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Obsession 15UC and 18UC

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#1 Gschnettler

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Posted 22 July 2021 - 05:16 PM

These scopes both look pretty cool…they seem very portable and people seem to get spectacular views with both of them.

I read as many reviews as I could. It seems as if people mostly like the 15UC very much, but not the 18UC. Is that really the case? They seem to be the same design.

For me, I don’t think the 15” would be enough of a step up over my 12”. But an 18” would be a nice big step up in aperture. And it would be more portable than my 12” so I could more easily take it farther to darker skies.

If I got one of these I suppose I would might get digital setting circles but not a motor drive. I think the appeal of this scope is the combination of large aperture, simplicity and small, lightweight portability.

What do you think?

#2 ButterFly

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Posted 22 July 2021 - 06:09 PM

I chose the 15 over the 18 for three reasons: it fits through standard doors with the bars attached; if fits in my trunk with the virtual mirror box attached to the base; and, it tracks a little faster because of the lighter weight, so I can track the ISS/Tiangong closer to zenith.

 

Tracking is a very nice feature.  If you only need equatorial tracking, a platform is enough.  To reach zenith comfortably, one can always raise the scope - lowering is harder.  Your height and the scope's height is another consideration, but it's not all that different.

 

If the 18 is within budgetary and other constraints, why not?  Make mock ups with sand as a good substitute for mirror and wood for wood, just to see what the weight and carrying points feel like.


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#3 ngc7319_20

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Posted 22 July 2021 - 06:14 PM

I had a 18UC for couple years... it had full go-to etc... 

 

Pluses:

- 18" dob that fits into couple airline approved cases

- very accurate goto

- big aperture

- fantastic once it was all assembled and working properly

 

Minuses:

- $1000 bag fees per trip

- complex assembly - there was much added complexity to make it fit the airline cases - lots of parts that had to come off

- collimation changed with pointing elevation and eyepiece weight -- the upper tube ring wasn't stiff enough

- so-so optics

- sometimes the elevation drive cable would jump out of the track, jam in the bearings, and then you were done for the night

- as my wife observed "for a portable scope its not very portable"


Edited by ngc7319_20, 22 July 2021 - 08:08 PM.

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#4 Bill Jensen

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Posted 22 July 2021 - 06:44 PM

You may want to join the Obsession Facebook group online, and get some input there as well. Perhaps there is an owner or two in your area, and you could try out the scopes in person. Pushing one around and seeing its size really helps in your purchase decision. 

 

Also, depending on when you are considering your purchase, in October, the Hidden Hollow Star party/conference is being held near Mansfield OH, not that far from Cincy. I went to that star party decades ago, and it attracted a good selection of speakers and larger scopes. 

https://wro.org/hidd...low-star-party/

 

I was seriously considering the 15 UC with just DSCs (the 18 UC was too heavy for me) back in June, but then a used 14.5 inch Starmaster in WV came available, and I grabbed it. I don't see that many used 15UCs come up on the market, but a used 18 UC comes up from time to time. 

 

you mentioned the 18 would be more portable than the 12 inch Orion that you have currently. According to the Orion web page, the heaviest component is 79 pounds for the XX12g. The Obsession website lists the heaviest component for the 18UC as 72 pounds (virtual mirror box and mirror combined) . While a bit lighter, it is not hugely different. In contrast, Obsession lists the heaviest component of the 15 UC as 45 pounds. 



#5 spaceoddity

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Posted 22 July 2021 - 07:15 PM

I've been curious about the 18UC too. It looks like it would be much more portable and lighter than my conventional 16" wooden truss. 18 would be as large as I could possibly transport and it would have to be a lightweight compact design. A few things concern me about the 18UC. 1. It only has 6 truss poles, not sure why you would want to cut corners in such a light part that contributes greatly to the rigidity of the scope. 2. I'd prefer a small, light but double ring UTA instead of a single ring. Again, rigidity and stability. 3. It has a 2" thick mirror. I'd prefer a little thinner mirror as thermal issues are always a concern where I live. 



