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Advice on large dob?

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#1 cweinhei

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Posted 26 September 2013 - 10:09 PM

I currently have a 10" Newtonian for imaging and I am thinking of getting a large "portable" truss based dob for visual. I'm considering either a 16" or 18" scope.

The Hubble UL18 scope is really appealing to me since one person can assemble,disassemble and move it. It looks like it stores easily which is another consideration.

I was only able to find one review, so I'm not sure how many of those actually have been sold. Also, there seems to be some debate on quality of the sandwich mirrors produced by Hubble Optics.

Please let me know if you have any advice for a first time large dob buyer.

#2 acochran

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Posted 26 September 2013 - 11:29 PM

If portability/weight/price are your biggest concerns, Hubble UL may be for you. Obsession also makes a UL, more expensive. Generally, the shorter the focal length, the stiffer the telescope body needs to be to hold collimation. If it were me, I'd buy premium quality optics (like Zambuto, etc) and have a light (lower OTA weighing something like 45-50 lbs hopefully) but stiff telescope made for it by someone like Teeter, or Dobstuff, or Astrogoods. Mediocre optics are ok at first, but if they don't perform like you want, like on planets, one starts to crave a better quality scope. The lower OTA of my 16" Lightbridge weighs 65 lbs, the limit of what I can move by myself.
Andy

#3 Jon Isaacs

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Posted 27 September 2013 - 04:39 AM

The Hubble UL18 scope is really appealing to me since one person can assemble,disassemble and move it. It looks like it stores easily which is another consideration.



This is true of just about any 16-18 inch Dob. Heck, it's even true of my 25inch F/5 Obsession. I leave it setup and ready to go but assembling and disassembling it is a one person job..

Jon

#4 Astrojensen

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Posted 27 September 2013 - 04:53 AM

This is true of just about any 16-18 inch Dob.



The lower tube assembly of the 16" Meade Lightbridge is borderline non-portable for quite a few people. The same goes for the Sky-Watcher 16" dobs. The premium makes (Obsession, Moonlight, etc) are much lighter and smaller.


Clear skies!
Thomas, Denmark

#5 Jon Isaacs

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Posted 27 September 2013 - 05:33 AM

This is true of just about any 16-18 inch Dob.



The lower tube assembly of the 16" Meade Lightbridge is borderline non-portable for quite a few people. The same goes for the Sky-Watcher 16" dobs. The premium makes (Obsession, Moonlight, etc) are much lighter and smaller.


Clear skies!
Thomas, Denmark


Probably but with some creativity, solutions can be found.., depending on the problem.

Light weight dobs typically suffer from stiffness issues, a stiff mechanical structure is an advantage, thin tubes, single ring upper assemblies, these make for possible vibration and collimation shift problems.

Jon

#6 kfrederick

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Posted 27 September 2013 - 08:00 AM

Put your money on the optics not the fancy ply-board . Go bigger than you can lift/ You will get stronger lifting it ! Ha Ha

#7 Dick Jacobson

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Posted 27 September 2013 - 08:33 AM

You should also take into account the zenith eyepiece height. Being unable to reach the eyepiece near zenith is a big annoyance. For me (5'11") a 14" f/4.5 is just the right size and a great step up from my previous 10" scope.

Regarding Hubble Optics, a friend ordered a 16" and was very unhappy with the mirror. He had it tested by a local expert who said it was the worst mirror he had ever seen. Hubble replaced it, but my friend is still not very happy with the scope, optically and mechanically.

#8 aatt

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Posted 27 September 2013 - 09:33 AM

Discovery telescopes are optically very good and the truss versions are mechanically decent.If you are thinking about Hubble optics, the Discovery scopes are roughly in that price range. An Obsession will be alot more. Some have problems with delivery times and communciation with the company. My experiences have been good.

#9 csrlice12

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Posted 27 September 2013 - 10:39 AM

If only they made a newt/dob with interchangeable mounts (EQ & Dob alt/az) where swapping it out would be easy to do.....

