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Obsession UC latest changes/updates

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#1 azfar72

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Posted 11 August 2021 - 04:37 PM

Hi everyone - I just noticed that obsession 15 inch UC is now available at F4  ( formerly F4.2 ) . The other change I saw was that the bearings are also available as a solid piece instead of foldable. I believe the buyer can now choose between foldable or solid piece upon purchase. I would like to know whether these changes were intended to make the scope hold collimation better ? i.e address flexing/collimation shift issues that some users have reported in the past. 

 

It would also be interesting to know if obsession plans to make the 18 inch UC an F4 as well. I would personally like to see that change since this would make the 18 inch UC eyepiece height at zenith lower and possibly no use of step stool for most users.  The 18 inch UC will also be more compact with the shorter truss poles. 



#2 ngc7319_20

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Posted 11 August 2021 - 05:27 PM

My experience with the 18UC was that the altitude motion had a sort of jump when it hit the fold in the side bearing.  It was possible to get it perfectly smooth, but took some extra time and attention during assembly.

 

I think collimation drift (on mine anyway) had more to do with flexure in the upper tube ring.  The focuser was located on UTA ring sort of equidistant between the truss poles.  So heavy eyepieces (+ coma corrector, etc.) bent the UTA and tweaked the truss geometry, etc.   For a while I looked into getting a carbon fiber UTA ring fabricated, but ultimately the cost didn't make sense.


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#3 spaceoddity

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Posted 11 August 2021 - 05:59 PM

Doesn't make much sense to offer the 15 in f/4 but not the 18. Don't see why anyone would want f/4 for a 15 unless they are really short. I was hoping the changes would be thinner mirrors available, 8 truss poles and a double ring lightweight UTA.


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#4 azfar72

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Posted 11 August 2021 - 07:17 PM

Yup agreed - F4.2 seems better suited for the 15 inch UC and F4 for the 18 inch UC. 



#5 ButterFly

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Posted 11 August 2021 - 07:39 PM

Doesn't make much sense to offer the 15 in f/4 but not the 18. Don't see why anyone would want f/4 for a 15 unless they are really short. I was hoping the changes would be thinner mirrors available, 8 truss poles and a double ring lightweight UTA.

A thinner mirror is a huge detriment to this design.  You would need to carry around counterweights.  So why not just keep it on the mirror instead?  The cooldown time of quartz is not much if the scope is stored outside.

 

The secondary inside the shroud is a much needed improvement.  I made a dew shield out of fabric held in place with a scaffold of paint stirrers secured by velcro.  Dew isn't much of a problem for me, but with the PVS14, every bit of baffling helps a lot.  Even with the weight and torque arm of the P2 + 67PP + PVS14, the upper ring holds up well.  With dew shield, and the above, I add a 2.5 lb weight on a threaded shaft to the wheelbarrow handle attachment hole, on the trunnion opposite the focuser.  Panning around Cygnus in H-alpha with a 15" from a dark site is supernal.

 

If collimating is a huge chore, practice until it's not.  I'm good until I get down to below 30 degrees, which isn't often.  Differential cooling takes about two hours to show up as a one-third vane shift on the autocollimator, which is when I bother to tweak.  I tweak collimation for planets always, because only crazy and/or arrogant people don't.  I like Jupiter at 750x+ and that happens quite often areound here.  Some tensioned cable on the opposite side of the focuser down to the trunnion is an easy solution for the lazy.  I just tweak collimation.

 

I am waiting to hear whether the single piece trunnions take care of the swinging trunnions when switching directions in alt.  With the ServoCAT, I notice a few minutes of swing when changing direction in alt above 60 degrees or so.  It's about ten minutes of swing below 45 degrees.  I never notice it hand tracking, but it's obvious with the SC.


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#6 briansalomon1

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Posted 14 August 2021 - 08:01 AM

"I am waiting to hear whether the single piece trunnions take care of the swinging trunnions when switching directions in alt.  With the ServoCAT, I notice a few minutes of swing when changing direction in alt above 60 degrees or so.  It's about ten minutes of swing below 45 degrees.  I never notice it hand tracking, but it's obvious with the SC."

