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Any 18"UC owners out there?

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#26 Mirzam

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Posted 12 June 2013 - 06:49 PM

One thing I do with my 14" travelscope, which uses a metal top ring but has some flexure inherent in the focuser, is to hang a small weight on the laser while collimating. This simulates the weight of an eyepiece in the focuser so that proper collimation in actual use is little closer. I call this "dynamic" collimation. Some scopes are more "dynamic" than others! :tonofbricks:

JimC

#27 Papa Taylor

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Posted 13 June 2013 - 10:51 AM

For about $200, a friend made me a 1/4 inch aluminum ring to affix to the birch ring for the upper assembly. That fixed my collimation problem.


Gene, did the aluminum ring go around the birch ring or was it the same size and shape (but 1/4" thick) and attached to the top or bottom of the birch ring?

#28 Papa Taylor

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Posted 13 June 2013 - 10:58 AM

Yes Jim, your smilie face with the bricks fall on it was well chosen and sums up my feelings right now but hey it's just a telescope. :foreheadslap: I'm trying to decide whether to fix the scope or trade it for something else. Since it's really too heavy for me to lift safely and too tall to look through without a ladder I'm leaning toward getting something else. The Portaball 12.5" is looking pretty good to me right now.

#29 Papa Taylor

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Posted 13 June 2013 - 02:15 PM

I have checked to see if the laser spot also moves when the telescope is tilted from vertical to near horizontal and it does quite a bit.

This is with the scope pointed straight up

Attached Thumbnails

  • 5918970-spot at vertical.jpg


#30 Papa Taylor

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Posted 13 June 2013 - 02:17 PM

And this is near the horizon. Spot moves gradually as I tilt the scope down then back to center when I point it back up.

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  • 5918974-spot near horizontal.jpg


#31 Papa Taylor

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Posted 13 June 2013 - 02:19 PM

The same thing happens with the focuser in, out or in the center of it's travel.

#32 Mirzam

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Posted 13 June 2013 - 05:35 PM

Try collimating with the scope pointed at about 45 degrees elevation. Then see how the spot moves as you go down to about 20 degrees and up to about 70 degrees. Hopefully, the spot movement will be a lot less over this commonly used range.

JimC

#33 GeneT

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Posted 13 June 2013 - 09:29 PM

For about $200, a friend made me a 1/4 inch aluminum ring to affix to the birch ring for the upper assembly. That fixed my collimation problem.


Gene, did the aluminum ring go around the birch ring or was it the same size and shape (but 1/4" thick) and attached to the top or bottom of the birch ring?


It went around the birch ring, i.e. was the same size and shape, and attached to the bottom. The 18UC has problems holding collimation throughout the viewing arc. The 18UC has an F4.2 mirror. It is important that collimation be held. The problem of holding collimation does not seem to exist with the 15 and 22 UC.

#34 GeneT

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Posted 13 June 2013 - 09:33 PM

I'm trying to decide whether to fix the scope or trade it for something else. Since it's really too heavy for me to lift safely and too tall to look through without a ladder I'm leaning toward getting something else. The Portaball 12.5" is looking pretty good to me right now.


I began this discussion stating that I had wished I had purchased the Obsession Classic 18 instead of the 18UC. The Classic is an excellent telescope--good value for the money. The 12.5 inch Portaball is an excellent telescope. However, if you want a larger one, maybe consider another 18.

#35 GeneT

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Posted 13 June 2013 - 09:43 PM

And this is near the horizon. Spot moves gradually as I tilt the scope down then back to center when I point it back up.


This is what I was saying. The 18UC does not hold collimation. An F4.2 mirror needs to be in collimation.

#36 wfj

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Posted 14 June 2013 - 02:02 PM

Thanks both of you, this is exactly the problem I'm chasing on a different scope of similar design.

For about $200, a friend made me a 1/4 inch aluminum ring to affix to the birch ring for the upper assembly. That fixed my collimation problem.


Gene, did the aluminum ring go around the birch ring or was it the same size and shape (but 1/4" thick) and attached to the top or bottom of the birch ring?


It went around the birch ring, i.e. was the same size and shape, and attached to the bottom. The 18UC has problems holding collimation throughout the viewing arc. The 18UC has an F4.2 mirror. It is important that collimation be held. The problem of holding collimation does not seem to exist with the 15 and 22 UC.


Gene,
Pardon me, but it sounds like you are describing a right angle cross section aluminum extrusion formed into a circle that encompasses the UTA ring, is welded at the seam and makes attachment along the bottom of the UTA with fasteners.

Did I get that correct?

If so, that suggests that the problem is parallelogram distortion of the UTA ring which "waffles" it out of the plane because the shearing force on the UTA ring bends it asymmetrically, tilting/shifting the diagonal. The "brace" you describe stiffens the ring to reduce this effect.

A thicker UTA (or cage), midpoint tension strings, more TE's, an I- or T-beam circle brace ... are among the structural elements also that would affect this.

