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Acceptable collimation shift?

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#1 Starman47

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Posted 11 December 2018 - 10:37 AM

I have a 16" f/4.5 dob that is based upon a classic design (i.e. not a lightweight). There is a bit of collimation shift. I have tightened everything (i.e., spider, secondary holder, focuser, truss tube connection points, and the focuser brake/lock), and I have checked the sling to see that it makes proper contact with the mirror. But there is a bit of collimation shift.

 

I collimate with the scope at 45 degrees. When i move it upward there is a bit of shift. When I move the telescope down there is a bit of shift. So, my question is simple. Is  a bit of shift to be expected in a larger telescope? And is there a point where one should learn to live with the shift? 

 

I have not tested the telescope to see if there is a degraded image due to the shift. And yes I know I really should do that.

 

 


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#2 GoFish

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Posted 11 December 2018 - 10:53 AM

I see this same issue with my 8” f/4 Newt. I’ll be watching this thread to see what, if anything, can be done to solve it (short of recollimating at each scope position). 

 

I’m presently scratching my head over who the bad actor(s) may be:

  1. Gravity-induced secondary mirror deflections
  2. Overall bending of the tube, like a beam supported in the middle with weights on both ends
  3. Distortion of the tube at the focuser attachment
  4. Bending of the focuser itself


#3 Starman1

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Posted 11 December 2018 - 10:56 AM

Shift in which alignment?

Do you mean a laser beam moves on the primary when you move the scope up and down?

Or do you mean the laser beam stays fixed on the primary but the return beam moves on the laser?

 

Have you tightened the spider?  Typically, vanes need to be piano-wire tight.  Even a bit of looseness will allow a laser beam to move on the primary.

If it's a classic design, did you use 1" poles?  That could allow enough sag to occur in the UTA to see a shift like that.

 

How is the primary mirror supported behind the back of the mirror?  Do all points actually contact the mirror or do, perhaps, you have a sticky triangle?

What kind of sling are you using under the primary mirror's edge?  How wide is the sling?

 

And last, whatever the answer to the first questions, how much shift occurs?  A millimeter or two or a centimeter?


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#4 Starman47

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Posted 11 December 2018 - 11:14 AM

Please read the original post.



#5 Starman1

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Posted 11 December 2018 - 12:00 PM

Sorry, but none of the questions I asked is answered by the first post.

The answers would help pin down a directed piece of advice.


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#6 Redbetter

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Posted 11 December 2018 - 02:21 PM

For the past 2 years I have been fighting some similar collimation shift gremlins with my 2003 vintage Obsession 20" f/5 classic.  Everything was tight, etc. and while I chased after suspected contributing problems, one that was variable and noticeable was the primary shifting around from the way it was positioned in the sling.  This was most noticeable when near vertical or near horizontal.  At intermediate positions the shift was less obvious and more variable--some nights negligible, sometimes quite problematic.  Since it was sometimes good, I assumed it was mostly a matter of getting the sling set just so...but I had a lot of reproducibility issues with that.

 

This month I believe I have fixed the problem.  I ordered a replacement sling from Dave Kriege and installed it.   I have only been able to use the scope twice since then (one of those ended almost immediately with high level cloud), but I had virtually no collimation shift with the Glatter Tublug in place running across the range of altitude...both times, after disassembly/transport/reassembly.  It hasn't behaved this well at any time in the past 2 years...knock on wood/Baltic birch plywood.   

 

My original sling was the old thin but broad yellow sling.  It had taken some wear from where the mirror moved around during transport (500 curves each direction to one of my dark sites.)  This had actually begun to wear through the upper portion of the belt where it was next to the bumper/rollers that the mirror clips attach to.  I finally wised up and began inserting pipe insulation around two of those rollers during transport so that the mirror can't move, but the damage was already done by the time I noticed. 

 

The new sling is a much thicker weave of different material, and doesn't appear to be quite as broad.  It was a little tricky to install because the original slots that it inserts through were very tight for the new thickness.  I had to do some beveling of the edges of the sling holder shaft slots to get the ends through without fraying.  And I had to use a hot putty knife like an iron to re-adhere and mash the ends a little since they had developed some separation--from the initial attempt before I modified the slots. 