 

I am waiting for an Obsession 15UC to be delivered. I have used motorized scopes before but didn't really love them so when I ordered this scope I left the servocat out, but I've never gone above 300X with my little refractors and I notice you mention 750X. I think that would require a drive system. If I motorize the scope I'll probably just get whatever Obsession recommends but I thought I'd ask, who makes the best servo drive units?


Edited by briansalomon1, 14 August 2021 - 08:03 AM.

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#7 George N

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Posted 14 August 2021 - 11:05 AM

Doesn't make much sense to offer the 15 in f/4 but not the 18. Don't see why anyone would want f/4 for a 15 unless they are really short. I was hoping the changes would be thinner mirrors available, 8 truss poles and a double ring lightweight UTA.

DaveK will never agree that a thinner mirror is "better".  wink.gif

Reason to buy a 15UC at F/4 = you can't fit longer truss poles in your vehicle. My recent experience with selling a Dob was - *three* deals fell thru when the truss poles were too long to fit in the potential buyer's relatively 'tiny' vehicle. I was suprized that they would even consider buying a telescope without first measuring their vehicle - or asking their spouse about her thoughts about riding around with telescope poles next to her head!

 

I've also heard from some 'big' Dob owners with kids -- who are not happy with the kids up a ladder - and thus with a requirement of 'short'. I've seen pictures from a friend of his 7 year old sitting observing on one of those adjustable chairs - my first thought - he *had* to lift her up there -- and, if she falls off there - she will be hospital bound - or worse.


Edited by George N, 14 August 2021 - 11:07 AM.


#8 George N

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Posted 14 August 2021 - 11:19 AM

"I am waiting to hear whether the single piece trunnions take care of the swinging trunnions when switching directions in alt.  With the ServoCAT, I notice a few minutes of swing when changing direction in alt above 60 degrees or so.  It's about ten minutes of swing below 45 degrees.  I never notice it hand tracking, but it's obvious with the SC."

 

I am waiting for an Obsession 15UC to be delivered. I have used motorized scopes before but didn't really love them so when I ordered this scope I left the servocat out, but I've never gone above 300X with my little refractors and I notice you mention 750X. I think that would require a drive system. If I motorize the scope I'll probably just get whatever Obsession recommends but I thought I'd ask, who makes the best servo drive units?

Right now the question might be - where do you get servo-drive units? ServoCat is somewhat back-ordered - like autos - because of chip supply. Bisque puts big adds for their control computer in the magazines - but they are back-ordered for something like 20 or more.

 

I've been impressed at how well they work on various scopes I've used for both slew accuracy and tracking. I once saw one keep M-57 centered for 20 minutes in an Obsession 18 Classic at 850x - not far from "Dobson's Hole" - very impressive.

 

On the other hand - they do require some 'regular' maintenance work - like cleaning the power pick-up track and spring-loaded pin, and add another layer of complexity - and stuff that can 'go wrong' on a dark remote observing field. Here in the Northeast at least - one 'issue' is - grass & weeds. Get that stuff caught in the Az drive and you will grind a dimple in your ground board - meaning expensive repairs that can take weeks - or longer - and some $$ too. I have one friend who brings a weed-whacker along on observing trips - and others with various ground-cover solutions that do not cause the scope to 'float' and lose slew accuracy. Also - installing/removing the alt drive cable can be a struggle. Put that all together with the uncertain availability - I decided to not get one.


Edited by George N, 14 August 2021 - 11:34 AM.

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#9 George N

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Posted 14 August 2021 - 11:31 AM

BTW - Dave orders mirrors a hundred at a time to save $$ - so is somewhat limited to what the mirror makers will provide. His whole 'business plan' is based on reduced cost from pre-stocking parts - but with several million $ tied up. While this saves $$ and reduces delivery times, it also means little to no ability to 'customize'. Teeter seems to going that way with his last 2022 offerings.

 

Does Dave still send new scopes out with the mirror in the original packaging from the maker, never even opened? He will certainly work with you if there is an issue with the mirror - but at least in the past there was no 'sky testing' before shipping.


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#10 a__l

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Posted 15 August 2021 - 03:27 AM

I have one friend who brings a weed-whacker along on observing trips - and others with various ground-cover solutions that do not cause the scope to 'float' and lose slew accuracy. Also - installing/removing the alt drive cable can be a struggle. Put that all together with the uncertain availability - I decided to not get one.

