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What is your favorite common-sized Dob (6-12")?

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#1 BlueTrane2028

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Posted 12 July 2021 - 04:26 PM

I've handled/owned quite a few of these things by now and I think I've picked up on a trend, wondering if others have noticed as well.

6" f/8 is always super sharp. 

8" f/6 feels like the 6 but brighter. 

10" f/4.8 feels fussy to focus and the views are bright but soft compared to the 6 and 8.
12" f/5 (I guess technically 4.9)... aperture is hard to say no to.  This is the only of the group that I haven't personally looked through more than two of the type, just one.

So, I put 12" in first, followed by 8" and 6" in a near tie, 10" in last.

Curious what others think.  Were the tens I touched subpar (two were Orion and one was Skywatcher)?  Would a custom mirror solve all ills and spike it toward the top?  Anyone strongly dislike the 12 or anything else I rate fairly high?


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#2 HasAnyoneSeenMyNebula

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Posted 12 July 2021 - 04:37 PM

Interesting question. I currently own a 5” Newtonian and for the price I like it. I’m picking up a newly used10” Orion Astrograph at the end of the month. I know you said Dob; I thought it might be close enough?

#3 wrvond

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Posted 12 July 2021 - 04:50 PM

I'd love a 22" Dob, but that just isn't in the cards for me. I've determined 12.5" is pretty much my physical limit these days. At the rate of decline, it may be 6" before I know it. undecided.gif

I have a 10" Orion that everyone that's used it agrees puts up very good views. I did change the secondary and have made other mods. In fact, the solid tube and primary mirror are about all that are unchanged. In any event I have to rank it higher than 8" or 6" simply because for me aperture rules when it comes to Dobs and I will always opt for the biggest I can handle. 


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#4 gwlee

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Posted 12 July 2021 - 04:54 PM

Although I have used larger dobs and appreciate their greater performance, I have always preferred to own 6”f8 and 8”f6 dobs because they are easier for me to handle. 


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#5 BlueTrane2028

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Posted 12 July 2021 - 05:24 PM

Interesting question. I currently own a 5” Newtonian and for the price I like it. I’m picking up a newly used10” Orion Astrograph at the end of the month. I know you said Dob; I thought it might be close enough?

It's likely the scopes you are referring to are shorter in focal length than the ones I have listed.  I'm specifically referring to the commercially available floor standing rocker box and tube Dobsonians.  For example, Orion makes an XT6, 8, 10 and 12.  Seems that 14"+ is a bit more rare to find randomly on Marketplace, it's usually one of those four smaller sizes that people have when they get a Dob for general use.

I'm definitely OK with discussing others.  I'm thinking 16"+ for my next aperture fever goal...



#6 BlueTrane2028

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Posted 12 July 2021 - 05:25 PM

Although I have used larger dobs and appreciate their greater performance, I have always preferred to own 6”f8 and 8”f6 dobs because they are easier for me to handle. 

I'd rather walk 100 yards with my 8 than my 12 (although it can be done with either).  About the same goes for my SCT setups as well.  C8 is no problem, but the 12"... I pull my truck up to the spot I intend to use, setup the tripod and just shuffle the fork from the tailgate to the tripod.  Not walking with that, haha.


Edited by BlueTrane2028, 12 July 2021 - 05:27 PM.

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#7 firemachine69

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Posted 12 July 2021 - 05:33 PM

I'd rather a 10-11" sct over a dob that size. The price difference just isn't sizeable nowadays. Only reason I'd want a dob would be monster class (16" and up). 


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#8 MeridianStarGazer

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Posted 12 July 2021 - 07:34 PM

It's likely the scopes you are referring to are shorter in focal length than the ones I have listed.  I'm specifically referring to the commercially available floor standing rocker box and tube Dobsonians.  For example, Orion makes an XT6, 8, 10 and 12.  Seems that 14"+ is a bit more rare to find randomly on Marketplace, it's usually one of those four smaller sizes that people have when they get a Dob for general use.