...also consider if you have enough room in case you want to take both scopes to a dark site....

#10 Jeff Morgan

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Posted 27 September 2013 - 04:04 PM

The Hubble UL18 scope is really appealing to me since one person can assemble,disassemble and move it. It looks like it stores easily which is another consideration.



This is true of just about any 16-18 inch Dob. Heck, it's even true of my 25inch F/5 Obsession. I leave it setup and ready to go but assembling and disassembling it is a one person job..

Jon



True. My 16" Dob can be set up in 10 minutes. Add 5 minutes for ServoCAT 2-star alignment and I'm in business. For perspective, that is about 3X faster than my equatorially mounted 4.7" refractor can be brought into action.

There are two ways to attack weight. I have been playing around with lightweight structures, 1/16" Finnish Birch plywood and balsa wood cores. You can do some really neat things with ordinary woodworking tools and glue.

But from a practical perspective, a pair of low-tech low-cost wheel barrow handles gives you enormous mechanical advantage (Archimedes was right). Once you have that, it really frees you from weight constraints. Suddenly an extra 10 pounds (or more) of glass weight in the mirror box is nothing, from a handling perspective. No need to take a high-dollar gamble on a sandwich mirror.

#11 cweinhei

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Posted 27 September 2013 - 07:28 PM

Someone locally is selling an older 18" F 4.5 Obsession.

http://www.astromart...ified_id=832744

What are the pros and cons of buying an older scope? I think this one was made in 1999.

How modern is the Gen 2 ServoCat GOTO drive system?

#12 Jeff Morgan

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Posted 28 September 2013 - 12:05 AM

Someone locally is selling an older 18" F 4.5 Obsession.

http://www.astromart...ified_id=832744

What are the pros and cons of buying an older scope? I think this one was made in 1999.

How modern is the Gen 2 ServoCat GOTO drive system?


Buying an old scope could be great it you have limited shop capabilities, want it next week, or just want to save some money.

Gen 3 is the current ServoCAT. I'm not familiar with Gen 2, but one of my friends has an early Gen 1, and it soldiers on quite well. Amazing system. Support from Gary Meyers is top-notch.

As to the structure - I would pay attention to any signs of water damage, wear on parts the move/flex (truss clamps), areas that bear a load (drilled holes, including the azimuth pivot) or warping (upper rings). I would largely ignore cosmetic issues - minor dents, scratches, etc. Inevitable on a scope that gets used a lot. Just get a good adjustment on the price ;)

If you're even minimally handy with tools, wood repairs are easy. At some point you may have to refinish it. Not rocket science, but can be tedious and take your scope out of service for a few weeks.

I would be most concerned about the optics. Very little intrinsic value in plywood, as you may note from Dobsonian resale values. The glass is where the value is.

#13 jrbarnett

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Posted 13 October 2013 - 10:38 AM

"What are the pros and cons of buying an older scope?"

It really depends on how well stored, maintained and cared for the scope is. I've seen Obsession stored outside in sheds that looked worse than wooden sailboats left in briny water for a decade. On the other hand, I've seen older Dobs that have been stored inside, and kept maintained (i.e., mirros coated when needed, wood retouched when varnish/clearcoat fails, etc.) that are as proficient as the day they were delivered.

There are a lot of used Dobs out there. Go have a look-see at this one and if it looks thrashed, skip it. With a little patience you'll get a well-loved, great condition Dob for a huge discount off of the new price.

Regards,

Jim

#14 acochran

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Posted 13 October 2013 - 06:11 PM

Someone locally is selling an older 18" F 4.5 Obsession.

http://www.astromart...ified_id=832744

What are the pros and cons of buying an older scope? I think this one was made in 1999.

How modern is the Gen 2 ServoCat GOTO drive system?

I have heard Obsession mirrors are thick. If so they will take more time to come to ambient temperature than a thinner mirror.
Andy

#15 nicknacknock

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Posted 14 October 2013 - 01:53 AM

I have heard Obsession mirrors are thick. If so they will take more time to come to ambient temperature than a thinner mirror.
Andy


Andy,

My observing partner has the same telescope and it only takes one hour or so to reach ambient temperature once he gets the fan going (said fan stays on for the duration of the session).

