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Equipment Discussions >> Reflectors

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Any 18"UC owners out there?
      #5898261 - 06/02/13 05:53 PM

I am working on initial setup and collimation of an Obsession 18" UC telescope and would like to speak with other owners of this scope about their experiences with setting up the alignment of their 18" UCs. I am using a Catseye Cheshire, Autocollimator and Sight Tube and a Glatter laser and Tublug. Will post some questions if I hear back from some members who own this particular telescope. I would greatly appreciate any help I could get setting up this instrument.

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5898796 - 06/03/13 12:08 AM

After a year, I sold my 18 UC. The setup is pretty straight forward--just follow the instructions. I used both CatsEye and Glatter with TuBlug to collimate. I found initial collimation to be very easy with the 18 UC. However, my telescope would not hold collimation through the viewing arch. 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.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5901306 - 06/04/13 08:50 AM

Thanks, Gene.
Could I ask a few questions about that aluminum ring, your initial setup and collimation? I am not new to collimation and have been using the catseye tools for years on my 12" Lightbridge. I purchased the laser and Vic Menard's book on collimation hoping they would help me collimate the UC but the laser and catseye tools are not agreeing. There seem to be some other strange things about this telescope. I bought it a few years ago and became pretty frustrated with it. It's been in it's case ever since and has really put a damper on my astronomy pursuits. I need to get it fixed or sell it. One thing I don't like at all is how sticky it is to move in altitude and azimuth. My Lightbridge is smooth and easy to move.


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UmaDog
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5901327 - 06/04/13 09:09 AM

You could try cleaning the bearings. Some waxes, etc, that are used to improve bearing motion can increase "sticktion" or static-friction. So perhaps give the bearings a clean first, in case they're coated with something.

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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5901794 - 06/04/13 01:16 PM

Quote:

...the laser and catseye tools are not agreeing.



Can you be more specific? Which tools are not agreeing, and how are they not agreeing?

Quote:

There seem to be some other strange things about this telescope.



I've collimated a few of these scopes in the field. The most common problem I've encountered is a torqued spider/UTA assembly. When this assembly is not geometrically correct, it's impossible to get the secondary mirror optimally aligned. If this is the issue with your scope, you'll need to loosen the spider connections to relieve the torque--you may also have to loosen the truss poles (which may be a contributing factor). Once you sort out the spider/secondary mirror geometry, you should be able to achieve good axial alignments with an optimized secondary mirror placement.

Quote:

One thing I don't like at all is how sticky it is to move in altitude and azimuth.



I didn't get to spend much time observing with the UCs I've collimated, so I can't help you much there, other than to suggest cleaning the bearing surfaces. Once everything is mechanically "tight", the motions should be smooth (IIRC, the transition on the altitude bearing was reasonably smooth when I was positioning a UC for a flexure/collimation check).

Let us know what progress you make and maybe we can get these issues resolved.


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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Vic Menard]
      #5902208 - 06/04/13 05:15 PM

I will start from the beginning with what I've done so far and work up to the disagreement between the laser and passive tools.
First, it appears that the focuser on this scope is not adjustable and that the spider also is not adjustable. I still wanted to verify that the focuser axis is perpendicular to the tube axis and did so by inserting the laser into the focuser, clamping a straight edge to the bottom of the birch ring that holds the "spider" and taking measurements as best I could from the laser beam to the straight edge on each side of the UTA. This seemed to check out. Next I checked to see if the beam appears to cross directly under the center hole in the spider where the secondary mirror holder mounts. Again this looks good. I then used the sight tube to adjust the secondary until it appears round and centered when looking through the sight tube.

What seems odd at this point is that the secondary mirror holder needs to be adjusted as far as it will go toward the primary in order for the secondary mirror to appear centered when looking through the sight tube.