Find a clean area. For good rotation of the roller Az. Attach the end of the cable to the elastic band and to the end of the Alt bearing after observation. You will have no problem with ServoCat for this reason.



#11 astrokeith

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Posted 15 August 2021 - 04:10 AM

We saw the 'solid' alt bearings in Ed Ting's video review. This will make a big difference. The 'bump' could with patience and skill be adjusted almost to zero, but the 'skating' action in azimuth caused by the full length altitude ptfe bearings surface was bad. The reversion to the classic 4 ptfe pads will solve this.

 

When I made my own 18" UC I started with solid alt bearings, which were manageable, just. Stowing the scope in the car or in storage at home was very difficult. On my new design they detach as one whole bearing. So I dont expect to see an Obsession 18" UC with solid bearings anytime soon.

 

The change from f4.2 to 4 is only going to reduce maximum eyepiece height by 3". Not sure if that is significant to anyone?

 

I always set my scope up on a breathable groundsheet. I'm usually set up for a week and dont want to kill the grass. The breathable groundsheets are now a requirement for many sites in the UK. They are also much more flexible (being woven) and I find the scope settles down nice and firmly. No puddles when it rains either!


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#12 Jon Isaacs

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Posted 15 August 2021 - 04:43 AM

The change from f4.2 to 4 is only going to reduce maximum eyepiece height by 3". Not sure if that is significant to anyone?

 

 

The Obsession spec's page lists the eyepiece height for the 18 inch F/4.2 at 70 inches.  I think reducing it to 67 inches would be helpful for many.  I am right at 6 feet and my friends 18 inch UC is a bit tall for me at the zenith.  My 16 inch F/4.4 is about 68 inches and just right.  

 

That 65-72 inch territory is critical for many observers and each inch makes a real difference.  

 

I just measured my 13.1 inch F/5.5 and the eyepiece height at the zenith is 72 inches, it seems much taller compared to the 16 inch F/4.4 but it's actually only 3-4 inches.

 

Jon


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#13 astrokeith

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Posted 15 August 2021 - 04:51 AM

The Obsession spec's page lists the eyepiece height for the 18 inch F/4.2 at 70 inches.  I think reducing it to 67 inches would be helpful for many.  I am right at 6 feet and my friends 18 inch UC is a bit tall for me at the zenith.  My 16 inch F/4.4 is about 68 inches and just right.  

 

That 65-72 inch territory is critical for many observers and each inch makes a real difference.  

 

I just measured my 13.1 inch F/5.5 and the eyepiece height at the zenith is 72 inches, it seems much taller compared to the 16 inch F/4.4 but it's actually only 3-4 inches.

 

Jon

You're probably right. I hadnt realised the current height was just the wrong side of being just right. 


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#14 George N

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Posted 15 August 2021 - 11:21 AM

Hi everyone - I just noticed that obsession 15 inch UC is now available at F4  ( formerly F4.2 ) . The other change I saw was that the bearings are also available as a solid piece instead of foldable. I believe the buyer can now choose between foldable or solid piece upon purchase. I would like to know whether these changes were intended to make the scope hold collimation better ? .....

 

As others have said - F/4.0 would make the 18UC ideal for a 6 foot owner ( not sure about the 15UC reasoning ). A friend has replaced the primary in his Obsession 18 Classic with a Zambuto F/4.0. He is maybe 5' 10" - and still needs a two-step stool for some observing. My eyeballs are at 68-inches and I still need one step to view comfortably with his 'Classic' high up (plus he always has a bino-viewer in it). I guess it is around 70 inches at Zenith? While I prefer to stand to observe with a Dob - one of those adjustable observing chairs will be on my "A'mart watch list". Not to mention - I really like to share views - and many of my friends do not 'live as close to the stars' as I do wink.gif  -- ie -- they be short people! << that includes my wife >> So a small ladder or Little Giant step-stool with big flat steps would still be nice to have along.

 

Perhaps the mirror suppliers just want to 'standardize' on F/4.0?

 

My understanding of the Obsession UC's use of the fold-alt bearing is -- the design was intended from the 'drawing board' to fit into small transport vehicles - and the fold is needed for that - ie, it needs to fold to fit in a small vehicle. I've read (but never experienced) that there is a small shift (clunk?) when the fold runs over the support pad. I've heard of people getting Dave to provide solid bearings to avoid the 'clunk' - for people who don't need the fold for transport loading. Seems he got enough requests to offer it as an option. Options are good!