#6 dustyc

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Posted 23 July 2021 - 10:29 AM

Anything above 12" requires a certain amount of "obsession" to get it out under the stars. I like my 15" UC because I can carry the mirror box out of the SUV and everything else assembles pretty quickly. A club member has the 22" with all the bells and whistles and he has to roll it out of his SUV with ramps. But he can keep the whole assembly with servocat as one piece. Then it's just the poles and top. It's big in terms of area but doesn't stick up very high. Same with mine. I haven't tried it yet but I think I could get the 15" into the trunk of my wife's Corolla.   

 

But, 12", then 16" then 24" will give you a big enough increase in light gathering to make it worthwhile. (double the area)


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#7 Bill Jensen

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Posted 23 July 2021 - 05:16 PM

a slightly used 18 UC just popped up on A--mart at a very attractive price. Florida. 



#8 CHASLX200

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Posted 23 July 2021 - 06:33 PM

a slightly used 18 UC just popped up on A--mart at a very attractive price. Florida. 

Where in FL? 



#9 Bill Jensen

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Posted 23 July 2021 - 07:33 PM

Where in FL? 

Jupiter. willing to drive to meet. I don't know the seller, just saw the ad. He  is throwing in some Ethoi. 


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#10 Astro-Master

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Posted 23 July 2021 - 08:37 PM

I wouldn't trade my 26 year old Obsession 18" Classic for a UC 18" even if you gave me an extra $3,000.  I love the easer and faster setup plus the better light baffling, and the bigger UTA that holds my Telrad, Stellarvue 60mm finder, and my Argo Navis Computer.

 

I have ramps and wheelbarrow handles and just ramp it in and out of my pickup truck, I'm only lifting about 15 lbs. at the handles, and at age 74 I should be able to continue well into my 80's God willing.  

 

I guess if I only had one car and couldn't ramp the scope into the car, I get the 18" UC, but I'd make improvements like better light baffling, and a 360 degree dew shield.  

 

I think you're smart not to get the Servo-Cat drive with the UC design, it is prone to problems with that design.


Edited by Astro-Master, 23 July 2021 - 10:37 PM.


#11 turtle86

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Posted 23 July 2021 - 09:00 PM

Jupiter. willing to drive to meet. I don't know the seller, just saw the ad. He  is throwing in some Ethoi. 

 

Just want to add that the scope is only a few months old, and also has an Ostahowski mirror, one of the very best.  The mirror alone is probably worth about 5K, so with the Ethos eyepieces thrown in, this sounds like a really good deal IMHO.  (I don't know the seller either.)


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#12 ButterFly

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Posted 23 July 2021 - 09:02 PM

I wouldn't trade my 26 year old Obsession 18" Classic for a UC 18" even if you gave me an extra $3.000.  I love the easer and faster setup plus the better light baffling. and the bigger UTA that holds my Telrad, Stellarvue 60mm finder, and my Argo Navis Computer.

 

I have ramps and wheelbarrow handles, and just ramp it in and out of my pickup truck, I'm only lifting about 15 lbs. at the handles, and at age 74 I should be able to continue well into my 80's God willing.  

 

I guess if I only had one car and couldn't ramp the scope into the car, I get the 18" UC, but I'd make improvements like better light baffling, and a 360 degree dew shield.  

 

I think you're smart not to get the Servo-Cat drive with the UC design, it is prone to problems with that design.

I would have gotten a 30" under those circumstances.

 

I track the ISS and Tiangong with my 15" UC.  Only a tool blames the tool.



#13 GeneT

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Posted 23 July 2021 - 11:22 PM

I wouldn't trade my 26 year old Obsession 18" Classic for a UC 18" even if you gave me an extra $3,000.  I love the easer and faster setup plus the better light baffling, and the bigger UTA that holds my Telrad, Stellarvue 60mm finder, and my Argo Navis Computer.

 

I have ramps and wheelbarrow handles and just ramp it in and out of my pickup truck, I'm only lifting about 15 lbs. at the handles, and at age 74 I should be able to continue well into my 80's God willing.  

 

I guess if I only had one car and couldn't ramp the scope into the car, I get the 18" UC, but I'd make improvements like better light baffling, and a 360 degree dew shield.  

 

I think you're smart not to get the Servo-Cat drive with the UC design, it is prone to problems with that design.

