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Any 18"UC owners out there?

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#51 GeneT

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Posted 16 June 2013 - 06:22 PM

In this scope, there are several possible causes of laser motion with altitude change:
1) focuser plate flex. While certainly possible with a Paracorr and an eyepiece, the angle of change wouldn't be exactly vertical (as it is).
2) Ring flexure. Still unlikely for the same reason as #1.
3) spider flexure. When the scope is vertical, the secondary's weight hangs under the spider. At the horizon, the weight hangs beside the spider and causes twist in the spider vanes. High likelihood of this one because the spider is not in tension. A couple possible cures: thicker vanes (not desirable because of weight and extra diffraction), or a small counterweight outside the spider (no extra diffraction because it's behind and smaller than the secondary holder) on the center bolt. Or, of course, some other way to mount the spider.
4) movement of the secondary center bolt in the spider hole. Try wrapping the bolt in plumbers tape until it fits tight in the center hole. Or, wedge toothpicks in the corners to make it tight in the spider.
5) sag in the poles. The long poles could easily sag when the scope is pointed low, allowing the UTA to move toward the ground. Though this is a light UTA, my impression, by grabbing the UTA and shaking the scope back and forth, is that this scope would have benefited from 1.25" poles. If you change them, though, to that diameter, it will throw off the balance quite a bit.
6) Flexure in the lower attachment brackets of the poles attached to the altitude trunnions or torsional twisting in the trunnions themselves. if I were to quantify this issue, it would be smaller, but it could be an issue since the pressure is very different when the scope is vertical than it is when horizontal.

All of these issues have solutions, but I'd start with #3, 4, 5 first.



Are these problems with all ultra compact scopes? I am considering buying an ultra light structure with a flat ring for the UTA.


There are some European Ultralight designs that would interest me. Also, we need to be careful generalizing about all of these types of telescopes. I've been reluctant to be overly critical in public forums because of my respect for Dave Kriege and Obsession telescopes. He will be looked upon as an innovator and one of those who moved ameratuer astronomy to new heights by offering excellent telescopes at reasonable prices.

The Upper Assembly is simply a Birch ring. Also, there are only three support points for an 18UC vs. four for the 22. That may be why the 22 holds collimation better. A simple check comparing the Obsession UC design with their classical telescopes reveals that the Upper Assembly is much more robust for the Classical than it is for the Ultra Compact.

In my opinion, the 18UC is not one of those telescopes that will be noted for their excellence. Maybe it is not a bad telescope, just not an excellent one that you would expect from Obsession. With my 18 UC, I had more issues than just the fact it would not hold collimation. I am just going to let it go at that for now.

I see that Teeter is coming up with an Ultra Compact/Light design. I see some design features in the Teeter that might play out better with his Lite telescopes.

In my opinion, there is a place for this type of telescope. I would bet that various design modifications will ultimately be made so that they are good performers.

For me, the extra portability of the 18UC did not outweigh or override the performance that an Obsession Classic would have provided. I thought I could just grab the 18UC and throw it in my SUV. I could not. I had to use wheelie bars and ramps. I could have used wheelie bars and ramps for an 18 inch Classic--but received excellent performance across the board.

If someone owns a Honda Accord or Toyota Camry or similar sized vehicle, then yes--you could fit an 18UC in the trunk of your car, something you could not do with an 18 inch Classic. So, yes there is a place for this telescope.

However, if one has a vehicle to haul a Classic, or if one does most of his or her viewing in the back yard, then I believe that person would be better served with the Classic Obsession--a mighty nice telescope.

#52 aceholgi

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Posted 18 June 2013 - 05:05 AM

I think ring flexure is the problem. On the 18"UC the poles are located too far from the biggest mass on the ring (focusser+paracorr+eyepiece). 6 pole designs cause issues. On the 22"UC the ring still is not adequately stiff, but the mass is handled much better beeing mounted close to one point of the 8-pole truss.

Holger.

#53 Papa Taylor

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Posted 18 June 2013 - 05:54 PM

I am not a telescope maker and have just a few more questions to which the answers might seem obvious but I'd rather ask them than be sorry I didn't.

But first I'd like to thank you all for helping me to nail this down. :bow: I could not have fixed this problem without all of your generous assistance.

I hope to find someone who can make me a 1/4" aluminum ring to reinforce the birch ring.

I understand that the aluminum ring should be the same exact size as the birch ring only 1/4 inch thick.

I'm assuming that the upper mounting brackets for the truss poles will then attach to the bottom of the aluminum ring , effectively moving the secondary mirror and focuser 1/4" further from the primary?

And holes will be drilled in the aluminum ring that exactly align with all the holes around the birch ring for the focuser bracket, spider/truss pole brackets, telrad mount and handle. I will need to buy longer bolts for most or all these items.