 

I don't see either of these addressing the reported 'collimation drift' - that seems more related to the strength of the single-ring UTA or other upper-tube issues.


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#15 SandyHouTex

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Posted 15 August 2021 - 11:34 AM

As others have said - F/4.0 would make the 18UC ideal for a 6 foot owner ( not sure about the 15UC reasoning ). A friend has replaced the primary in his Obsession 18 Classic with a Zambuto F/4.0. He is maybe 5' 10" - and still needs a two-step stool for some observing. My eyeballs are at 68-inches and I still need one step to view comfortably with his 'Classic' high up (plus he always has a bino-viewer in it). I guess it is around 70 inches at Zenith? While I prefer to stand to observe with a Dob - one of those adjustable observing chairs will be on my "A'mart watch list". Not to mention - I really like to share views - and many of my friends do not 'live as close to the stars' as I do wink.gif  -- ie -- they be short people! << that includes my wife >> So a small ladder or Little Giant step-stool with big flat steps would still be nice to have along.

 

Perhaps the mirror suppliers just want to 'standardize' on F/4.0?

 

My understanding of the Obsession UC's use of the fold-alt bearing is -- the design was intended from the 'drawing board' to fit into small transport vehicles - and the fold is needed for that - ie, it needs to fold to fit in a small vehicle. I've read (but never experienced) that there is a small shift (clunk?) when the fold runs over the support pad. I've heard of people getting Dave to provide solid bearings to avoid the 'clunk' - for people who don't need the fold for transport loading. Seems he got enough requests to offer it as an option. Options are good!

 

I don't see either of these addressing the reported 'collimation drift' - that seems more related to the strength of the single-ring UTA or other upper-tube issues.

Carl Z charges a 25% premium for f/4 above his f/4.5 prices.  So they must be more difficult to make to Carl’s standards.


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#16 astrokeith

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Posted 15 August 2021 - 11:45 AM

As others have said - F/4.0 would make the 18UC ideal for a 6 foot owner ( not sure about the 15UC reasoning ). A friend has replaced the primary in his Obsession 18 Classic with a Zambuto F/4.0. He is maybe 5' 10" - and still needs a two-step stool for some observing. My eyeballs are at 68-inches and I still need one step to view comfortably with his 'Classic' high up (plus he always has a bino-viewer in it). I guess it is around 70 inches at Zenith? While I prefer to stand to observe with a Dob - one of those adjustable observing chairs will be on my "A'mart watch list". Not to mention - I really like to share views - and many of my friends do not 'live as close to the stars' as I do wink.gif  -- ie -- they be short people! << that includes my wife >> So a small ladder or Little Giant step-stool with big flat steps would still be nice to have along.

 

Perhaps the mirror suppliers just want to 'standardize' on F/4.0?

 

My understanding of the Obsession UC's use of the fold-alt bearing is -- the design was intended from the 'drawing board' to fit into small transport vehicles - and the fold is needed for that - ie, it needs to fold to fit in a small vehicle. I've read (but never experienced) that there is a small shift (clunk?) when the fold runs over the support pad. I've heard of people getting Dave to provide solid bearings to avoid the 'clunk' - for people who don't need the fold for transport loading. Seems he got enough requests to offer it as an option. Options are good!

 

I don't see either of these addressing the reported 'collimation drift' - that seems more related to the strength of the single-ring UTA or other upper-tube issues.

From my albeit limited experience, the 15" doesn't seem to suffer from the top end issues?



#17 azfar72

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Posted 16 August 2021 - 01:16 AM

I own a 15 inch UC f4.2 and it does suffer from collimation drift ( both secondary and primary mirror collimation ). In order to reduce collimation drift, I usually collimate when the scope is pointing at a 45 degree angle. I also asked someone to machine me an aluminium upper ring which has also reduced collimation drift . I was not able to machine a ring thick enough as it would unbalance the scope. These are my thoughts : 

 

- Reducing the focal length from f4.2 to f4, would result in shorter poles. Which as a result would buckle less and reduce pressure applied to the ring. This might improve things

 

- My primary mirror collimation drift I think is caused by the primary mirror slightly lifting off when slewing the scope towards horizantal. I do not think its the mirror cell as the triangles seem to float without restriction, but it would be nice if I can get someone else to check it out one day for me. What I think it is is that the foldable bearings bend a tiny bit under the weight of the primary mirror and cause the mirror the lift off and also the poles to buckle. This is why probably obsession introduced those "stabilizer bars " that connects both points of the bearing ends. So once again, Ij think with the shorter poles, they would be stiffer and less buckling. 

