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Equipment Discussions >> Reflectors

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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Starman1]
      #5922773 - 06/15/13 08:02 PM

Quote:

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.obsessiontelescopes.com/telescopes/18UC/index.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.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5922782 - 06/15/13 08:10 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.


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: Any 18"UC owners out there? new [Re: Starman1]
      #5923176 - 06/16/13 02:38 AM

Quote:

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.


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alexvh
sage


Reged: 07/29/07

Loc: South Africa
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5923190 - 06/16/13 03:25 AM

Quote:

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!



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?


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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Any 18"UC owners out there? new [Re: alexvh]
      #5923374 - 06/16/13 08:33 AM Attachment (15 downloads)

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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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5923398 - 06/16/13 08:53 AM Attachment (17 downloads)

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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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Starman1]
      #5923444 - 06/16/13 09:19 AM Attachment (12 downloads)

Quote:


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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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5923448 - 06/16/13 09:22 AM Attachment (15 downloads)

and the secondary with the scope in the same position

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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Any 18"UC owners out there? new [Re: alexvh]
      #5923529 - 06/16/13 10:41 AM

Quote:

Quote:

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.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5923534 - 06/16/13 10:43 AM

Quote:

Quote:


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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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: alexvh]
      #5924272 - 06/16/13 07:22 PM

Quote:

Quote:

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.


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aceholgi
member


Reged: 04/20/11

Re: Any 18"UC owners out there? new [Re: GeneT]
      #5926868 - 06/18/13 06:05 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.


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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: aceholgi]
      #5928083 - 06/18/13 06:54 PM Attachment (12 downloads)

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. 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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Mirzam
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5928172 - 06/18/13 07:55 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


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Seldom
scholastic sledgehammer


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5928356 - 06/18/13 10:22 PM Attachment (12 downloads)

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).

Edited by Seldom (06/18/13 10:29 PM)


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wfj
Pooh-Bah
*****

Reged: 01/10/08

Loc: California, Santa Cruz County
Re: Any 18"UC owners out there? new [Re: Seldom]
      #5930022 - 06/19/13 08:37 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.


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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5930374 - 06/19/13 11:51 PM

Quote:

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.


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UmaDog
Carpal Tunnel
*****

Reged: 09/15/10

Loc: Basel, Switzerland
Re: Any 18"UC owners out there? new [Re: Seldom]
      #5932063 - 06/20/13 09:09 PM

Quote:

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.


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Seldom
scholastic sledgehammer


Reged: 08/05/12

Loc: N of Cedar City Light Dome
Re: Any 18"UC owners out there? [Re: UmaDog]
      #5932113 - 06/20/13 09:44 PM

Quote:

Quote:

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?


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UmaDog
Carpal Tunnel
*****

Reged: 09/15/10

Loc: Basel, Switzerland
Re: Any 18"UC owners out there? [Re: Seldom]
      #5932186 - 06/20/13 10:21 PM

The "bare laser" measures the focuser axial alignment error. The tolerances for that for high power viewing are listed here: http://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf

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


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