I'm definitely OK with discussing others.  I'm thinking 16"+ for my next aperture fever goal...

Determining ratios is not so simple. How many get sold used might not be the same ratio as how many get bought new.

How many get brought to a dark sky site does not say much. Maybe a small one is not worth bringing, or a big one is too heavy to bring.

Only the manufacturer knows how many are made.

 

 

 

As for views:

6" f8 gave me my best view of Jupiter.

8" definitely resolves more stars in M13

10" takes the stars and makes them direct vision.

 

 

I've not compared 12" to 10" side by side. The 10" is no slouch. The 8" is a good intro, making DSO glow instead of just appear as in a 4.5". But the 10" is where you start to see detail in the DSO.

 

I lifted a 12" once and re-hurt my already injured back. It does pull in lots of light though. I've not tried it or a 10" on the planets.

 

 

As for 6" being sharpest, it does take the least amount of skill to make sharp. But the mirror sells for half as much, too. So maybe the 8" has a good mirror too.

 

I don't know what my favorite is. Weight is an issue though.


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#9 Jon Isaacs

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Posted 12 July 2021 - 07:46 PM

This is the way it works for me:

 

I've owned several 6 inch F/8 and 8 inch F/6 Dobs. Currently I own a 10 inch F/5 GSO that I've had for 18 years, a 12.5 inch F/4.06 that I've had for 20 years and a 13.1 inch I've had for 8 years.. plus 2 larger Dobs.

 

My 10 inch F/5 outperformed the 6 inch and 8 inch Dobs, more planetary detail, near Dawes limit doubles, going deeper on the DSOs. Clean sharp views. I pay careful attention to collimation and thermal equilibrium.. I typically use a Paracorr and Nagler type 6 eyepieces for the planets.

 

It's a sweet spot, biggest tube Dob I'd want to own. The 12.5 inch and 13.1 inch are truss Dobs. The 13.1 inch is F/5.5 with a premium mirror.. Great planetary-double star views.

 

Jon


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#10 areyoukiddingme

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Posted 12 July 2021 - 08:11 PM

Having recently got a new primary for my 12.5", I've been using my 8" F7 as a comparison. 

 

My conclusion so far is that the only reason not to prefer the 12.5" is the extra attention needed to get the thing working to full potential, which in this case is because of the thin primary (19mm) being in a cell that is in a sphere, and that has to move through multiple angles. 

 

My solution of 6 nylon tipped bolts just about touching at the center of gravity is working pretty well, but I'm getting a tiny bit of pinching when at 'side-to-side' angles on the supports.

 

This is hinted at in looking at doubles at high power and planets, but it's very subtle, and diffiult to separate effects of cooling and seeing on the two scopes. So I'm still in the testing phase and have not yet drawn a solid conclusion.

 

Another way of looking at it is that my 8" F7 is a reference standard (also thin quartz, but shows zero signs of any optical gremlins) that is incredibly good, and quick to get up and running (faster than small refactors).


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#11 HasAnyoneSeenMyNebula

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Posted 12 July 2021 - 08:18 PM

This is the way it works for me:

 

I've owned several 6 inch F/8 and 8 inch F/6 Dobs. Currently I own a 10 inch F/5 GSO that I've had for 18 years, a 12.5 inch F/4.06 that I've had for 20 years and a 13.1 inch I've had for 8 years.. plus 2 larger Dobs.

 

My 10 inch F/5 outperformed the 6 inch and 8 inch Dobs, more planetary detail, near Dawes limit doubles, going deeper on the DSOs. Clean sharp views. I pay careful attention to collimation and thermal equilibrium.. I typically use a Paracorr and Nagler type 6 eyepieces for the planets.

 

It's a sweet spot, biggest tube Dob I'd want to own. The 12.5 inch and 13.1 inch are truss Dobs. The 13.1 inch is F/5.5 with a premium mirror.. Great planetary-double star views.

