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Equipment Discussions >> Reflectors

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Vic Menard
Post Laureate
*****

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
Post Laureate
*****

Reged: 07/21/04

Loc: Bradenton, FL
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
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
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
member


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
member


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
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
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
member


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
member


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
member


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
member


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
member


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
Post Laureate
*****

Reged: 04/01/08

Loc: Lovettsville, VA
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
*****

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
*****

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
*****

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
Pooh-Bah
*****

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
*****

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
member


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
*****

Reged: 11/07/08

Loc: South Texas
Re: Any 18"UC owners out there? [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)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: Any 18"UC owners out there? [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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