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#7 txmnjim

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Posted 11 December 2018 - 02:33 PM

I have a 16" f/4.5 dob that is based upon a classic design (i.e. not a lightweight). There is a bit of collimation shift. I have tightened everything (i.e., spider, secondary holder, focuser, truss tube connection points, and the focuser brake/lock), and I have checked the sling to see that it makes proper contact with the mirror. But there is a bit of collimation shift.

 

I collimate with the scope at 45 degrees. When i move it upward there is a bit of shift. When I move the telescope down there is a bit of shift. So, my question is simple. Is  a bit of shift to be expected in a larger telescope? And is there a point where one should learn to live with the shift? 

 

I have not tested the telescope to see if there is a degraded image due to the shift. And yes I know I really should do that.

I have the same issue w my new (to me) 17.5" truss dob. I will be watching this thread closely...



#8 Starman1

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Posted 11 December 2018 - 02:43 PM

For the past 2 years I have been fighting some similar collimation shift gremlins with my 2003 vintage Obsession 20" f/5 classic.  Everything was tight, etc. and while I chased after suspected contributing problems, one that was variable and noticeable was the primary shifting around from the way it was positioned in the sling.  This was most noticeable when near vertical or near horizontal.  At intermediate positions the shift was less obvious and more variable--some nights negligible, sometimes quite problematic.  Since it was sometimes good, I assumed it was mostly a matter of getting the sling set just so...but I had a lot of reproducibility issues with that.

 

This month I believe I have fixed the problem.  I ordered a replacement sling from Dave Kriege and installed it.   I have only been able to use the scope twice since then (one of those ended almost immediately with high level cloud), but I had virtually no collimation shift with the Glatter Tublug in place running across the range of altitude...both times, after disassembly/transport/reassembly.  It hasn't behaved this well at any time in the past 2 years...knock on wood/Baltic birch plywood.   

 

My original sling was the old thin but broad yellow sling.  It had taken some wear from where the mirror moved around during transport (500 curves each direction to one of my dark sites.)  This had actually begun to wear through the upper portion of the belt where it was next to the bumper/rollers that the mirror clips attach to.  I finally wised up and began inserting pipe insulation around two of those rollers during transport so that the mirror can't move, but the damage was already done by the time I noticed. 

 

The new sling is a much thicker weave of different material, and doesn't appear to be quite as broad.  It was a little tricky to install because the original slots that it inserts through were very tight for the new thickness.  I had to do some beveling of the edges of the sling holder shaft slots to get the ends through without fraying.  And I had to use a hot putty knife like an iron to re-adhere and mash the ends a little since they had developed some separation--from the initial attempt before I modified the slots. 

You mention the new sling isn't quite as broad, which I infer means it is narrower than the edge of the mirror.

Bear in mind that the new sling should not be positioned exactly centered on the edge of the mirror, but a millimeter or two closer to the rear of the mirror than the front.

The front/rear center of gravity is closer to the back than the front

(see: http://www.cruxis.co...ecalculator.htm  to calculate how much)

and if the belt is centered, this will induce astigmatism in the star images when the scope points low.

You can see from the calculator what effect this has when it is even slightly off.

 

Sounds like a big improvement, though.


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#9 havasman

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Posted 11 December 2018 - 02:57 PM

Well then "a bit" is just too much. Or about right. Either way.

None is good. Any is bad. That too.


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#10 airbleeder

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Posted 11 December 2018 - 04:46 PM

Sorry, but none of the questions I asked is answered by the first post.

The answers would help pin down a directed piece of advice.

   I agree with Don. 


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#11 niallk

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Posted 11 December 2018 - 05:17 PM

I had observed a small shift in the returned beam with a barlowed laser: one pivot point on a floating cell triangle not moving freely it was a smidgen tight. Just a fraction of a turn of the acorn nut and all was freely moving: collimation rock solid over full altitude range :) (Obsession 15" Classic).

Edited by niallk, 11 December 2018 - 05:18 PM.