 

Jon

Jon,

If you don’t mind my asking, what is the focal length of the 10”?


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#12 Tom Stock

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Posted 12 July 2021 - 08:35 PM

If the 10 is soft my guess is its out of collimation or not sufficiently cooled.  Could also be a bad cell, pinched mirror, or a variety of other things including secondary or secondary mounting.

 

The 10 should out perform the 8 and 6 although it will be more sensitive to collimation and focus.


Edited by Tom Stock, 12 July 2021 - 08:37 PM.

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#13 Jon Isaacs

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Posted 12 July 2021 - 08:37 PM

Jon,

If you don’t mind my asking, what is the focal length of the 10”?

 

F/5.. It's just a standard GSO Dob manufactured in Taiwan 19 years ago.

 

GSO Dob Base refinished.jpg

 

Jon


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#14 MeridianStarGazer

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Posted 12 July 2021 - 10:05 PM

Having recently got a new primary for my 12.5", I've been using my 8" F7 as a comparison. 

 

My conclusion so far is that the only reason not to prefer the 12.5" is the extra attention needed to get the thing working to full potential, which in this case is because of the thin primary (19mm) being in a cell that is in a sphere, and that has to move through multiple angles. 

 

My solution of 6 nylon tipped bolts just about touching at the center of gravity is working pretty well, but I'm getting a tiny bit of pinching when at 'side-to-side' angles on the supports.

 

This is hinted at in looking at doubles at high power and planets, but it's very subtle, and diffiult to separate effects of cooling and seeing on the two scopes. So I'm still in the testing phase and have not yet drawn a solid conclusion.

 

Another way of looking at it is that my 8" F7 is a reference standard (also thin quartz, but shows zero signs of any optical gremlins) that is incredibly good, and quick to get up and running (faster than small refactors).

Ah, yes. Ball scopes need different mirror cells. One more reason I won't attempt to make one.

 

 

And I agree. Bigger can be better, but it requires getting everything working right. Bigger usually means thicker mirror, which needs more cooling time, and is faster and needs more careful collimation. But get that all right, and bigger shows better.


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#15 25585

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Posted 13 July 2021 - 08:36 AM

My current most used is my 10" F6.  I have a 10 F5 & 12 F5 as well as 8 F6. All solid tube.

 

For travel the 10" F5 or 8" F6 are easier. 10" to me is 12-lite moreso than 8+, bigger difference between 8 & 10, than 10 & 12 for light gathering. Handling of 10" is less painful, not only through weight but physical dimensions. 12" = 14" OTA outer tube diameter & proportionately sized mount, while a 10" = 12" etc.

 

SCTs are different beasts, very long FL, I like wider field viewing than SCTs give. My C8 hardly ever gets used.


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#16 Tom Stock

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Posted 13 July 2021 - 10:44 AM

My favorite size is the biggest one.

 

The most practical one is the 10".  It fits in a car, can be easily carried assembled, shows much of what my 16" can show, and cools down fairly quick.


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#17 xvariablestarx

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Posted 13 July 2021 - 10:52 AM

My favorite Dob is one I can lift and carry ;)


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#18 George N

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Posted 13 July 2021 - 04:13 PM

I've looked thru a lot of 'commercial' Dobs - going back to a very early example of a Coulter 10.1" (owned by a local public observatory, used on many a night there and at home). I've only owned ( until this coming October ) an Obsession 20 Dob (but 3 other Newts) - but the 20" is not in this size range and is a 'premium' Dob.

