conditions, quality of equipment, the objects being observed and maybe most leading of all: personal preference..

What seems to be agreed upon at least, is that dobs can achieve the same or at least close to performance as the refractors for a fraction of the price. Though be it the views would be slightly different due to their differences in design.

All in all I don’t think there is any good or bad choice to be made and it all comes down to what works for you. It kind of stinks i’m not able to “see for myself” as easily these days. But i think a reasonably sized dob, let’s day 12”, companioned by a high end 5” APO would be a nice versatile couple to have. Separate as wel some might say.

Price always plays a factor and i’m not in a situation where i can easliy finance an APO 6” or above. So the claims of 7” do-it-all APO’s don’t apply to me. Also portability wise it’s a no go. As i said earlier: it’ll probably come down to personal preference and what you like to see and consider nice views.

If we’re being real i’ll probably save up for a nice 5” APO and buy a 10 or 12” dob not long after because i’ll wonder what views i’m missing.. or vice versa.

Thank you all for the great insights and please add to the topic if you feel like there’s more to add.

Best,

Ruben

I suggest spending a few hundred dollars to buy an inexpensive, good quality 80mm ED scope and using it for a couple years. It will give you the personal, hands-on experience to evaluate the many different opinions given here, and an 80mm doublet is often a handy, lifetime scope for people who own big dobs and big refractors. 


Edited by gwlee, 27 September 2020 - 06:41 PM.

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#102 John Fitzgerald

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Posted 27 September 2020 - 06:49 PM

In retrospect, about 10 to 12 years ago, I should have just gone for a 300mm f/9 apo in a dome, and been done with most astro purchases for a lifetime.   If we had not bought this house.... lol.gif cool.gif


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#103 gnowellsct

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Posted 27 September 2020 - 07:00 PM

Neither is really "practical" for most skygazers. 

 

A which is most practical would be tabletop dob vs small ed refractor.

I don't like tabletops.  They tend to be unsteady or too low to the ground.   I wouldn't mind one that was higher up on a reasonable tripod.  I like the Vixen RS130 for this reason.

 

I think an ED refractor is a good investment if one puts it on a good mount.  But I add that qualifier to everything.  

 

Greg N



#104 gnowellsct

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Posted 27 September 2020 - 07:05 PM

OP, out my 50+ years in the hobby, I was a Refractor Snob for the first 44 years.  Then I bought a used 1971 Criterion RV-6... that one Old Newt changed this hobby for me.  A good thing, because that Orion XT12g almost turned me off reflectors... what a big floppy heavy mess it was.  Yes, it pulled in more faint fuzzies, but high-power planetary on that AZ platform?  Nope.  I prefer GEMs for serious observing (30 minutes or more studying 1 object).  So, I meant it about my 1980s Meade 826 -- it does everything well.  It's easy to set up, take down, and use -- easier than my much more $$$$ 2017 APM 152ED.  (In fact, I'm thinking of selling the Big ED.)  A light 8" F6 Newt on a solid GEM with good visual tracking up to 400x... what's not to love??

 

In just a few years, I went from 0 Reflectors to 5... 3 Newts + 2 Casses.  Very happy camper.  Not turning in my Refractor Fan Membership Card, but I'm enjoying this mix of lenses & mirrors.  Just not this one...

 

attachicon.gifOrion XT12g - First Setup S02.jpg

Yeah I really much prefer GEMs.  Doesn't change the fact that two Skywatcher dobs (10" and 12") beat the pants off my 5" apo one year.  Even with the tappa tappa tappa to keep Jupiter in the field of view, it was a blow out.  Skywatcher 2, Astro-physics zero on that particular object on that particular night.  

 

But the 130mm is a scope I really like and I wouldn't trade it for a 12" SW dob.  

 

Greg N


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#105 CHASLX200

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Posted 27 September 2020 - 07:07 PM

If someone wants a good 6" Refractor then the Sky watcher 6" ED would be a top pick on price and bang for buck. But will need a big mount G11 or bigger.  That is where we get into the money part as big and good mounts are not cheap.  I had a AP 800 mount for my SW150ED. But it was a 6 or 7 trip scope to set up since my house is not set up to roll it out all set up.