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5902212 - 06/04/13 05:16 PM

here is top of the secondary mirror holder

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5902213 - 06/04/13 05:18 PM Attachment (54 downloads)

okay let's try that again

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5902217 - 06/04/13 05:19 PM Attachment (49 downloads)

here is the view through the sight tube

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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5902393 - 06/04/13 06:49 PM

Quote:

Thanks, Gene.
Could I ask a few questions about that aluminum ring, your initial setup and collimation? I am not new to collimation and have been using the catseye tools for years on my 12" Lightbridge. I purchased the laser and Vic Menard's book on collimation hoping they would help me collimate the UC but the laser and catseye tools are not agreeing. There seem to be some other strange things about this telescope. I bought it a few years ago and became pretty frustrated with it. It's been in it's case ever since and has really put a damper on my astronomy pursuits. I need to get it fixed or sell it. One thing I don't like at all is how sticky it is to move in altitude and azimuth. My Lightbridge is smooth and easy to move.




I did not have stiction issues with my 18 UC, however many did. Notice that there is a Teflon band all along the rocker box. That is not the typical Dob solution. On all other Dobs that I have seen, there are two Teflon patches on each rocker box. To be honest, I was disappointed with my 18 UC and wished I had bought the 18 inch Obsession Classic. That is an excellent telescope.

Here is a posting I made awhile back. My post is about the 8th one down. http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflector...
Hope all this sorts out for you.
GeneT


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Vic Menard
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Reged: 07/21/04

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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5902459 - 06/04/13 07:22 PM

Quote:

...First, it appears that the focuser on this scope is not adjustable and that the spider also is not adjustable. I still wanted to verify that the focuser axis is perpendicular to the tube axis and did so by inserting the laser into the focuser, clamping a straight edge to the bottom of the birch ring that holds the "spider" and taking measurements as best I could from the laser beam to the straight edge on each side of the UTA. This seemed to check out. Next I checked to see if the beam appears to cross directly under the center hole in the spider where the secondary mirror holder mounts. Again this looks good.



Preliminary mechanicals--good.

Quote:

I then used the sight tube to adjust the secondary until it appears round and centered when looking through the sight tube.



Your image taken through the sight tube looks pretty good. I'll probably be able to tell more when you uncover the primary mirror (I'm just guessing but I suspect when you uncover the primary mirror and try to adjust the secondary mirror tilt with the laser, concentricity is lost).

Quote:

What seems odd at this point is that the secondary mirror holder needs to be adjusted as far as it will go toward the primary in order for the secondary mirror to appear centered when looking through the sight tube.



The attachment looks good to me, the wingnut threads are fully utilized, and there's room to make any necessary secondary mirror adjustments.


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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Vic Menard]
      #5903787 - 06/05/13 12:14 PM

Thank you everyone for the responses so far. I have read the thread where Gene posted his excellent review of issues he had with the 18 UC. Several other good posts there too, all helpful. I plan to continue with the collimation process hopefully tonight. I realized that I have not yet checked to see if the hole that the secondary mirror holder mounts to is centered on the birch ring. Dave Kriege told me that this setup is self centering but perhaps I should check it anyway? I could get a threaded rod and insert it where the secondary mirror holder goes and then measure from the edge of the birch ring to the threaded rod in several points around the birch ring. Do you folks feel that this is necessary or should I assume that the spider is in fact self centering?

I am very excited that I might be able to collimate this telescope soon and start enjoying the wonderful views that I have read this telescope is capable of providing.


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906492 - 06/06/13 05:42 PM Attachment (25 downloads)

Okay, I have adjusted the diagonal to aim the laser at the mirror center spot.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906497 - 06/06/13 05:44 PM Attachment (24 downloads)

and adjusted the primary to return the beam to the center of the back side of the laser

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906503 - 06/06/13 05:46 PM Attachment (34 downloads)

this is now the view through the sight tube.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906509 - 06/06/13 05:50 PM

I am noticing at this point that two of the primary mirror clips are well visible while one (bottom left of reflection) is just barely visible...

Does this indicate that something is improperly adjusted?