 

First - my fav of this group - again based on using scopes that belong to others - would be a 10-inch with DSCs and maybe GoTo - probably Sky Watcher's 10 - great views of a zillion deep sky objects, decent field of view - great on planets. Who could want more? << well maybe more lite weight >>

 

I find the 6" and 8" solid tube Dobs backbreakers to observe with - even with a chair. Nice views - but too much bending for a 6 foot guy (great for smaller people, including mid-age kids)

 

I find the 'commercial' 12" too big and heavy to deal with - even tho a 12 is on the borderline between 'normal' and 'big' size and opens up the sky to level that some would call 'serious' observing. It could be a 'life-time telescope' for visual observing - but again - the commercial Dobs in that size are too big and bulky for me to fully enjoy the view. Now make it a 'premium' truss tube 12 - especially of the current 'lite' verity - and I'm fully on board - but that was not the question asked - so it's a 10" for me.


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#19 Bill Jensen

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Posted 13 July 2021 - 06:42 PM

My favorite "now" of that size range is my 10 inch f/5 Teeter STS (tube), given my storage/viewing conditions at home. I have owned both tube and truss dobs in the 8 and 10 inch sizes. The tube dob has its compromises yet works well for my very light polluted back yard, with tons of neighbor's lights of a long row of townhouses.

 

The 10 inch has a premium mirror cell (Aurora precision) along with other premium components. Compared to the Orion 10 inch I briefly owned, the overall ease of use of the Teeter is well worth the extra cost I paid. That said, the optics in the Orion XT10i were actually quite good. Our DC burbs do not have the best seeing, so that plays a role, not just the optics. 

 

Perhaps the OP's "soft" views through a 10 inch set of commercial dobs was due in part to seeing conditions at the time, or the scopes not being carefully collimated, or not reaching ambient temps or a cell that was too tight on the mirror, or a secondary mirror that needed upgrading, or.... multiple factors all at once. 

 

A 12.5 inch premium truss dob is probably the largest I would use at home. Ryan Goodson of NMT just gave an example online of a 14 inch f/4t thin mirrored russ scope that had a 40 pound heaviest component. So I would make an exception for a scope like that! smile.gif Plus his wood work looks amazing. 

 

The 16 inch f/4 that I own that is stored out West is probably my sweet spot, followed now closely by the 14.5 inch Starmaster that I recently bought. Both are feet on the ground scopes, good for darker skies. Have  I tried the 14.5 inch from home? Sure, but it is not practical without a garage and the glare from all the houses. It is also not the OP's list of range of 6-12 inches. 

 

Now my real favorite would be a 20 inch f/3.3 scope, but that is another thread ;-)

 

The 10 inch can be wheeled out to my back deck family easily, and set up for cooling (or now it would be warming up) within minutes. It holds collimation very well, just needing a tweak after checking with the Glatter set of tools. As I get older, keeping it simple is a good thing. 


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#20 vtornado

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Posted 13 July 2021 - 09:59 PM

That is a hard question, especially if we are talking about stock dobs.

 

I prefer an 8 over a 6 due to increased light capture and it is not "much" heavier and larger.

I had a 10 and traded it for a 12 to get 1mm exit pupil at 300x.   I have to admit that sometimes the 12

is too much of a hassle to take out.  Access to open sky means I have to take it across a large, bumpy lawn or pack it up in the car.

I keep it in my garage, but my garage is insulated and can be 20 degrees warmer or colder than the outside air.  On those days cooling takes planning.

I have the skywatcher flex tube so it fits in my car and makes it easier to carry due to shorter tube.

Best would be two dobs an 8 and 12.  So if limited to one dob, that means a 10 is best???


Edited by vtornado, 14 July 2021 - 10:40 AM.

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#21 areyoukiddingme

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Posted 13 July 2021 - 11:44 PM

Ah, yes. Ball scopes need different mirror cells. One more reason I won't attempt to make one.

 

 

And I agree. Bigger can be better, but it requires getting everything working right. Bigger usually means thicker mirror, which needs more cooling time, and is faster and needs more careful collimation. But get that all right, and bigger shows better.

Well, maybe you should. . . I filed the nylon tips on my side supports a little, sanded smooth . . . now my primary has a tiny bit of wiggle room in all directions that it did not before.