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#106 Jon Isaacs

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Posted 27 September 2020 - 07:23 PM

People should read the complete article linked. Skip to the last 8 paragraphs for somewhat of a summary.  HERE is the link.

 

Using the word “sacred text” is not only laughable but is a complete misrepresentation of this comprehensive article with studies done at professional observatories over many years. How seeing is evaluated by professionals and the “probabilities” derived are of interest. I have not even quoted some of the more interesting findings.

 

Have fun.

 

Bob

 

Observatories are not practicing visual astronomy.

 

Jon


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#107 gnowellsct

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Posted 27 September 2020 - 07:31 PM

 

 

There is an place for aesthetics but not at the expense of seeing detail.  The 25 inch F/5 with an aperture mask could be a 10 inch F/12.5 with no CO. 

 

jon 

OMG Jon don't say it.  I knew someone with a 25 inch.  I can't go there.  and he sold his.  And you sold yours.  I get tired just thinking about setting it up and standing on a ladder all night then packing it up again.


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#108 Jon Isaacs

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Posted 27 September 2020 - 07:34 PM

Splitting doubles I would also vastly prefer the refractor. Much easier to make out even small elongations of really right pairs under most conditions with the 7” the larger aperture scopes I had access to,

 

 

Something ain't right. The Dawes limit for a 7 inch is 0.65", the Rayleigh criterion, 0.79".  These are both difficult because the Airy disks are overlapping. With a 10 inch, they're wider, with my 13.1 inch, they're wide.. of course seeing allowing. 

 

When you say "larger scopes you had access to", these were not scopes you owned and setup?

 

There's no doubt Newtonians require careful attention and looking through someone else's scope is rarely a good way to evaluate the capabilities of a scope, refractor or reflector. 

 

Jon



#109 Mitrovarr

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Posted 27 September 2020 - 07:37 PM

Big Dob mounts are as simple as small Dob mounts, it doesn't get any simpler.

In my experience, Newtonians on GEMs are not such a great idea. A GEM is far more complex than any DOB mount and the observer faces a number of issues. The biggest is the eyepiece position. As the scope moves about the sky, the tube rotates. The eyepiece ends up in the wrong place and it's necessary to rotate the tube. This makes "rotating rings" necessary. They work OK but it's easy to lose alignment.

In general, a larger Newtonian on a GEM is a real beast. Heavy, awkward, time consuming to setup. The Dobsonian really did revolutionize the Newtonian.

Jon


Another thing is, once you are ready to spend the kind of big $$$ it would take to get a decent large GEM for a newtonian and good quality rotating rings (very spendy) you could just get a motorized dobsonian mount.
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#110 Jon Isaacs

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Posted 27 September 2020 - 08:23 PM

My location is frequently 1.8 to 2.0 arcsec seeing, at least according to: 

 

https://www.goodtostargaze.com/

 

Tonight's prediction is 1.8 to 2.1 at different hours. For comparison San Diego is .6 to .8 (lucky buggers!).

 

These are, of course, average estimates. So even if a 10" Dob has a Dawes limit of about .5 arcseconds, my bet is that where I live there are moments and little 20-30s runs on the seeing that would get me close to that resolution.

 

Given that planetary viewing is a patience game, I'm more than happy to wait for those moments.

 

 

Thank you for posting that link. I like it.. 

 

Jon



#111 gnowellsct

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Posted 27 September 2020 - 08:23 PM

Another thing is, once you are ready to spend the kind of big $$$ it would take to get a decent large GEM for a newtonian and good quality rotating rings (very spendy) you could just get a motorized dobsonian mount.

You would definitely want the motorized Dob over the Newt on a GEM.  I did that experiment.  Had the Newt on its native 1960s mount, and then on the AP900, and also tried it out on the G11.  I was surprised that it did very well on the G11, and used that several times because, being smaller than the AP900, it reduced the overall (impossible) chore of setting up and operating a Newt on a GEM.  And this wasn't a very big Newt: only a 10" f/6.  Big enough to be a star party king in the 1960s but not much now.

