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Any 18"UC owners out there?

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#1 Papa Taylor

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Posted 02 June 2013 - 04:53 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.

#2 GeneT

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Posted 02 June 2013 - 11:08 PM

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.

#3 Papa Taylor

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Posted 04 June 2013 - 07:50 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.

#4 UmaDog

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Posted 04 June 2013 - 08:09 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.

#5 Vic Menard

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Posted 04 June 2013 - 12:16 PM

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


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

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.

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.

#6 Papa Taylor

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Posted 04 June 2013 - 04:15 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.

#7 Papa Taylor

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Posted 04 June 2013 - 04:16 PM

here is top of the secondary mirror holder

#8 Papa Taylor

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Posted 04 June 2013 - 04:18 PM

okay let's try that again

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#9 Papa Taylor

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Posted 04 June 2013 - 04:19 PM

here is the view through the sight tube

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#10 GeneT

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Posted 04 June 2013 - 05:49 PM

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.cloudynig...rd=reflector...
Hope all this sorts out for you.
GeneT

#11 Vic Menard

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Posted 04 June 2013 - 06:22 PM

...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.

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).

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.

#12 Papa Taylor

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Posted 05 June 2013 - 11:14 AM

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.

#13 Papa Taylor

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Posted 06 June 2013 - 04:42 PM

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

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#14 Papa Taylor

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Posted 06 June 2013 - 04:44 PM

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

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#15 Papa Taylor

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Posted 06 June 2013 - 04:46 PM

this is now the view through the sight tube.

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#16 Papa Taylor

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Posted 06 June 2013 - 04:50 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?

#17 Papa Taylor

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Posted 06 June 2013 - 04:53 PM

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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#18 Papa Taylor

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Posted 06 June 2013 - 04:56 PM

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

#19 Papa Taylor

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Posted 06 June 2013 - 05:05 PM

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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#20 Fred1

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Posted 06 June 2013 - 05:29 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.

#21 Vic Menard

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Posted 06 June 2013 - 05:54 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.

#22 Vic Menard

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Posted 06 June 2013 - 06:10 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...

#23 Mirzam

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Posted 06 June 2013 - 06:11 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

#24 Papa Taylor

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Posted 12 June 2013 - 05:42 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.

#25 Papa Taylor

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Posted 12 June 2013 - 05:45 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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