 

Tested on the double double in Lyra just now, got intermittently good seeing, and ended up running out of options for more power at 723x. The stars danced, but I saw no evidence of astigmatism, so the cell is working suprisingly well.

 

I still want to do more testing on the planets at different angles when the chance arises, but this is looking very promising, and it was surprisingly easy to do. I also tried out a boundary layer fan rather than a fan running on the back. And I also tried out a shroud I made out of reflectix with black flocking stuck to the inside.

 

More experimenting will be needed, but I didn't get the impression that cooling was a problem either.

 

Back to the topic at hand . . . if this behavior continues, the 12.5" will be preferred to my essentially perfect 8"


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#22 havasman

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Posted 14 July 2021 - 01:03 AM

I had a 5" tabletop Dob, a OneSky. I have an XT10i and two premium truss Dobs, 12.5" f4.7 and 16" f4.49. That's probably too many Dobs for my needs but that's another matter.

 

To me the sweet spot in the Orion (and similar) lines is the 10" tube Dob because they're relatively lightweight, compact, powerful and simple. The bases of larger mass-market scopes I have dealt with start getting heavy in apertures over 10".

 

The best scope to use is the 16" Starmaster. The 12.5" is a tradeoff I haven't had that long and I'm still coming to terms with it, but slowly. 

 

Even the 5" made a powerful observing tool. The 10" is much more so and the grasp of the aperture always keeps increasing. So you just have to decide how much physical and $$$ burden you want to bear and you have your personal sweet spot. But you should not be confused about the advantages of aperture or about the burden of large mass market scope structures.


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#23 seasparky89

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Posted 14 July 2021 - 05:50 PM

I have 3 dobs, split between my 2 homes; 10”, 14” and 15”.  The one used most is my 10” by far.  It is a great do-it-all scope for visual.  Relatively easy to transport, even in my Accord sedan.  I never run out of things to see with the 10”.  Of course, the other larger scopes will show more and brighter objects, but I always weigh the logistics vs performance when I travel to darker sites.  All 3 have DSCs and EQ platforms.  But as I get older, that 10” gets more and more used.  Oh, the views in that 10” are very sharp.


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#24 clusterbuster

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Posted 20 July 2021 - 09:30 AM

I've handled/owned quite a few of these things by now and I think I've picked up on a trend, wondering if others have noticed as well.

6" f/8 is always super sharp. 

8" f/6 feels like the 6 but brighter. 

10" f/4.8 feels fussy to focus and the views are bright but soft compared to the 6 and 8.
12" f/5 (I guess technically 4.9)... aperture is hard to say no to.  This is the only of the group that I haven't personally looked through more than two of the type, just one.

So, I put 12" in first, followed by 8" and 6" in a near tie, 10" in last.

Curious what others think.  Were the tens I touched subpar (two were Orion and one was Skywatcher)?  Would a custom mirror solve all ills and spike it toward the top?  Anyone strongly dislike the 12 or anything else I rate fairly high?

I absolutely love my Z12, when I first got it, I had no idea that the whole Telescope was going to be so good, OPTICS, MECHANICAL MOVEMENTS, EASE OF USE....

I was pleasantly surprised. Jupiter and Saturn are STUNNING in this Dob.

(that is why I branded it as a TAKAHASHI,(lol)).

 Mark


Edited by clusterbuster, 20 July 2021 - 09:32 AM.


#25 Bean614

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Posted 20 July 2021 - 08:36 PM

Jon,

If you don’t mind my asking, what is the focal length of the 10”?

Jon answered your question, but also answered it in his first post, when he stated he had a 10 inch, f/5.  It appears you might be unaware that the Focal Length divided by the Aperture Diameter equals the Focal Ratio, which in this case is f/5.

  So, if you KNOW the Diameter, and you KNOW the Focal Ratio, then you KNOW the Focal Length! (Multiply FR x D = FL).


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