 

My only observation about the three or four servo equipped dobs I've seen is that they handle very clumsily in comparison to a GEM.  They do the job but one has to modify one's expectations.  The object doesn't always stay centered at high powers, and there is backlash/delay between pressing the control and getting the thing to do what you want, and a tendency to overshoot.  I suppose you get better as you work with it but when I was taking in views on these things I would ask the host to do the adjustments.  Using a paddle is second nature to me but not on that kind of system.  And I may never learn these systems because the people who were bringing them into my life have gotten GEM mounted SCTs.  :(  

 

A good star party needs a variety of well functioning kit.  I mean, you know, if I want to look through an SCT on a GEM I can furnish that experience myself.  

 

Greg N    


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#112 Peter Natscher

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Posted 27 September 2020 - 09:14 PM

A 8" APO is basically not transportable at 60lbs+ along with 150lbs.+ of mount, counterweights and pier. if that is important.  Plan to have an observatory for it and that adds to the expense.  A 16" Dob is still very transportable in a small to mid-size SUV.  This allows the Dob to be used at better observing sites.

 

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

 



#113 Mitrovarr

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Posted 27 September 2020 - 09:29 PM


A good star party needs a variety of well functioning kit.  I mean, you know, if I want to look through an SCT on a GEM I can furnish that experience myself.  

 

Greg N    

For all the disadvantages to SCTs I find myself using them more than any other kind of scope due to the sheer practicality.


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#114 bobhen

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Posted 28 September 2020 - 07:35 AM

Observatories are not practicing visual astronomy.

 

Jon

Along with the general takeaway for the visual astronomer that if seeing is as indicated at some observatories that have exceptional seeing, how might that correlate to the typical seeing in my backyard? The article also addresses issues that are of interest to the visual astronomer.

 

Here is just one such quote from the article…

 

"As a simplification most useful to the visual astronomer, the optical effects of this turbulence layer can be contrasted as three types of distortion in the star diffraction artifact (diagram at right)."

 

Bob



#115 Bomber Bob

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Posted 28 September 2020 - 08:05 AM

You do any real side by sides with my 826 you bought and that 6" ED?  By heck that 826 Meade ran neck and neck with a 6" APM and Skywatcher 150ED.

Yes, after I upgraded the 826 focuser to the Lumicon helical, I put it up against my APM152ED and saw for myself with each scope on a StarFinder EQ.  The ED can go 75x per inch, and the Galilean are sharper disks; and, it presents tight doubles with better clarity.  But, the 826 shows more belt colors on Jupiter & Saturn, more belt to zone intrusions, and overall both disks are brighter.  For DSOs the 826 pulls way ahead.  $300 investment vs. $3000...



#116 Bomber Bob

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Posted 28 September 2020 - 09:31 AM

‘If we’re being real i’ll probably save up for a nice 5” APO and buy a 10 or 12” dob not long after because i’ll wonder what views i’m missing.. or vice versa.”
 

That’s a good common sense solution that I went with — except no DOB mount, I prefer EQ.  I have a mount that’ll carry a 10” / 12” fast Newt, but I would not be hauling one that large out to the country...

 

These type threads get messy because of the scope type comparisons, and the differences between experienced observers.  And, we have to try & figure out your observing plans, seeing, logistics, etc. that you may or may not have considered.  If we didn’t care about this hobby, we wouldn’t be here, so it’s a mixed blessing...



#117 turtle86

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Posted 28 September 2020 - 09:51 AM

Thank you for posting that link. I like it.. 

 

Jon

 

Me too.  The app is on my iphone now.



#118 DeanS

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Posted 28 September 2020 - 09:55 AM

Bottom line, eventually get one of each, or two of each, or three.............:)


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#119 turtle86

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Posted 28 September 2020 - 10:00 AM

Yeah I really much prefer GEMs.  Doesn't change the fact that two Skywatcher dobs (10" and 12") beat the pants off my 5" apo one year.  Even with the tappa tappa tappa to keep Jupiter in the field of view, it was a blow out.  Skywatcher 2, Astro-physics zero on that particular object on that particular night.  

 

But the 130mm is a scope I really like and I wouldn't trade it for a 12" SW dob.  

 

Greg N

 

The few times I've had my 12.5" Starmaster and AP 130 GT out at the same time, the Starmaster has always won on planets.  The AP 130 GT sure isn't going anywhere though.


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#120 DeanS

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Posted 28 September 2020 - 10:30 AM

The few times I've had my 12.5" Starmaster and AP 130 GT out at the same time, the Starmaster has always won on planets.  The AP 130 GT sure isn't going anywhere though.