 

But overall I really enjoy the scope. With the aluminum ring, collimation drift are acceptable. And as mentioned by someone else in previous posts , when I do planetary viewing I also collimate right at the exact angle. Sometimes I also hang a weight on the focuser when Im using my autocollimator from cats eye. I think the autocollimator is too light, so I try to mimic the weight of paracorr/eyepiece combo as much as I can. With a relatively flexible upper ring, I think hanging a weight helps during collimation


Edited by azfar72, 16 August 2021 - 01:52 PM.


#18 George N

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Posted 16 August 2021 - 09:26 AM

Carl Z charges a 25% premium for f/4 above his f/4.5 prices.  So they must be more difficult to make to Carl’s standards.

"People time" drives up the cost for 'pro' far more than "machine time".

 

The faster the mirror, the more the required paraboloid has to deviate from a sphere - and that means more 'hands on' testing and figuring - and just good-old skill needed.

 

Thin large mirrors are more likely to be astigmatic - again requiring skilled figuring to correct. Being able to correct astigmatism is often what separates the 'pros' from the very good amateur mirror makers. 



#19 a__l

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Posted 16 August 2021 - 09:52 AM

The above conclusions are controversial (#18). These are products on the assembly line. Made in a limited time with limited quality.  These conclusions are from the interferometric study of mine (14.5) and other mirrors.
The amateur always has time to fine-tune the mirror. If he has the tools to study the mirror and sufficient patience.


Edited by a__l, 16 August 2021 - 09:53 AM.


#20 ButterFly

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Posted 16 August 2021 - 10:34 AM


- My primary mirror collimation drift I think is caused by the primary mirror slightly lifting off when slewing the scope towards horizantal. I do not think its the mirror cell as the triangles seem to float without restriction, but it would be nice if I can get someone else to check it out one day for me. What I think it is is that the foldable bearings bend a tiny bit under the weight of the primary mirror and cause the mirror the lift off and also the poles to buckle. This is why probably obsession introduced those "stabilizer bars " that connects both points of the bearing ends. So once again, Ij think with the shorter poles, they would be stiffer and less buckling. 

Check the sling.  If it's not at the center of gravity (COG) around the mirror, it can twist the mirror around during motion.  Here is a calcualtor: Mirror Edge Support Calculator.  The attachment points should also be placed vertically on the post such that the cable sling is parallel to the COG.  When looking down the tube, one side higher than the other on the posts is a twist from left to right.  The sling not on the COG is a twist up and down.  I give it a good jiggle when I set up, then go up and down a few times. 
 

The cell is the cell and the sling is the sling.  The mirror should be centered on the cell and support uniformly by the sling.  If after collimating, and using the sight tube, there is residual shift, adjust from the focuser.

 

 

Sometimes I also hang a weight on the focuser when Im using my autocollimator from cats eye. I think the autocollimator is too light, so I try to mimic the weight of paracorr/eyepiece combo as much as I can. With a relatively flexible upper ring, I think hanging a weight helps during collimation

Yes!  I ranted about a light autocollimator a while ago, but I have changed my mind since.  One can always add weight, right?
 



#21 azfar72

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Posted 16 August 2021 - 01:38 PM

Check the sling.  If it's not at the center of gravity (COG) around the mirror, it can twist the mirror around during motion.  Here is a calcualtor: Mirror Edge Support Calculator.  The attachment points should also be placed vertically on the post such that the cable sling is parallel to the COG.  When looking down the tube, one side higher than the other on the posts is a twist from left to right.  The sling not on the COG is a twist up and down.  I give it a good jiggle when I set up, then go up and down a few times. 
 

The cell is the cell and the sling is the sling.  The mirror should be centered on the cell and support uniformly by the sling.  If after collimating, and using the sight tube, there is residual shift, adjust from the focuser.