Gene T also mentioned bolts with wing nuts somewhere on the ring assembly. I'm wondering where these extra fasteners would be positioned, perhaps here? Should I use just two extra fasteners or four? I would assume that the same bolt size should be used for these as those that mount the truss brackets to the ring.

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#54 Mirzam

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Posted 18 June 2013 - 06:55 PM

I obtained my aluminum ring by going to a metal fabrication/welding shop. They used a water jet tool to cut the ring from a sheet of aluminum. I had some other parts cut out at the same time so I'm not exactly sure what the ring alone would cost. A guess would be about $125-150. The weight penalty of adding the ring to the UTA will be significant--requiring a lot of counterweight at the opposite end. It would be helpful to add some lightening holes as you can see in my photo above.

You are correct about the focus change--the additional thickness will pull the focal plane inward by the same amount as the ring thickness. Perhaps you can remount the focuser support bracket slightly inward to compensate? Another possibility is to extend the collimation bolts of the primary mirror, but this has the very undesirable side effect of moving the center of gravity in the wrong direction. Not sure how practical it is to shorten the truss poles, but that would be the way most ATM's would solve the focus change problem.

JimC

#55 Seldom

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Posted 18 June 2013 - 09:22 PM

I don't think I'm seeing the deflection that Gene talks about in my 18" UC, at least not with a Barlowed laser. The photos were taken at angles noted. I did have the optional stiffener bars in place on the azimuth bearings. When I removed the Barlow from the laser I did notice some movement in the laser, but I think that it came from movement of the laser within it's own housing. I'd have thought that the Barlowed measurement would be more accurate, but someone more knowledgeable please advise.

Apologies for the less than perfect centering. I thought this was good enough to show movement (or lack of it).

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#56 wfj

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Posted 19 June 2013 - 07:37 PM

You can get a scope stiff enough to find no deflection.

The harder example is to be able to track down the source of the deflection. This is true no matter what the mechanical structure is.

My suggestion to "debug" the structure to isolate the problem before engineering/applying a "fix". This requires however great patience/skill, which might not be present at this time.

The general strategy is to measure deflection across the each of the structural members, while increasing the offset loads. So you start with nothing on the UTA, and add on til the deflection becomes present.

Once you determine the nature of this deflection, you again strip down to nothing, but incrementally increase load at a single point which is at the balance point of all the others added to the UTA - this confirms the finding of a single load/thrust path that it likely is - you want a simplest example to work with.

Next, you inspect/mitigate/engineer around this issue - by adding load or stiffening components to discern the nature of the structural issue. You may find other issues in the process, like perhaps a damaged structural member (if all TE's but one accept the load, and moving the single TE around moves the issue, perhaps there's a flaw in the TE?).

Nothing quite so frustrating as anticipating a problem with a "fix". only to find ... another problem elsewhere ... that the "fix" didn't address.

Just some thoughts.

#57 GeneT

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Posted 19 June 2013 - 10:51 PM

I'm wondering where these extra fasteners would be positioned,


Ours were positioned in the same places as the birch ring, i.e. the holes were drilled in the same place--through the birch ring and aluminum ring.

#58 UmaDog

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Posted 20 June 2013 - 08:09 PM

I'd have thought that the Barlowed measurement would be more accurate, but someone more knowledgeable please advise.


It's accurate, but it's only one of the two errors that matter (measures primary tilt errors). What happens to the laser spot on the primary when you shift the scope in elevation? Motion of the laser across the primary measures secondary tilt errors.

#59 Seldom

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Posted 20 June 2013 - 08:44 PM

I'd have thought that the Barlowed measurement would be more accurate, but someone more knowledgeable please advise.


It's accurate, but it's only one of the two errors that matter (measures primary tilt errors). What happens to the laser spot on the primary when you shift the scope in elevation? Motion of the laser across the primary measures secondary tilt errors.


The bare laser wanders from one side of the donut to the other, but stays in the donut. I'm not using a Glatter laser, and I thought this was due to internal mounting movement inside the laser, but I guess it could also be caused by deflection of the secondary mounting (half inch allthread rod). How much deviation is acceptable with the bare laser?

#60 UmaDog

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Posted 20 June 2013 - 09:21 PM

The "bare laser" measures the focuser axial alignment error. The tolerances for that for high power viewing are listed here: http://www.catseyeco... Tolerances.pdf

The motion could be due to a number of things. I think Don listed candidates earlier in the thread.

#61 GeneT

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Posted 20 June 2013 - 10:34 PM

The weight penalty of adding the ring to the UTA will be significant--requiring a lot of counterweight at the opposite end.