I have owned both a 20 inch Obsession Classic and an 18 inch Ultra Compact. I did not like the build of the 18 UC, but loved the build of the 20 inch Classic. There is no comparision between the builds of the two. True, the Classic weighs more and has a larger footprint. But, those qualities actually enhance the differences between the two. Take a look at the UC's upper assembly. https://www.obsessiontelescopes.com/ The secondary above the upper assembly and light shroud. It only has a tongue like feature sitting across from the eyepiece holder. I constantly had stray light problems with my UC because the baffle is not 360 degrees as it is with the Classic. The stray light on a UC was annoying and would show up when light got through the area covered by the tongue like feature. Also, the cover around the primary mirror on a UC is flimsy. The mirror box or lower assembly on the Classic is very solid.  https://www.obsessiontelescopes.com/18UC-vs-20-Classic/index.php The major difference in weight between the two designs shows up in the mirror box. The Classic's mirror box weighs more than twice that of the UC. https://www.obsessio...ssion_Specs.pdf However, again that provides more stability. I liked the handling of the Classic for more than that of the Ultra Compact. I sold my 20 inch Classic because I did not have a vehicle to transport it. I sold my Ultra Compact because I did not like the telescope, feature by feature, especially when compared to the Classic. I recommend looking at other Ultra Lites such as Waite research  http://www.waiteresearch.com/.   


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#14 CHASLX200

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Posted 24 July 2021 - 07:22 AM

I wouldn't trade my 26 year old Obsession 18" Classic for a UC 18" even if you gave me an extra $3,000.  I love the easer and faster setup plus the better light baffling, and the bigger UTA that holds my Telrad, Stellarvue 60mm finder, and my Argo Navis Computer.

 

I have ramps and wheelbarrow handles and just ramp it in and out of my pickup truck, I'm only lifting about 15 lbs. at the handles, and at age 74 I should be able to continue well into my 80's God willing.  

 

I guess if I only had one car and couldn't ramp the scope into the car, I get the 18" UC, but I'd make improvements like better light baffling, and a 360 degree dew shield.  

 

I think you're smart not to get the Servo-Cat drive with the UC design, it is prone to problems with that design.

Same here. I would take the Ethos.



#15 CHASLX200

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Posted 24 July 2021 - 07:24 AM

Just want to add that the scope is only a few months old, and also has an Ostahowski mirror, one of the very best.  The mirror alone is probably worth about 5K, so with the Ethos eyepieces thrown in, this sounds like a really good deal IMHO.  (I don't know the seller either.)

How much is the whole ball of wax?  Maybe sell the mirror and junk the scope or sell for parts. Just bought another scope and just don't have room for another.



#16 Bill Jensen

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Posted 24 July 2021 - 09:11 AM

How much is the whole ball of wax?  Maybe sell the mirror and junk the scope or sell for parts. Just bought another scope and just don't have room for another.

$6500 including DSCs and 3 Ethos eyepieces. 

 

I can't lift that much safely any longer, and the scope needs to remain in my SUV due to lack of storage. Ramps don't do me a lot of good since I won't leave the mirror in the scope while it is stored in the back of the SUV (I can remove the mirror/cell with the Starmaster). If I did have a garage, this would be tempting, even to work around some of the challenges posed by the more compact design. 

 

Put another way, if I had some decent storage, I would already be on I-95 South today ;-)  


Edited by Bill Jensen, 24 July 2021 - 09:11 AM.


#17 CHASLX200

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Posted 24 July 2021 - 10:47 AM

Way too much for my blood.



#18 astrokeith

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Posted 24 July 2021 - 11:19 AM

There's other differences between the 15" & 18" besides aperture.

 

The bigger has the folding altitude bearings. These come with a whole host of issues. Biggest issue for me is the full length teflon bearing surface on the 18" which leads to problems in stability and 'feel'. (the scope 'skates' on the bearing rather than being positively supported. With the 6 poles, offset pole end brackets, single OTA ring, exposed secondary and rear of focuser, shroud fits were it touches at the bottom, poor quality metal fittings that corrode in dew. I cant recommend it.

 

The 15" is a lot more stable.

 

The ServoCat altitude driving can be challenging too. But definitely I would add the Nexus DSC Pro.

 

Classic Obsessions are good, especially with the better mirrors. Obsession UC's are a poor compromise IMO.


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#19 ButterFly

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Posted 25 July 2021 - 09:08 AM

My 15" has folding altitude trunnions as well.  The size and weight are just right for me, at present, that I never fold them.  The teflon bearings are also full length.