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906523 - 06/06/13 05:53 PM Attachment (23 downloads)

I also noticed that although I measured the squareness of the focuser axis to the tube axis to the best of my ability, when I racked the focuser all the way in (initial alignment was made with focuser racked all the way out), the laser spot moves about 1/8 inch relative to the primary mirror center spot.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906530 - 06/06/13 05:56 PM

And also that I could not see the edges of the primary mirror through the sight tube.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906546 - 06/06/13 06:05 PM Attachment (18 downloads)

The only way I can figure to adjust the axis of this focuser is to perhaps place some shims under the inside edge of the bracket that it is mounted in, between the bracked and the birch ring. The moving laser spot would seem to indicate that the focuser needs to be tilted up a little, toward the top of the telescope. Am I on the right track?

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Fred1
Carpal Tunnel
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Reged: 09/19/07

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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5906589 - 06/06/13 06:29 PM

Hmmm... it doesn't appear to have a collimatable base. Before getting too experimental, try calling Starlight Instruments to see what they suggest. I've found them to be very responsive.

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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Any 18"UC owners out there? new [Re: Fred1]
      #5906625 - 06/06/13 06:54 PM

The laser beam should not move as the focuser is racked in and out. This indicates that either the laser is not aligned to the focuser drawtube, or the drawtube is not aligned to the focuser body.

Two questions:
Have you checked the laser alignment through several rotated positions? (Be sure to resecure the laser with the same locking screw each time.)

When you say you racked the focuser all the way in and out, did you run the focuser to the absolute extreme? If the misalignment only occurs at the extreme in or out position--it's possible that you've exceeded the milled flat track that the drive shaft engages. As a rule, I try to keep the focuser motion "inside" these extreme positions to minimize torquing the drawtube.

Looking at your view through the sight tube, I (think I) see a tilt/rotation error. Surrounding the dark silhouette shadow of the secondary mirror, I can see two mounting screws near twelve o'clock and one mounting screw at about eight o'clock, but I can't see the mounting screw at four o'clock.

The sight tube cross hairs appear to be a little high and to the left of the primary mirror center spot (this is the same alignment as aligning the laser dot to the primary mirror center spot). I suggest shortening the sight tube about 3/4 inch and racking the focuser closer to see the primary mirror in the secondary mirror.

Edited by Vic Menard (06/07/13 08:39 AM)


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Vic Menard
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Reged: 07/21/04

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Re: Any 18"UC owners out there? new [Re: Fred1]
      #5906655 - 06/06/13 07:10 PM

Just giving a second look at your view in the sight tube.
It's possible that the four o'clock screw head is illuminated causing it to blend in to the primary mirror background.
Assuming the primary mirror clips intrude equally around the circumference of the primary, you could tilt the secondary mirror (the adjustment screw behind the two paired screws at twelve o'clock) to get a better centering.

I'll wait to hear if you have sorted out why the laser moves when the focuser is racked in and out...


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Mirzam
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Re: Any 18"UC owners out there? new [Re: Vic Menard]
      #5906659 - 06/06/13 07:11 PM

Or it may mean that the top ring is flexing and causing collimation shift due to the weight of the laser.

Otherwise the initial collimation looks okay to me.

JimC


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5917513 - 06/12/13 06:42 PM

Sorry for the delay. Grandkids visit and 29th wedding anniversary are over. I'm ready to get back to work on my telescope. Can someone please tell me how to pull quotes so that I may respond to individual posts? I have looked around but haven't found out yet how to do it.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5917518 - 06/12/13 06:45 PM

I have tried applying a small amount of pressure to the end of the laser and also to the edge of the birch ring below the focuser and this does cause the laser beam to move the same as if I rack the focuser in and out. I'm thinking that the birch ring is in fact flexing from the weight of the laser.

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Mirzam
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5917605 - 06/12/13 07:49 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!

JimC


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5918598 - 06/13/13 11:51 AM

Quote:

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?


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5918609 - 06/13/13 11:58 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. 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.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5918970 - 06/13/13 03:15 PM Attachment (19 downloads)

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


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5918974 - 06/13/13 03:17 PM Attachment (15 downloads)

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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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5918977 - 06/13/13 03:19 PM

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

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Mirzam
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5919307 - 06/13/13 06:35 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


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5919680 - 06/13/13 10:29 PM

Quote:

Quote:

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.