It's a solid built, keeps collimation during the night and it is a beautiful scope. Heavy to move, but it's the price you pay for aperture.

Nick

#16 Kevdog

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Posted 14 October 2013 - 10:39 AM

Someone locally is selling an older 18" F 4.5 Obsession.

http://www.astromart...ified_id=832744

What are the pros and cons of buying an older scope? I think this one was made in 1999.

How modern is the Gen 2 ServoCat GOTO drive system?


In July I bought a 1997 Obsession and it is in great shape. All the mechanicals are fine and he just had the optics recoated last year. On a scope that old, that's probably your main worry. If they haven't been recoated then they may be due for one soonish (2-4 years or so).

The scope I bought had been upgraded quite a bit. The motion is still buttery smooth (no ServoCat). I feel I got a steal at $2900 for the condition it was in. It doesn't have ArgoNavis either, but has the 8192 encoders installed.

#17 TechPan6415

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Posted 14 October 2013 - 02:09 PM

Regarding Hubble Optics, a friend ordered a 16" and was very unhappy with the mirror. He had it tested by a local expert who said it was the worst mirror he had ever seen. Hubble replaced it, but my friend is still not very happy with the scope, optically and mechanically.


What specifically was the issue that made it the "Worst mirror he had ever seen"?

There are reports of good Hubble mirrors from people on this very site and others who have bought just the mirror and then put it in their own tube assembly, that is why I even considered a UL-16. So far, I am not at all unhappy with the views in my 16", seeing and good collimation makes all the difference in my case really. I still need to do a more conclusive star test though.

#18 GeneT

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Posted 15 October 2013 - 07:56 PM

I recommend taking a look at several 18 inchers--Obsession, Teeter, Star Structure and others. These are all truss type telescopes. I prefer the traditional or classic truss design. There are a lot of options--good options to consider.

#19 Sean Cunneen

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Posted 15 October 2013 - 08:07 PM

I would strongly advise to try any large scope before you commit to buying. I have viewed through only a handful of decent large scopes. Ordering a scope that large new will either be from a mass producer in which case the optics are a very expensive roll of the dice or a premium maker who will need a very long lead time. Buy used and be picky, take your time. After many years in this hobby I have come to suspect that only iffy optics ever come up for sale, hence the mandate to use before you buy!

There is a third option, make it yourself, or get chummy with a glasspusher.

#20 Achernar

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Posted 16 October 2013 - 10:25 AM

I would attend a few regional and or national star parties, and while there try out the large Dobs others will be bringing there. That way you could get direct hands on test drives to see what aperture, focal ratio and design is what you're looking for. A 15 or 16-inch will got into many cars, and for someone who is about six feet tall no ladder will be needed even while looking at the zenith. A 18 or 20-inch needs a six foot step ladder, and a 30-inch required a tall ladder. Larger telescopes might lead to you fearing ladders in the dark. Take into account your age and health and how much climbing you are comfortable with. Even though you can roll most of them, you'll still have to get it into and out of your vehicle, and where you'll be storing it. If you have to keep it in the house as I do, you will find a 16-inch just about takes up a standard doorway with the handles on as you roll it in or out of the house. One reason why I opted for a 15-inch was the fact I am middle aged and do not care to climb ladders in the dark. Or if you have the means, opt for a sub-F/4 Dob to reduce the size of the scope and the height of the eyepiece. Another consideration to think about is whether or not you plan to use digital setting circles. If you do, be aware most, but not all Dobs can be equipped with them.

Taras

#21 dpwoos

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Posted 16 October 2013 - 11:47 AM

I currently have a 10" Newtonian for imaging and I am thinking of getting a large "portable" truss based dob for visual. I'm considering either a 16" or 18" scope.

The Hubble UL18 scope is really appealing to me since one person can assemble,disassemble and move it. It looks like it stores easily which is another consideration.