My interest is in understanding the nature of the flaw better, and how it compromises the design.

#37 GeneT

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Posted 14 June 2013 - 03:34 PM

I don't know how my friend made the aluminum ring. All I know it was 1/4 inch thick and fit around the birch ring perfectly. The screws were drilled through the birch ring and aluminum ring and joined with wing nuts. Standing back, it looked like a single unit. When looking up close, you see two units bonded into a perfect unit. 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.

#38 Papa Taylor

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Posted 14 June 2013 - 06:04 PM

All I know it was 1/4 inch thick and fit around the birch ring perfectly.


Did the ring resemble any of these drawings? I'm still unclear about the shape of the ring.

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  • 5921088-aluminum ring.jpg


#39 GeneT

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Posted 15 June 2013 - 04:20 PM

All I know it was 1/4 inch thick and fit around the birch ring perfectly.


Did the ring resemble any of these drawings? I'm still unclear about the shape of the ring.


Aluminum disc fits under the birch ring.

#40 Starman1

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Posted 15 June 2013 - 05:30 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.

#41 GeneT

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Posted 15 June 2013 - 07:02 PM

2) Ring flexure. Still unlikely for the same reason as #1.


I worked through the flexure issue with my 18UC with several others who had this problem. If you look at a picture of the ring, notice the heavy elements all located on about 25 percent of the ring. http://www.obsession...ndex.php#photos
The solution was a reinforcing ring. An aluminum reinforcing also solved the flexing problem of several others. However, the flexure problem was only one element that I did not like about the telescope. So I sold it.

#42 Starman1

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Posted 15 June 2013 - 07:10 PM

Gene,
The movement of the laser dot appeared to be purely vertical in his case.
Differential flexure of the focuser plate and/or the ring it sits on would likely move the laser dot at an angle. Since it moved vertically, I suspect something in the scope that causes direct up-down movement in the laser dot.
Not that the ring is adequately stiff, of course, if you point out that it helped to stiffen it up. I can also see the focuser plate sagging quite a bit when a Paracorr + heavy eyepiece are added. And the extra weight would cause a torsional twisting of the entire ensemble, too.
It's very hard to make a scope stiff enough, but it's even harder to make an ultralight stiff enough. I'm currently reading Highe's book on dob construction, and it's a daunting series of compromises.

#43 alexvh

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Posted 16 June 2013 - 01:38 AM

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.

#44 alexvh

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Posted 16 June 2013 - 02:25 AM

One thing I do with my 14" travelscope, which uses a metal top ring but has some flexure inherent in the focuser, is to hang a small weight on the laser while collimating. This simulates the weight of an eyepiece in the focuser so that proper collimation in actual use is little closer. I call this "dynamic" collimation. Some scopes are more "dynamic" than others! :tonofbricks:



JimC



I am really beginning to see the downsides of the ultralight design here, particularly the UTA. Anyone have any thoughts or experience on this?

#45 Mirzam

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Posted 16 June 2013 - 07:33 AM

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

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  • 5923374-5572525-scope-14b.jpg


#46 Mirzam

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Posted 16 June 2013 - 07:53 AM

Here's the top ring. It is made of aluminum and the self supporting spider is welded aluminum. The spider and the focuser mounting bracket are very robust. The only flex left comes from the focuser itself, which is an ultra light weight KineOptics focuser that uses a single nylon pressure screw to hold the drawtube against the bearings. Flexure in the focuser is why I still do the dynamic collimation.

JimC

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  • 5923398-14-inch ring.JPG


#47 Papa Taylor

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Posted 16 June 2013 - 08:19 AM

The movement of the laser dot appeared to be purely vertical in his case.


Just for clarification, these pictures better show the movement of the laser spot when the scope is tilted from vertical to about 40 degrees from the horizon. I held my camera in a vertical orientation in each.

It is a gradual movement beginning when I first begin to tilt the scope. When the scope is vertical, the spot is centered on the primary and both spots converge on the diagonal mirror.

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  • 5923444-primary spot.jpg


#48 Papa Taylor

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Posted 16 June 2013 - 08:22 AM

and the secondary with the scope in the same position

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  • 5923448-diagonal spot.jpg


#49 Starman1

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Posted 16 June 2013 - 09:41 AM

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.

Yes, these are potential problems with all ultralights, but i aimed this at the UC owner.

#50 Starman1

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Posted 16 June 2013 - 09:43 AM


The movement of the laser dot appeared to be purely vertical in his case.


Just for clarification, these pictures better show the movement of the laser spot when the scope is tilted from vertical to about 40 degrees from the horizon. I held my camera in a vertical orientation in each.

It is a gradual movement beginning when I first begin to tilt the scope. When the scope is vertical, the spot is centered on the primary and both spots converge on the diagonal mirror.


Ah, so the spot does move at an angle. That enhances the likelihood of ring and/or focuser plate flexure.


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