 

I do feel that another crossbeam at the 0-10 degree elevation side would enhance stability.  When using SC, and switching the direction of motion in altitude, there is some motion in azimuth due to the trunnions swinging on the bearings.  Adding a counterweight to the handle bar hole on the trunnion opposite the eyepiece side has reduced that, but it's still there.  It makes a difference on the Moon, where I would want to just pan around while tracking.  The swing is about five minutes at higher elevation, and goes up to about 10-15 minutes at very low elevation.  Buckling of the trunnion tips is the best source I have come up with.

 

When hand tracking, this swing is too small to ever notice.  When using the slowest motion with SC in altitude only, and switching, then it's noticable.



#20 astrokeith

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Posted 25 July 2021 - 09:26 AM

My 15" has folding altitude trunnions as well.  The size and weight are just right for me, at present, that I never fold them.  The teflon bearings are also full length.

 

I do feel that another crossbeam at the 0-10 degree elevation side would enhance stability.  When using SC, and switching the direction of motion in altitude, there is some motion in azimuth due to the trunnions swinging on the bearings.  Adding a counterweight to the handle bar hole on the trunnion opposite the eyepiece side has reduced that, but it's still there.  It makes a difference on the Moon, where I would want to just pan around while tracking.  The swing is about five minutes at higher elevation, and goes up to about 10-15 minutes at very low elevation.  Buckling of the trunnion tips is the best source I have come up with.

 

When hand tracking, this swing is too small to ever notice.  When using the slowest motion with SC in altitude only, and switching, then it's noticable.

Interesting - did you see Ed Ting's review of the 15" UC? https://www.youtube....h?v=GbwL116hvEg

That example was fixed, which led me to think they all were. Looking at the Obsession website it shows only the folding versioning no mention of a fixed option.



#21 Bill Jensen

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Posted 25 July 2021 - 10:13 AM

Interesting - did you see Ed Ting's review of the 15" UC? https://www.youtube....h?v=GbwL116hvEg

That example was fixed, which led me to think they all were. Looking at the Obsession website it shows only the folding versioning no mention of a fixed option.

When I contacted Dave K in June, he advised that the 15 inch UC being sold now did not have a folding bearing. It is one piece. 


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#22 GeneT

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Posted 25 July 2021 - 07:13 PM

There's other differences between the 15" & 18" besides aperture.

 

The bigger has the folding altitude bearings. These come with a whole host of issues. Biggest issue for me is the full length teflon bearing surface on the 18" which leads to problems in stability and 'feel'. (the scope 'skates' on the bearing rather than being positively supported. With the 6 poles, offset pole end brackets, single OTA ring, exposed secondary and rear of focuser, shroud fits were it touches at the bottom, poor quality metal fittings that corrode in dew. I cant recommend it.

 

The 15" is a lot more stable.

 

The ServoCat altitude driving can be challenging too. But definitely I would add the Nexus DSC Pro.

 

Classic Obsessions are good, especially with the better mirrors. Obsession UC's are a poor compromise IMO.

I have owned an 18 UC and agree with your comments.



#23 dustyc

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Posted 25 July 2021 - 08:24 PM

That folding bearing goes a long way to making it "ultra compact". I guess the other way to do it would be to take the bearings off like the Teeter Journey scope. But it means more things to fiddle with. 


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#24 ButterFly

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Posted 25 July 2021 - 09:31 PM

It's a nice feature to have, even though I don't need it now. The counterweight on the trunnion is helpful for the Paracorr, 67 PP, PVS14 anyway. For aligning and tracking, I just do sidereal last motions.

I agree that it's easier to cut wood than to mend it. I considered adding a bar between the two with a stabilizing flag (with tension adjustment) to ride the inside bearing. Too much work for too little gain for me.

The only fix I felt was necessary so far is a detent for the altitude cable. It prevents the cable for unwinding, or dragging under the trunnion. String is fine enough, and easily undone.

#25 GeneT

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Posted 25 July 2021 - 11:42 PM

That folding bearing goes a long way to making it "ultra compact". I guess the other way to do it would be to take the bearings off like the Teeter Journey scope. But it means more things to fiddle with. 

You can order a solid (non folding) bearing and it still is an ultra compact. That will not allow you to fold it up in the optional carrying case. 


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