I was only able to find one review, so I'm not sure how many of those actually have been sold. Also, there seems to be some debate on quality of the sandwich mirrors produced by Hubble Optics.

Please let me know if you have any advice for a first time large dob buyer.


Have you hung out with a dob of this size? Are you confident that you know what you will be getting at the eyepiece, and what that will cost in effort?

If not, then join your local astro club and observe with these folks. On the other hand you could just go for it and sell the scope if it doesn't live up to your expectations, which is one reason why the used scope market is very active.

#22 TechPan6415

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Posted 16 October 2013 - 12:36 PM

My criteria was super simple, get as big a scope as I could carry the components down stairs by my self, have it take up minimal room in a small apartment when stowed and view comfortably at the zenith while standing.

I did not need to drive 100+ miles to a "local" star party or "Local" club to check out some guys million pound 18" Obsession Classic or million dollar 18" UC to figure out that I was either going to be building the scope I needed or taking my chances on a UL-16. I might have went for the UL-18 if they offered it at the time because it is still a manageable beast as far as I am concerned.

It's just not that hard to look at the specs and figure out what you want, obviously bigger is better, but to a point in terms of managing it.

I did my research and as far as I am concerned, I totally nailed what I wanted, it's not hard to figure out.

#23 GeneT

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Posted 16 October 2013 - 01:35 PM

A 15 or 16-inch will got into many cars, and for someone who is about six feet tall no ladder will be needed even while looking at the zenith. A 18 or 20-inch needs a six foot step ladder, and a 30-inch required a tall ladder.


One's height and upper body strength figures into this decision. I have a friend who is 6' 3". He hefts an 18 inch Dob as easily as I do with my 12.5 incher. I am 5' 10". We both view flat footed at zenith. My 6' 3" friend also has an easy time loading, unloading, and setting up his telescope. A few years ago, I thought an Obsession 18 inch Ultra Compact would be perfect for me. Even with its F4.2 mirror, I needed a two step stool to view at zenith. Being older, standing on a stool looking up, at odd angles, causes me to loose my balance. The 18 UC also was hard for me to pick up and load. Its handles rested about two inches from the floor. I had to make an upward jerking movement, which I felt would ultimately cause me to pull something in my back. Therefore, I had to use the wheely bars and ramps to load the telescope. Wheely bars and ramps take up a lot of room in a vehicle. I originally thought that I would not have to use them. Where I finally ended up in all this was with my 12.5 inch Portaball. I have a small three wheeled transporter so all I have to do is lift the telescope up about six inches and onto the transporter, then into my vehicle. The upper assembly is encased/nests in the sphere and easily rides on one of the seats.

There are many factors to consider regarding the size of a telescope. Sometimes we don't get it right the first time. Sometimes our circumstances change such as where we live, or our physical health. Due to these kind of factors, I have bought and sold several telescopes over the years. I am happy where I ended up.

#24 cw00

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Posted 16 October 2013 - 02:18 PM

There are many things to consider when taking on a large dob. One thing is whether you are going to use it in your backyard most of the time or you are going to transport it to a dark site. If using primarily in the yard, how bad is light pollution there? The UC design may be compact but it may not be ideal for your observation site if there is plenty of stray light coming in from all directions. When considering a classic model, the size of the mirror box will be one deciding factor. I can fit my 20" Obsession classic in my Honda CRV but it is pretty tight. And the mirror box clears my front door by a hair. So I measured everything multiple times before placing an order. In the end, I have no regrets with all the hassle because the view of the large dob at a dark site is simply priceless.

#25 Dick Jacobson

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Posted 16 October 2013 - 03:44 PM

Regarding Hubble Optics, a friend ordered a 16" and was very unhappy with the mirror. He had it tested by a local expert who said it was the worst mirror he had ever seen. Hubble replaced it, but my friend is still not very happy with the scope, optically and mechanically.


What specifically was the issue that made it the "Worst mirror he had ever seen"?

The Strehl ratio was 0.3 as I recall, meaning it was nowhere near diffraction limited.






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