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Of coma & 120mm ED refractors

reflector refractor
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#1 CollinofAlabama

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Posted 07 May 2018 - 12:30 PM

Coma is essentially negligible at F/8.  It's there, and can be seen in my 2" widefield eyepieces, but it's very muted, even compared to my F/6.24 8" GSO, to say nothing of the multitude of F/5 and faster mirrors out there.  The SkyWatcher 6" traditional dobsonian makes a nice lightweight alternative when I want something quick to setup, but with enough aperture to wow people on the planets and such.  The SW6 makes owning a 5" refractor obsolete, in my opinion, all while providing the great dobsonian stability that handles the West Texas winds so much better than anybody's refractor that's not in an observatory, or using a mount that's ridiculously heavy and expensive (to say nothing of the accompanying 120mm+ ED glass tube).  Yes, it will have less thermal stability, like all reflectors compared to refractors, but that's a problem one might resolve with a cross-mirror fan, and would be a whole lot cheaper to implement than a big mount, ED glass, etc, without affecting general portability very much.
 
I like reflectors, and especially dobsonians, for their ease of setup and use.  I have always preferred the eyepiece-at-the-top-&-angled kind of design ergonomically, and the general dobsonian design, with the weight at the bottom of the tube, cannot be emphasized enough how wind resistant it is compared to a refractor's flying in the air like a flag setup.  This comparative difference was demonstrated to me Saturday night, when I had out my SW6" for its maiden sky-voyage and an often-used Kunming 102mm F/7 refractor on the GSO SkyView Deluxe Alt-Az mount.  Although it wasn't very windy that evening, we still had some, and every wind produced a light dance in the refractor, and only a little wiggle in the dobsonian, which dissipated much, much more quickly than the spasmodic gyrations of the image in the refractor.
 
I will have to decide if I want to sell my most excellent Z8 and replace it with a GSO-10"-dob-and-Coma-Corrector or not.  That's a different story, and would involve comparing dobsonian performance to 6"+plus refractors, which are, to my line of thinking, insane and off the table, cost and mount options considered.  But I believe the stories I've read here on CN, that a 6" reflector can keep up, visually, with 5" ED refractors.  I've seen for myself how much better Saturn appeared one evening long, long ago, in a LB8" dob compared to an Orion 120mm ED scope.  No comparison, really, the 8"-er was that much better, so I'm sure one would have to move into the refractor stratosphere to continue competing with dobsonians above 6", and why I'd never own such a refractor.  But the SW6, especially if I can upgrade the rather inferior Synta 2" rack and pinion it comes with, puts all the performance of a 120-127mm refractor into an easier to manage, more stable package, at a fraction of the cost.

Edited by CollinofAlabama, 07 May 2018 - 09:57 PM.

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#2 gwlee

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Posted 11 May 2018 - 01:11 PM

Coma is essentially negligible at F/8.  It's there, and can be seen in my 2" widefield eyepieces, but it's very muted, even compared to my F/6.24 8" GSO, to say nothing of the multitude of F/5 and faster mirrors out there.  The SkyWatcher 6" traditional dobsonian makes a nice lightweight alternative when I want something quick to setup, but with enough aperture to wow people on the planets and such.  The SW6 makes owning a 5" refractor obsolete, in my opinion, all while providing the great dobsonian stability that handles the West Texas winds so much better than anybody's refractor that's not in an observatory, or using a mount that's ridiculously heavy and expensive (to say nothing of the accompanying 120mm+ ED glass tube).  Yes, it will have less thermal stability, like all reflectors compared to refractors, but that's a problem one might resolve with a cross-mirror fan, and would be a whole lot cheaper to implement than a big mount, ED glass, etc, without affecting general portability very much.
 
I like reflectors, and especially dobsonians, for their ease of setup and use.  I have always preferred the eyepiece-at-the-top-&-angled kind of design ergonomically, and the general dobsonian design, with the weight at the bottom of the tube, cannot be emphasized enough how wind resistant it is compared to a refractor's flying in the air like a flag setup.  This comparative difference was demonstrated to me Saturday night, when I had out my SW6" for its maiden sky-voyage and an often-used Kunming 102mm F/7 refractor on the GSO SkyView Deluxe Alt-Az mount.  Although it wasn't very windy that evening, we still had some, and every wind produced a light dance in the refractor, and only a little wiggle in the dobsonian, which dissipated much, much more quickly than the spasmodic gyrations of the image in the refractor.
 
I will have to decide if I want to sell my most excellent Z8 and replace it with a GSO-10"-dob-and-Coma-Corrector or not.  That's a different story, and would involve comparing dobsonian performance to 6"+plus refractors, which are, to my line of thinking, insane and off the table, cost and mount options considered.  But I believe the stories I've read here on CN, that a 6" reflector can keep up, visually, with 5" ED refractors.  I've seen for myself how much better Saturn appeared one evening long, long ago, in a LB8" dob compared to an Orion 120mm ED scope.  No comparison, really, the 8"-er was that much better, so I'm sure one would have to move into the refractor stratosphere to continue competing with dobsonians above 6", and why I'd never own such a refractor.  But the SW6, especially if I can upgrade the rather inferior Synta 2" rack and pinion it comes with, puts all the performance of a 120-127mm refractor into an easier to manage, more stable package, at a fraction of the cost.

My 34#, Orion XT6, 6”f8 Dob is my most used telescope because it strikes the best balance between aperture for optical performance and the portability required for tree dodging at this site. I found that it easily bested the optical performance of a premium 4” APO while equaling its portability. Have not compared it to a 5” APO. 

 

However, the SW6 traditional dob that I bought weighed as much as my Orion XT8 though, so I returned it immediately. 