#12 Starman1

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Posted 11 December 2018 - 05:24 PM

I had observed a small shift in the returned beam with a barlowed laser: one pivot point on a floating cell triangle not moving freely it was a smidgen tight. Just a fraction of a turn of the acorn nut and all was freely moving: collimation rock solid over full altitude range smile.gif (Obsession 15" Classic).

Excellent.  This could also be an issue for the OP if that's where he sees the shift.


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#13 havasman

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Posted 11 December 2018 - 05:29 PM

Sorry, but none of the questions I asked is answered by the first post.

The answers would help pin down a directed piece of advice.

Hence my "answer" in post #9.

 

And - NUMBER NINE  NUMBER NINE NUMBER NINE NUMBER NINE NUMBER NINE NUMBER NINE


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#14 Starman1

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Posted 11 December 2018 - 05:43 PM

slap.gif

Tsk.



#15 Redbetter

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Posted 12 December 2018 - 04:13 AM

You mention the new sling isn't quite as broad, which I infer means it is narrower than the edge of the mirror.

Bear in mind that the new sling should not be positioned exactly centered on the edge of the mirror, but a millimeter or two closer to the rear of the mirror than the front.

The front/rear center of gravity is closer to the back than the front

(see: http://www.cruxis.co...ecalculator.htm  to calculate how much)

and if the belt is centered, this will induce astigmatism in the star images when the scope points low.

You can see from the calculator what effect this has when it is even slightly off.

 

Sounds like a big improvement, though.

Yes, it has been an improvement.  I am able to place the sling where it "feels right" again, and it doesn't seem to result in mirror tilt at extreme angles (still need some sessions to confirm, but the first impressions have been good.)  The other issue I had was the need to run the collimation up as high as it would go for the 31T5.  The pins holding the sling don't move with the collimation travel, so this means the attachment point has been well behind where I had used it on the mirror in the past.  I account for this when positioning the sling.  This combined with the separation of the top third of the sling were creating an unworkably sensitive/critical sling position, and at times I was seeing astigmatism from the sling as well as the collimation shift.  I had a feel for and could visualize where and why the tilt was occurring, but I had too many dependent variables to try to balance with some stretch as well as tear/wear of the upper portion of the sling.  That is why I finally decided to stop fighting it and order the new sling. 



#16 Starman1

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Posted 12 December 2018 - 10:26 AM

If you get there again in a few years (and it sounds like it'll be a few years), I recommend a Glatter wire sling, available from Teeter's Telescopes.

With a wire, there is no possibility of a variable stretch in the material or a pressure point that varies from front to back on the belt.

Like all slings except the aluminum one used in Starstructure telescopes, the mirror can still swing side to side, so use of the scope on a tracking platform

is not good for collimation and having a level base is desirable.



#17 iKMN

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Posted 12 December 2018 - 11:20 AM

I've had collimation shift in ALT movement with practically every newt I've owned, especially truss.    I've only owned cheap commercial newts though not the fancy custom types.  For the most part, I think I 'worried about it too much'.  Unless you're really getting bad views from it I wouldn't be too concerned.  I know it's easy said then done and I too ran through the typical tightening of everything to make sure and it did decrease but not eliminate completely the shifting.  FWIW, and I'll get roasted for this... last star party my mirror completely came out of the cell when I walked away from an unbalanced scope that nosedived on me.  I snapped it back in place and loe and behold the image was still good enough that I didn't want to bother with collimation again.  I could tell it was a bit off but it still put up good views - lol.  YMMV. 

 

k


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#18 Kunama

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Posted 12 December 2018 - 08:27 PM

If you get there again in a few years (and it sounds like it'll be a few years), I recommend a Glatter wire sling, available from Teeter's Telescopes.

With a wire, there is no possibility of a variable stretch in the material or a pressure point that varies from front to back on the belt.

Like ALMOST all slings except the aluminum one used in Starstructure telescopes, the mirror can still swing side to side, so use of the scope on a tracking platform

is not good for collimation and having a level base is desirable.