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5919688 - 06/13/13 10:33 PM

Quote:

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.


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5919699 - 06/13/13 10:43 PM

Quote:

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.


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wfj
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Reged: 01/10/08

Loc: California, Santa Cruz County
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5920757 - 06/14/13 03:02 PM

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

Quote:

Quote:

Quote:

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.


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: wfj]
      #5920896 - 06/14/13 04:34 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.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5921088 - 06/14/13 07:04 PM Attachment (18 downloads)

Quote:

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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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5922572 - 06/15/13 05:20 PM

Quote:

Quote:

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.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: Any 18"UC owners out there? new [Re: GeneT]
      #5922665 - 06/15/13 06:30 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.


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GeneT
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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
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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


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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Reged: 08/05/12

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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
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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
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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
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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

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Re: Any 18"UC owners out there? new [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
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Re: Any 18"UC owners out there? new [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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GeneT
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Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5932295 - 06/20/13 11:34 PM

Quote:

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.


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Mirzam
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Re: Any 18"UC owners out there? new [Re: GeneT]
      #5932637 - 06/21/13 08:34 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


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Papa Taylor
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Re: Any 18"UC owners out there? new [Re: wfj]
      #5935022 - 06/22/13 05:18 PM

Quote:

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.


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Papa Taylor
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Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5935035 - 06/22/13 05:24 PM

Quote:

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.


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Papa Taylor
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Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5935043 - 06/22/13 05:29 PM

Quote:

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.


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Papa Taylor
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Re: Any 18"UC owners out there? new [Re: GeneT]
      #5935048 - 06/22/13 05:35 PM Attachment (13 downloads)

Quote:

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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GeneT
Ely Kid
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5935329 - 06/22/13 08:53 PM

Quote:

Quote:

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.


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GeneT
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5935334 - 06/22/13 08:55 PM

Quote:

Quote:

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?


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Mirzam
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Re: Any 18"UC owners out there? new [Re: GeneT]
      #5935479 - 06/22/13 10:29 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


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wfj
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Reged: 01/10/08

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Re: Any 18"UC owners out there? new [Re: GeneT]
      #5935496 - 06/22/13 10:40 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.


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Doug McI
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5942815 - 06/27/13 10:10 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.

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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Doug McI]
      #5950425 - 07/01/13 09:55 PM

Quote:

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.


Quote:

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.

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.


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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: wfj]
      #5950460 - 07/01/13 10:16 PM Attachment (5 downloads)

Quote:

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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Papa Taylor
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Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5950498 - 07/01/13 10:42 PM

Quote:

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.


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Mirzam
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Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5950905 - 07/02/13 08:45 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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Papa Taylor
member


Reged: 09/04/11

Loc: western N.C.
Re: Any 18"UC owners out there? new [Re: Mirzam]
      #5951174 - 07/02/13 11:28 AM

Quote:

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




Thanks, Jim
I will take the scope out and check this if it ever stops raining.


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wfj
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Reged: 01/10/08

Loc: California, Santa Cruz County
Re: Any 18"UC owners out there? new [Re: Papa Taylor]
      #5971512 - 07/15/13 12:27 AM

Quote:

Quote:

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?



Correct. Realize that stiffening affects the entire component and does not have to be over the entire surface to work. Also, stiffness comes from the surface/shape of a geometric support, not the volume/weight of material.
Quote:

The parts of the aluminum 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?



Unlikely but remotely possible. By the way, you should check the brackets as well as possibly cracked.

I would also inspect the bracket to see if it was not flat but convex - that would be an example of how you might get the appearance of flexure without any breaks, because the thrust bearing angle of the TE's might rotate/tilt because the bracket isn't fixed against the ring, allowing freedom of movement.

The general theme is to "stiffen" a structural member and see if the deflection amount changes - sometimes you don't need to stiffen everything.


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Almagne
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Reged: 02/25/08

Re: Any 18"UC owners out there? new [Re: wfj]
      #6021082 - 08/12/13 03:44 PM

Although this thread is a month dead, here is a link to the ATM forum, where my fix for the upper ring flexing is shown:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/6017607/page...


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