#3 Eddgie

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Posted 11 May 2018 - 04:27 PM

A top quality 10" reflector is an awesome planetary scope and difficult to beat with any refractor costing less than $100,000

 

Now the operative words here are "top quality."

 

The only issue with a 10" GSO or similar scopes is that the optical quality is variable and even the best ones will fall short of a scope with an excellent premium mirror.

 

If planets are a very high priority target for you, then what I would recommend would be to look for a used scope with a Zambuto or other high quality mirror.  Now it will cost more, but these scopes made by people like Starmaster typically are far better made than something like the GSO, much lighter than something like the GSO, and will generally have movement in the alt and az planes that is buttery smooth.

 

Failing that, if you buy the 10" GSO, find a mirror service that can touch up the mirror to a high standard (if it needs it.  Some of these mirrors will be quite good).  Now the issue with this approach is simply that by the time you buy the scope and have the mirror refinished, you may not be that far behind the price of a used Starmaster, and you will see the pain when you go to re-sell it, where the premium mirror scope will hold pretty much all of the value in dollars you spent on it.

 

Two paths to get to outstaning planetary performace though, and if the mirror is excellent, it is difficult to do much better than a 10" reflector because to do better with a larger aperture, seeing needs to be perfect and the bigger you go, the more that matters.  10" is a superb size for serious planetary because for many observers, there are a decent number of nights a year where a 10" scope will work near its full potential.


Edited by Eddgie, 11 May 2018 - 04:33 PM.

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#4 Eddgie

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Posted 11 May 2018 - 04:31 PM

This is sold, but it is an example of  what you can find with patience:

 

https://www.cloudyni...or-plus-extras/

 

Stick one of these in a used Craigslist dob and get ready for the oohs and ahhhs.


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#5 Richard Whalen

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Posted 11 May 2018 - 05:05 PM

A top quality 10" reflector is an awesome planetary scope and difficult to beat with any refractor costing less than $100,000

 

Now the operative words here are "top quality."

 

The only issue with a 10" GSO or similar scopes is that the optical quality is variable and even the best ones will fall short of a scope with an excellent premium mirror.

 

If planets are a very high priority target for you, then what I would recommend would be to look for a used scope with a Zambuto or other high quality mirror.  Now it will cost more, but these scopes made by people like Starmaster typically are far better made than something like the GSO, much lighter than something like the GSO, and will generally have movement in the alt and az planes that is buttery smooth.

 

Failing that, if you buy the 10" GSO, find a mirror service that can touch up the mirror to a high standard (if it needs it.  Some of these mirrors will be quite good).  Now the issue with this approach is simply that by the time you buy the scope and have the mirror refinished, you may not be that far behind the price of a used Starmaster, and you will see the pain when you go to re-sell it, where the premium mirror scope will hold pretty much all of the value in dollars you spent on it.

 

Two paths to get to outstaning planetary performace though, and if the mirror is excellent, it is difficult to do much better than a 10" reflector because to do better with a larger aperture, seeing needs to be perfect and the bigger you go, the more that matters.  10" is a superb size for serious planetary because for many observers, there are a decent number of nights a year where a 10" scope will work near its full potential.

I can think of a lot of scopes that will out perform a 10" newtonian for way less than 100k including refractors.

You can buy a 10" TEC for 50k, most 7 to 9" high quality apos will do better, as will 8 to 10" high quality MCTs and MNTs.

Not that I dont like 10" Newtonians, I do. But lets be realistic......


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#6 Jon Isaacs

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Posted 12 May 2018 - 02:33 AM

I can think of a lot of scopes that will out perform a 10" newtonian for way less than 100k including refractors.

You can buy a 10" TEC for 50k, most 7 to 9" high quality apos will do better, as will 8 to 10" high quality MCTs and MNTs.

Not that I dont like 10" Newtonians, I do. But lets be realistic......

 

I agree. 

 

Let's be realistic , Ed's $100,000 refractor comment was hyperbole and needs to taken as such. 

 

Realistically,  in a thread discussing telescopes costing well under $1000, $50,000 scopes, 7-9 inch apos, 8-10 inch MNT's are as unrealistic as a $100,000 telescope.. Collin ain't buying one.

 

Jon


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#7 SteveG

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Posted 12 May 2018 - 02:47 AM

My main scope is a 10" f5 with very good optics.

 

My grab and go scope is the ED 120. I love having both!


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#8 CollinofAlabama

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Posted 12 May 2018 - 11:28 AM

Steve, can't argue with that combination, other than to say that, for portability and cost, a 6" dob will completely keep up with your 120mm ED scope.  That said, the 120mm ED scope will have a big advantage thermal-equilibrium-wise, and at your latitude, that can be an overriding concern.  Just the same, judicious installation of fans might make it's otherwise large advantage in that department somewhat minor.  But in my part of the country, wind is a bigger and more consistent obstacle even than fluctuating temperatures (tho we have that, too).  Summer's already started here, so thermal issues are gone until October.

 

Jon Isaacs -- word!

 

GW, couldn't say about the 6" SkyWatcher relative to the Orion XT6, but like the fact that the 2" hole is already cut into the tube (tho I might need to drill two, three or four small ones for a 2" GSO Crayford upgrade).  I can say it's a good deal easier to move about than my Z8.

 

Ed, I might go for some kind of enhanced mirror, but that would only be after I determined it wasn't as good as some of the best ones in our observing group.  We've got one guy with an XT10i that has a simply fantastic mirror.  It's been compared to custom optics already and lacks nothing at the eyepiece.  But I agree about variable quality with mass-produced optics, some are not as good as others (also verified at the eyepiece).


Edited by CollinofAlabama, 12 May 2018 - 11:31 AM.



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