 

 

Yes!  I ranted about a light autocollimator a while ago, but I have changed my mind since.  One can always add weight, right?
 

 Yup I'm going to double check the sling.  I failed to mentioned that mine came with a kevlar belt when I purchased the scope on the used market and I upgraded it to a glatter sling since then. I used an online calculation to wrap the wire around the centre of gravity of the mirror. I have a gained a lot with this new sling, esp in terms of uniform cooling ( which I verified during a star test ) and potential potato chipping. Long story short, I have noticed an improvement at the eyepiece when observing. But I feel that the mirror slightly lifting off issue was better mitigated using the kevlar belt as it was attached to a fixed point and seemed to wrap around and hold the mirror backwards better. I will definitely check if the mirror is well in the centre  of the mirror cell by measuring the distance of the edge of the mirror to the floating triangles from all sides. Will also double check the positioning of the sling


Edited by azfar72, 16 August 2021 - 01:41 PM.


#22 GeneT

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Posted 16 August 2021 - 05:39 PM

My experience with the 18UC was that the altitude motion had a sort of jump when it hit the fold in the side bearing.  It was possible to get it perfectly smooth, but took some extra time and attention during assembly.

 

I think collimation drift (on mine anyway) had more to do with flexure in the upper tube ring.  The focuser was located on UTA ring sort of equidistant between the truss poles.  So heavy eyepieces (+ coma corrector, etc.) bent the UTA and tweaked the truss geometry, etc.   For a while I looked into getting a carbon fiber UTA ring fabricated, but ultimately the cost didn't make sense.

Agree with the above. I recommend getting the solid side bearing. Look at where the secondary mirror rests. It rests above the upper ring assembly and above the shroud, with only a tongue like device on the opposite side. That is not enough to keep out stray light. I don't believe the difference between F4 and F4.2 is a big deal. One thing I believe Obsession made in its telescope line is keeping two inch thick mirrors instead of moving to thin mirror technology. Thin mirrors when mounted properly are just as accurate in figure as the thicker mirrors. The thinner mirrors reach and keep ambient temperatures much better than the two inch jobs. Lastly, in my opinion that Obsession Classic Dobs are excellent value for the money; not so for the Ultra Compacts.



#23 Bob S.

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Posted 16 August 2021 - 07:07 PM

Agree with the above. I recommend getting the solid side bearing. Look at where the secondary mirror rests. It rests above the upper ring assembly and above the shroud, with only a tongue like device on the opposite side. That is not enough to keep out stray light. I don't believe the difference between F4 and F4.2 is a big deal. One thing I believe Obsession made in its telescope line is keeping two inch thick mirrors instead of moving to thin mirror technology. Thin mirrors when mounted properly are just as accurate in figure as the thicker mirrors. The thinner mirrors reach and keep ambient temperatures much better than the two inch jobs. Lastly, in my opinion that Obsession Classic Dobs are excellent value for the money; not so for the Ultra Compacts.

Gene,

Not sure what you mean about a "tongue like device on the opposite side"? My 2021 18" UC has a rather robust UTA kydex light baffle opposite the focuser which seems to do pretty well even in my suburban environment where each of our homes has driveway entrance lights for street lighting. I don't have the optional fabric shroud and Kydex primary mirror shrouds on the scope in the picture below. I have owned a bunch of thin mirrored Newtonians throughout my 20 years in the hobby and am frankly not as enamored with them as I used to be. As we know, the thinner the mirror, the better the primary mirror cell has to be to prevent astigmatism from showing up. I actually like the back-weighted balance of the 18" UC I recently bought and find that it works extremely well with a Paracorr 2 and heavier ep's such as my 21mm Ethos. It also allows for a 50mm correct image finder on the UTA along with a Telrad which is kind of fun to use each finder. I like the fact that Obsession included 4 counterweights with the system to create all kinds of balance options for different types of ep's that I might put in the scope. The 2" thick quartz Ostahowski mirror performed magnificently on my one and only outing with it so far but  1:9 thickness ratio is certainly not too flabby IMO. 