This is true. However, I found it to result in a better balance solution for the telescope. With the aluminum ring/birch assembly attached to the top of upper assembly, I attached two PVC pipes filled with BB's to the undercarriage, i.e. under the mirror. I experimented on how many BB's to fill the PVC pipe until I achieved perfect balance. This arrangement meant that I did not have to use the tube and BB's on the upper assembly. This switch actually resulted in a better functioning telescope.

#62 Mirzam

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Posted 21 June 2013 - 07:34 AM

I am not familiar with the details of the UC's, but on my own scope I use an elastic band as a partial counterweight. The band runs from the back of the mirror box to the front of the rocker box. It is easy to adjust the tension by retying the knot.

It's still a good idea to get the balance fairly close, but the elastic works really well and does save some weight.

JimC

#63 Papa Taylor

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Posted 22 June 2013 - 04:18 PM

My suggestion to "debug" the structure to isolate the problem before engineering/applying a "fix". This requires however great patience/skill, which might not be present at this time.


I do have a lot of patience but you are right about the skills. I do not know how to measure deflection. Can you recommend a resource I could use to learn?

Many have mentioned that ring flexure was the main problem with their 18"UCs so I will probably try getting an aluminum ring made to attach to the birch ring. Since this telescope requires a good bit of weight to balance the top end, I will first try aluminum ring without extra lightening holes in it. Can always take it back to be lightened if need be.

#64 Papa Taylor

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Posted 22 June 2013 - 04:24 PM

I would not despair too much. I love my 14-inch travelscope (I made it myself). It went to Chile with me and worked beautifully. I don't think there is a problem with the ultralight concept--just in the execution by some commercial vendors.

Everything fits into a couple large suitcases with plenty of room left for socks.

JimC


Jim, you comments have been very helpful. And that is a very nice looking telescope you made there. Although I don't have the time at this stage of my life, I can definitely see myself trying to build my own Dobsonian telescope one day. I enjoy woodworking and have learned a lot from this thread about how a newtonian works.

#65 Papa Taylor

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Posted 22 June 2013 - 04:29 PM

You are correct about the focus change--the additional thickness will pull the focal plane inward by the same amount as the ring thickness. Perhaps you can remount the focuser support bracket slightly inward to compensate? Another possibility is to extend the collimation bolts of the primary mirror, but this has the very undesirable side effect of moving the center of gravity in the wrong direction. Not sure how practical it is to shorten the truss poles, but that would be the way most ATM's would solve the focus change problem.

JimC


I believe it would be fairly simple to trim the truss poles by 1/4 inch on this telescope.

#66 Papa Taylor

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Posted 22 June 2013 - 04:35 PM

The extra weight of the aluminum ring did change the balance point of the telescope--for the better. I did not have to use the device holding BB shot on the UA to affect balance. I hooked up two PVC tubes with BB shot and affixed them to the underside, below the mirror, and the telescope was in perfect balance.


Gene, do you know if your aluminum ring had lightening holes drilled in it like in this picture of the ring that JimC made or was your ring solid with no lightening holes?

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#67 GeneT

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Posted 22 June 2013 - 07:53 PM

You are correct about the focus change--the additional thickness will pull the focal plane inward by the same amount as the ring thickness. Perhaps you can remount the focuser support bracket slightly inward to compensate? Another possibility is to extend the collimation bolts of the primary mirror, but this has the very undesirable side effect of moving the center of gravity in the wrong direction. Not sure how practical it is to shorten the truss poles, but that would be the way most ATM's would solve the focus change problem.

JimC


I believe it would be fairly simple to trim the truss poles by 1/4 inch on this telescope.


Be careful trying to trim the truss poles yourself. Dave Kriege will do it at no charge. You will have to pay shipping. Also, you will have to tell Dave exactly the length to make the truss poles. E-mail him for exact directions.

#68 GeneT

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Posted 22 June 2013 - 07:55 PM

The extra weight of the aluminum ring did change the balance point of the telescope--for the better. I did not have to use the device holding BB shot on the UA to affect balance. I hooked up two PVC tubes with BB shot and affixed them to the underside, below the mirror, and the telescope was in perfect balance.


Gene, do you know if your aluminum ring had lightening holes drilled in it like in this picture of the ring that JimC made or was your ring solid with no lightening holes?


Mine did not have the holes drilled in it like Jim's. His may also be a way to solve the problem. Jim--did your aluminum ring solve your collimation problem?

#69 Mirzam

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Posted 22 June 2013 - 09:29 PM

Keep in mind my scope was not an Obsession UC, but yes, the aluminum ring helped a lot. As mentioned above, the remaining flexure is due to the focuser itself. However, this is not enough to be a serious problem.

