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Help regarding primary mirror clips

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#26 wrnchhead

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Posted 07 December 2018 - 09:50 AM

From what I learned in this thread, I took my GSO cell back out, loosened the clips until they were nanometers (estimate) from the top of the mirror. I noticed some gap in the side-to-side, so I used some card stock to shim the sides. So it can rotate but not slide. I did the laser tilting test, no shift in collimation. 
 

Thanks for the info guys. 


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#27 havasman

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Posted 07 December 2018 - 09:54 AM

Parallel to the ground.

Strongly recommend your initial collimation be at a pointing angle of approximately 45 to 70 degrees. No significant observing occurs at ground level and your primary is not maximally seated in the mirror cell when the scope is parallel to the ground. I suspect that may solve 90% of your issue. You want your datum point to be legitimate and the mirror cell needs to be engaged for that to be possible.

Basically you have created the problem by using poor technique. You make a single poor argument against standard practice and have based it on real, but essentially flawed experience.

If your primary mirror support system will not hold your mirror in place it is not because the standard is flawed. It could be because your mirror cell just doesn't work. But that too is unlikely because it is just one of many thousands like it. Use the tool as it is designed to be used and it will work.


Edited by havasman, 07 December 2018 - 10:04 AM.


#28 Astrohoven

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Posted 07 December 2018 - 10:37 AM

Strongly recommend your initial collimation be at a pointing angle of approximately 45 to 70 degrees. No significant observing occurs at ground level and your primary is not maximally seated in the mirror cell when the scope is parallel to the ground. I suspect that may solve 90% of your issue. You want your datum point to be legitimate and the mirror cell needs to be engaged for that to be possible.

Basically you have created the problem by using poor technique. You make a single poor argument against standard practice and have based it on real, but essentially flawed experience.

If your primary mirror support system will not hold your mirror in place it is not because the standard is flawed. It could be because your mirror cell just doesn't work. But that too is unlikely because it is just one of many thousands like it. Use the tool as it is designed to be used and it will work.

You don't get the point... and I'm not sure but I feel some aggressiveness from your side. I hope I'm wrong. Say I collimated at a 45 degree angle. First, collimation will move as I'm balancing the scope. Say I collimate after i slew to the target right before imaging, I'm sure collimation will move as the mount tracks. I'm also positive that if I collimate at a 45 degree angle and slew the scope up to 70 degrees, collimation will move as well. Change in collimation might be smaller in that case but still it's not ideal.



#29 havasman

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Posted 07 December 2018 - 10:51 AM

You don't get the point... and I'm not sure but I feel some aggressiveness from your side. I hope I'm wrong. Say I collimated at a 45 degree angle. First, collimation will move as I'm balancing the scope. Say I collimate after i slew to the target right before imaging, I'm sure collimation will move as the mount tracks. I'm also positive that if I collimate at a 45 degree angle and slew the scope up to 70 degrees, collimation will move as well. Change in collimation might be smaller in that case but still it's not ideal.

Ideal comes at a price. Perfect is unobtanium. Try using your system as it is designed to be used and see what it will do. If you can be unsure of the outcome, you may learn what the actual result may be. It is likely that your results will not approach the result of similar testing with, for instance, a Planewave CDK. But that's not news. Optimize your system and then see what it will do for you. At least try it.

 

Actually, I think I do get the point.


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#30 spencerj

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Posted 07 December 2018 - 12:34 PM

Collimation may not "move" as much as you think.  A good part of the issue you are seeing comes from collimating with the OTA horizontal.  When you do this, the mirror is likely tipped out of the cell at the time you are collimating.  This is literally the worst possible condition to collimate the scope, because the mirror is going to tip back to the proper seated position when you point the scope at the sky.  That tipping of the mirror is what makes your collimation look off--it instantly invalidates any adjustment you just made.  When pointing at the sky, the mirror tips back to where it is supposed to be.  In that position, gravity holds it in place and the mirror is supported by the cell.

 

So if you collimate at 60* and then move the OTA to different orientations through balancing and pointing to different targets, the mirror may tip a bit, but should settle when you get to where you are going and gravity is holding the mirror properly against the cell.  When the scope goes back to pointing up, the mirror should be back to where it started unless it is sliding and rotating as well as tipping.  If that is the case, you can choose to either do the best you can with what you have, upgrade the mirror cell or visit the refractor forum.  


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#31 Mike G.

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Posted 07 December 2018 - 04:28 PM

"or visit the refractor forum"

 

Heresy I say!  pure heresy!  Moderator, lock the thread!!


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#32 airbleeder

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Posted 07 December 2018 - 05:29 PM

You don't get the point... and I'm not sure but I feel some aggressiveness from your side. I hope I'm wrong. Say I collimated at a 45 degree angle. First, collimation will move as I'm balancing the scope. Say I collimate after i slew to the target right before imaging, I'm sure collimation will move as the mount tracks. I'm also positive that if I collimate at a 45 degree angle and slew the scope up to 70 degrees, collimation will move as well. Change in collimation might be smaller in that case but still it's not ideal.

    Don't be so sensitive. IMO, Havasman gave you good advice. Had you not rather given the collimation drift that it still remain within tolerance?

   If your primary cell is good and well secured to the OTA, your edge supports are sturdy,  properly spaced and at the COG of the mirror. Stiff springs. Havasman's and others advice is about all you have to try. You may be "happier" surprised.

    I had a steel tubed newtonion on a GEM that was loose all over. Focuser, secondary, primary. I would alternate with an autocollimator, combo tool and laser while moving ota in all it's various positions to track the problems. I got it all tight, no play. When it came to the primary flopping around, I made some steel edge supports, placed them 120 degrees apart at the center of gravity of the mirror and added stiffer springs. No more movement. 

   Try not get getting so frustrated that you can't think it through. It's easy to do. I have forgotten, but I believe your scope is a dob. Just be glad it's not on a GEM like mine was. If I wasn't so cool, calm and collected, I'd have probably gotten upset myself. Good luck.


Edited by airbleeder, 07 December 2018 - 05:33 PM.


#33 Starman47

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Posted 07 December 2018 - 07:14 PM

Are you guys saying that most large Dobs will have a bit of collimation shift, but it is only a concern if this shows up at the eyepiece!



#34 wrnchhead

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Posted 08 December 2018 - 12:18 AM

I think the point that's being missed here is that the angle to get something so flat and heavy to tip at all has to be 90°+. Going from 0-85° isn't going to tip the mirror. Shimming the sides keeps any slide from happening, so collimation shift in that zone has to be caused by something else. 


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#35 havasman

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Posted 08 December 2018 - 12:31 AM

Are you guys saying that most large Dobs will have a bit of collimation shift, but it is only a concern if this shows up at the eyepiece!

I'm not saying that.

 

Both my smaller mass produced Dob and medium premium Dob repeatedly exhibit stable collimation throughout a session at any useful pointing angle as do other larger and much larger premium Dobs other folks own that I have gotten to use. I had a 130mm Dob that had a bit of collimation shift but that was a $200 telescope and still quite good for the $.




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