Hard to beat a Zambuto mirror ;)


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#121 DeanS

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Posted 28 September 2020 - 11:08 AM

The best of both worlds wink.gif

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#122 Bomber Bob

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Posted 28 September 2020 - 11:43 AM

I like the wheel-barrow solution!   IF I ran a fever, and bought a 12" or larger DOB, it would have to have an all-manual mount with real steel bearings.  No cheap plastic gears.  No Go-To.  Smooth alt/az with adjustable friction -- sorta like me & my Dad made for that 8" F4.5 Newtonian that I ground & polished the mirror for back in 1979...  Only optics I've ever made myself... NOT anywhere near Zambuto quality!

 

The craptastic plastic gearing on that Orion XT12g went out shortly after the warranty period -- How Convenient -- which soured me on this type mount for good.

 

------------  HOW THESE TYPE THREADS GO  --------------

 

Refractor Fans:  My Tak FSQ-MOUSE 100XD$$ whooped an Apertura-fell-off-the-truck 30" DOB.

 

Reflector Fans:  My $6K Obsession 24" with signed mirrors whooped an ES FL-102AR.

 

SCT Fans:  My C8... is still temperature adapting... But, once it's ready, watch out!!

 

MCT Fans:  We're outside observing, y'all carry on...


Edited by Bomber Bob, 28 September 2020 - 11:56 AM.

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#123 gnowellsct

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Posted 28 September 2020 - 11:58 AM

A 8" APO is basically not transportable at 60lbs+ along with 150lbs.+ of mount, counterweights and pier. if that is important.  Plan to have an observatory for it and that adds to the expense.  A 16" Dob is still very transportable in a small to mid-size SUV.  This allows the Dob to be used at better observing sites.

Well I think the 8 inch APO is unwieldy and I would be hard pressed to put one into my Accord.  But on the other side of this argument I regularly transport 150 lbs of mount, counterweights, and pier.  Mount: two sections of 15 lbs each, counterweights, 18 lbs each, counterweight shaft, 20 lbs, ATS pier (looks big but all aluminum) 30 lbs, OTA C14 45 lbs, battery 50 lbs (that's the worst part of my rig), apo 12 lbs, eyepiece box 30 lbs, observing and two folding chairs, 30 lbs = 30+54+20+ 30+45+50+12+30+30 = ~300 lbs, though I suspect I overestimated a few items and would peg it at 250 lbs.  

 

It's my favorite set up but not when the observing window is short or the forecast is iffy.  It is definitely my wife's favorite setup.   It is so nice to have a spouse who likes astronomy I try to cater to her preferences.  

 

The reason the APO is a problem is that it would likely take up two seats, as I found out when I transported a 10" f/6 Newt.  I had to put the front passenger seat down and slide the tube in from the rear passenger door.  That would leave no room for wife and dog.   The stupid Accord designers decided to save ten bucks by not making the rear seat split 60/40, either the whole thing goes down or it stays up.  But even if it was 60/40 there still would be a problem of a lost seat and lost trunk space, causing some items from trunk to have to move into the rear seat and now room for wife but no dog.  

 

But with a C14 it all works out for wife and dog and a rig worthy of the Normandy invasion.  And with great performance capabilities.  smile.gif

 

Greg N

 

C14+CFF at Chimney mountain 6-20.jpg


Edited by gnowellsct, 28 September 2020 - 12:18 PM.

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#124 gnowellsct

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Posted 28 September 2020 - 12:03 PM

The craptastic plastic gearing on that Orion XT12g went out shortly after the warranty period -- How Convenient -- which soured me on this type mount for good.

 

 

 

I had a very regrettable experience with a craptastic CG5 and have stayed off mainland exports as much as possible ever since.  Still, some of the optics are pretty good.  It is difficult to pick out which will be which.  I think the servoCAT dob systems are pretty reliable.  But they do add considerable complexity to set up.

 

GN



#125 gnowellsct

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Posted 28 September 2020 - 12:21 PM

Hard to beat a Zambuto mirror wink.gif

Twelve inches of Synta mirror (that's SW right?) is apparently more than enough.  Zambuto is very good, but overkill in terms of putting together a mirror that can beat a 5 inch apo.    




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