Old 14-08-2019, 01:30 PM
Hijynx (Benn)
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Hard decision

Hi all. I have been looking for a decent midsized dob for some time. Currently I have the opportunity to purchase either a 18 UC Obsession at f4.2 or a 16 SDM at f4.5. The SDM is considerably cheaper. Both are second hand and both are running the same 32k guiding and other features. What would you do?
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Old 14-08-2019, 01:45 PM
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There is not much difference in size between 18" and 16"..

Only 26% in area.
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Old 14-08-2019, 02:39 PM
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FlashDrive (Col)
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SDM would be my pick ... but that's just me..!!
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Old 14-08-2019, 05:28 PM
Rainmaker (Matt)
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Originally Posted by FlashDrive View Post
SDM would be my pick ... but that's just me..!!
Me too....
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Old 14-08-2019, 06:33 PM
Ukastronomer (Jeremy)
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Please don't take it the wrong way (oh NICE by the way)

but to me I don't know what it is but hand made scopes always look......... well....... hand made, and that puts me off.

My choice would be

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Old 14-08-2019, 06:46 PM
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iborg (Philip)
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Personally, I like the hand made look.

As to which to choose - some of the questions I would like answers to -
What are they like to assemble?

Do you know what they are like when packed?
How heavy are the seperate sections?
How well would each fit into car?

Assuming I could justify the money to myself, I think weight of the heaviest item would be upper most in my mind.

Whichever you choose, I hope you enjoy.

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Old 14-08-2019, 07:34 PM
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Not a bad choice to have. I have an 18" F4.5 Obsession classic. At the size you are looking at I would be looking at the quality of the mirror and equally important at the way the scope is built. How easy is it to transport? How well does it hold collimation when you move it from zenith to say 15 degrees? Both will provide a lifetime of views. Does one have a ServoCat? If not if you want to add one which will be easier? Which one requires less baffling of the UTA to cut down on stray light? A big Dob is a joy but there is a lot to think about - much of which is not obvious until you have one. Happy to discuss more by PM.
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Old 14-08-2019, 07:36 PM
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Don't be torn..buy them both.
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Old 14-08-2019, 07:37 PM
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alocky (Andrew lockwood)
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You'd probably find the 18" UC a lot easier to transport and setup than the SDM 16, and it's pretty easy to see the difference in the eyepiece between an 18 and a 16 side by side. You can be pretty confident the optics in either will be good, both SDM and Obsession use reputable mirror makers. The only downside to the UCs is the potential for the collimation to drift - but I haven't heard anyone on the Obsession group complain about it for a long time now, so I guess it's been sorted.
Either way - nice problem to have!
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Old 14-08-2019, 09:34 PM
Hijynx (Benn)
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Yes it sure is a nice problem. Both have 32k encoders, servocat nexus, Argo Navis, light shrouds etc. both have good mirrors. I like the custom look but can also appreciate the simple, military style of the ultra compact. It is a nice decision to sit and think about.
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Old 15-08-2019, 07:05 AM
glend (Glen)
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First off let me say that I completely understand the role that "passion" has in buying decisions; heck it keeps Shannon's in business ( ie cars). You don't give us any background on your experience with astronomy, and that is a factor in any advice people might offer. My personal opinion, based on chasing my passions for seven decades is buy neither, those are, in my opinion, overly expensive scopes with limited resale market interest. You will always lose money on selling scopes, and frankly neither of them is much better visually than say a 16" GSO dob or a Skywatcher. Minimise your investment and potential resale loses. Sure they are nice to look at but your buying a tool, an instrument to look at the night sky.
Be honest about why you want a large dob. Are you buying it to impress people at a star party in the daylight, or you just want something that works. It is easy to add guiding systems like Astro Devices Nexus to a GSO dob or go for the Skywatcher Goto version.

Disclosure, I owned a 16" GSO dob for years, and on occasion have looked through similar "custom, ie expensive" offerings, including very large SDMs.

Last edited by glend; 15-08-2019 at 08:06 AM.
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