My scope also uses 8 truss poles rather than 6. I also do not use extremely heavy eyepieces with my travel scope, although the Paracorr by itself is pretty hefty.

JimC

#70 wfj

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Posted 22 June 2013 - 09:40 PM

You can also have the stiffening plate avoid the mechanical interference with the strut fasteners and then not have to modify the length of the TE's.

As to observing deflection, that's what the laser is being used for, although sometimes we use microscopes with measuring reticles to measure smaller deflections carefully. You are just using the mirror to magnify the combined effect through the primary's optical axis.

If I direct you to an engineering text, you'll be drowned in too much detail. The idea is to measure the effect in isolatable components such that you know the source of the problem, then investigate the components. A fractured birch ring might also be the source here.

I'm worried that if you go through the trouble of fabricating/modifying, and you still have the issue anyways, you might be a little annoyed.

The rule I work by is that I only modify/redesign when I've proved the source of the issue. The virtue of this is I don't modify too quickly. I've still been wrong a few times, but one feels better about appreciating the intent of the initial design.

All scopes, including original Obsessions, have had quality control issues, some that can be buried in something like a materials flaw. The puzzle is in finding/explaining the flaw.

One time, I was able to diagnose a flaw on a telescope with two C-clamps and a two foot piece of bar stock. I simply clamped the bar in various places, and noticed the deflection vanished in one position. It was a crack.

Not bad for a glorified accountant, with some past engineering skills.

#71 Doug McI

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Posted 27 June 2013 - 09:10 AM

Having owned one I had the same problems. Try a product called McLube. It for sail boats and works great on the bearing surfaces. Next make sure your focuser is tighten. Mine was only finger tight and was moving creating lots of problems. A laser and a auto collimanator was all I used. Excuse the spelling. Failed English in High School.

#72 Papa Taylor

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Posted 01 July 2013 - 08:55 PM

Having owned one I had the same problems. Try a product called McLube. It for sail boats and works great on the bearing surfaces.


Thanks, I'll give that a try.


Next make sure your focuser is tighten. Mine was only finger tight and was moving creating lots of problems.


I checked my focuser bracket and sure enough the three screws that hold it to the birch ring were not tight. I tightened them and decided to then check the other fasteners on the telescope as well. Nearly all of them were loose except for the ones that hold the top and bottom of each truss tube to its bracket. With finger crossed, I tightened all the loose fasteners hoping this might have been the source of the problem all along. It helped some but unfortunatelly there is still quite a bit of movement in the laser spot when I tilt the telescope up and down. :foreheadslap:

Seems like it must be ring deflection since I can gently push and pull on the birch ring under the focuser and observe the same movement in the spot as when I tilt the telescope. I can also actually see the ring flexing. I realize there could be a crack in the ring but all 3 clear sections of the ring flex pretty easily and evenly. That ring just does not appear to robust enough to do the job.

#73 Papa Taylor

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Posted 01 July 2013 - 09:16 PM

You can also have the stiffening plate avoid the mechanical interference with the strut fasteners and then not have to modify the length of the TE


I like this suggestion. So are you saying that the aluminum ring could be pierced to allow an open space for the top truss brackets to still contact the birch ring directly? The parts of the alumimum ring that would then be left going around the truss mounting brackets would not be very big. Might this be a place where deflection would still occur?

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#74 Papa Taylor

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Posted 01 July 2013 - 09:42 PM

You are correct about the focus change--the additional thickness will pull the focal plane inward by the same amount as the ring thickness. Perhaps you can remount the focuser support bracket slightly inward to compensate? Another possibility is to extend the collimation bolts of the primary mirror, but this has the very undesirable side effect of moving the center of gravity in the wrong direction. Not sure how practical it is to shorten the truss poles, but that would be the way most ATM's would solve the focus change problem.

JimC


Suppose I remove the birch ring entirely and simply replace it with the solid 1/4" thick aluminum ring?

I could insert a aluminum spacer between each truss pole mounting bracket and the aluminum ring to raise the top of the aluminum ring back up to the same height as the top of the original birch ring. This should bring the focal plane back to where it is right now with the unmodified birch ring.

Do the engineers and ATMs in the group feel that the 1/4" thick aluminum ring would be robust enough to do the job without the birch ring installed above it? I'm thinking leave it solid, no lightening holes, only mounting holes for truss tube brackets, focuser bracket, etc.

#75 Mirzam

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Posted 02 July 2013 - 07:45 AM

This is hard to predict. Because the UC-18 only uses 6 truss poles instead of 8 the aluminum ring has somewhat longer unsupported arcs.

Here is something you can check. Determine how much in-focus range there is remaining when viewing a star (or other object at infinity). If you have 1/2" and you loose 1/4" after combining the two rings together then problem solved.

JimC






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