Twin 90º slings do not allow side to side movement like a single 180º one...... and if fitted with 4 linear bearings, they put no strain on the mirror during collimation or use....

 

Regards the OP's question, if it is a secondary holder like the Astrosystems one it may need a counterweight on the end of the mounting rod to counteract the moment of the mirror hanging on one side of the spider... most spiders will deflect a little with the scope at elevations lower than 60º.  

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Edited by Kunama, 12 December 2018 - 10:51 PM.

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#19 Starman47

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Posted 13 December 2018 - 09:52 AM

Thank you all for your informative posts.  I now hope to sell the 16" telescope, and to investigate different designs while at the next star party.  If the telescope does not sell, then I may look into selling the telescope without optics and building a more portable 16" telescope using Albert Highe's design information. (Comments are welcome on the plan)

 

Oh my 12" Dobstuff - Easy Travel Telescope (over 24,000 miles and counting - mostly from moving to Australia and then back to Tennessee) has almost no flex and almost no collimation shift.. My desire is to change out the current secondary and add a 1/20 wave Antares secondary into the system. I am hesitant now to use an Astrosystem spider and spider holder due to the possibility of flex caused by the heavy secondary holder.(see post #18). The good news is that if the new spider introduces more flex, then I can return to the original 3 vane spider and perhaps use the Antares secondary in the original holder. So, if the changes do not work, then no harm, no foul

 

Just FYI, the GOS pm in the 12" Dobstuff was tested. I was gob smacked to learn that it had an excellent Strehl = .96. In fact, I was told that the mirror was an excellent example of what GSO can produce, and it would be a waste of money to refigure the mirror. My views are only limited by the quality of the secondary (and of course "seeing," "transparency," and my older eyes - smile.gif ). 



#20 Starman1

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Posted 13 December 2018 - 10:28 AM

The Astrosystems secondary and spider combinations are available with different thicknesses of vanes, and flexure can be kept to a minimum by

keeping the secondary up close to the spider vanes.  Or by counterweighting it, as was suggested.



#21 Kunama

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Posted 13 December 2018 - 03:05 PM

For a truly portable 16" scope (that still holds collimation) I suggest you contact 'bratislav' on the forum for details of his custom build......


Edited by Kunama, 13 December 2018 - 03:05 PM.

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#22 MitchAlsup

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Posted 13 December 2018 - 09:39 PM

Twin 90º slings do not allow side to side movement like a single 180º one......

I fixed my mirrors lateral movements with 2 nylon 1/4-20 screws, and allowing the mirror 0.010 movement at 45ºF.

20" F/4 DOB semi-lightweight.


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#23 Redbetter

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Posted 14 December 2018 - 12:23 AM

3 nights now (2 of them full sessions, unabbreviated) with the replacement sling and the primary collimation has been very stable.  It is back to collimate and observe, rather than rechecking/recollimating when observing low or near zenith.  Five hours last night and I never had to adjust it.  I was observing targets from about 25 to 85 degrees during the session.


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#24 Starman1

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Posted 14 December 2018 - 12:45 AM

I fixed my mirrors lateral movements with 2 nylon 1/4-20 screws, and allowing the mirror 0.010 movement at 45ºF.

20" F/4 DOB semi-lightweight.

That would work fine.



#25 Redbetter

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Posted 14 December 2018 - 03:20 AM

If you have Obsession type edge support (belt + 2 fastening screws for belt).
The heavy mirror pulls the belt (any) and bends the screws. It is treated by replacing the belt on the cable and fix the cable to the frame of the mirror cell (cable without loops at the ends).

It works well for me, including an aluminum mirror cell (mirror weight ~18 kg or 40 lb) or steel mirror cell (mirror weight ~25 kg or 55 lb).

No, I doubt it is bending the bolts for the belt on the 20" with its 2" thick mirror, or at least not a noticeable amount.   Those bolts are through the frame bars near where they tee.  If there is bending there, it isn't showing up with Barlowed laser running the elevation up and down with a new thicker belt.   Even with the mirror vertical each would be holding less than 20 lbs and with less than 2" of bolt length to the center of where the force is acting relative to the frame.




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