 

In terms of thermal equilibration, I am fortunate to live in Florida where the temperature swings are not as pronounced as many other parts of the country during much of the viewing year. I have heard tales in the past of UC's with ServoCATS experiencing some difficulties but fortunately found an almost brand-new scope without any drives and altitude bearing metal stifferners which is exactly what I was looking for. I am much more into simplicity these days than most anything else and the UC pretty much seems to fit the bill for that. Additionally, at age 72, I sure as heck don't see myself lifting up components of an 18" Classic into my vehicle with as much ease as the UC components present in terms of relative lightness for the heaviest components which are the Virtual Mirror Box/Primary Mirror. 

 

Could it be that your 18" UC was one of the earlier ones where some of the "bugs" hadn't been worked out yet? This latest iteration of the UC seems to be pretty reasonable from a functionality standpoint. The laser beam of my collimator does not drift outside of the center ring when I collimate it at about 50 degrees and then swing the scope through 30-80 degrees of arc. I would agree with anyone arguing that a Classic Obsession will always be more rigid than a UC but the skeletonized UC is mostly air except for the VMB.

Attached Thumbnails

  • 18UC.jpg

Edited by Bob S., 17 August 2021 - 02:58 PM.

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#24 GeneT

GeneT

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Posted 17 August 2021 - 06:16 PM

attachicon.gif18UC.jpg

Gene,

Not sure what you mean about a "tongue like device on the opposite side"? My 2021 18" UC has a rather robust UTA kydex light baffle opposite the focuser which seems to do pretty well even in my suburban environment where each of our homes has driveway entrance lights for street lighting. 

 

Could it be that your 18" UC was one of the earlier ones where some of the "bugs" hadn't been worked out yet? This latest iteration of the UC seems to be pretty reasonable from a functionality standpoint. The laser beam of my collimator does not drift outside of the center ring when I collimate it at about 50 degrees and then swing the scope through 30-80 degrees of arc. I would agree with anyone arguing that a Classic Obsession will always be more rigid than a UC but the skeletonized UC is mostly air except for the VMB.

Not sure what you mean about a "tongue like device on the opposite side"? My 2021 18" UC has a rather robust UTA kydex light baffle opposite the focuser which seems to do pretty well even in my suburban environment where each of our homes has driveway entrance lights for street lighting.

 

My 18 UC was not one of the early ones built. The UC had been out a few years. My secondary rested considerably above the upper ring. Your secondary is under the ring, not above it like mine. Also, your light baffle covers much more of the ring area than mine did. As I described it, mine was a tongue like device. What is your upper ring made of--metal? Mine was birch wood. Mine would not hold collimation. I spent $200 for a replacement aluminum upper ring to solve the problem. Maybe Dave listened to some of our complaints and made some needed fixes. I am happy that you are benefitting from these upgrades making the UC a better product. We can disagree on this, but I am sold on thin mirrors. In addition to the problems I mentioned on my 18 UC, I had problems reaching and maintaining thermal equilibrium. I believe that Obsession is one of the few vendors still using two inch mirror thickness. 


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#25 Bob S.

Bob S.

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Posted 17 August 2021 - 07:44 PM

Gene,

Actually, my secondary mirror is still above the ring with the three struts holding it in place. The picture I sent gives a false impression that the secondary mirror is below the UTA ring. I tapped the ring and it feels like solid wood to me? 

 

In terms of holding collimation, I always take into account that when I am making large moves with a light weight truss system that I am bound to flex the structure and see that flexure in a laser beam. I also want to put in my autocollimator and do the same movements as the autocollimator is generally more predictive of how much loss of collimation is occurring with the movements. However, I owned a lot of Starmasters in my time and the builder/designer, Rick Singmaster used to get on my case about complaining about flexure when making large moves with his four pole more compact Starmasters. I have not had a chance to experiment with how well the 18" UC holds collimation between large and small movements at this time. It may be that it flexes such that you can really see a laser dot move when making large moves? With Ethos ep's, I don't really have to perturb the scope as much because of the wider fields of view and so have not noticed distorted images when it comes to holding a target like a planet or a globular cluster in the FOV. Again, having had only a few hours with this scope over one night and I do not have a solid appreciation of what it can and cannot do in a plethora of different scenarios. I look forward to seeing what it is capable of. BTW, I have owned about 6 Portaballs over the years and they are terrific scopes. Luckily, Dave Jukem, the owner of Mag 1 Instruments has vowed to continue to support the scopes in the field with current and former parts but has recently discontinued production of those fun instruments. 

Bob




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