38 replies to this topic

[Salmja79]

Hi,

 

I  have some collimation woes with a 10" GSO Newtonian (Stellalyra F/4)and was hoping the community would be able to advise. The scope loses collimation when slewing in RA. I upgraded the collimation  springs on the primary mirror which has improved the situation but not resolved it (link to springs used is below).  It may be flexture so I wanted to see if anyone else has had this issue with a large Newt and how they have managed to fix it (if they did). I currently think I have two options:-

 

1) Replace the locking bolts with 3 additional collimation springs (link to springs below This is the cheapest option but hear there may be some downsides (stress on the mirror cell, collimation bolts etc) and it may not resolve the issue

2) replace the stock GSO secondary spider with a CNC machined one (link below). This is expensive so I wanted to be sure it will fix the issue before I take the plunge.

 

Collimation springs - https://www.firstlig...ry-mirrors.html

 

Seconday spider - https://www.backyard...50-cnc-machined

 

Or are there other things I should try?

 

Thanks in advance

 

J

Edited by Salmja79, 22 April 2025 - 07:20 AM.

[Avgvstvs]

I'm assuming the mirror cell has additional screws to lock the mirror down when collimated. Instead of more springs you could add washers to the spring assembly to compress the springs a little more perhaps. Or add one extra turn to each screw. Pictures could help.

Edited by Avgvstvs, 22 April 2025 - 05:24 AM.

[mrowlands]

"1) Replace the locking bolts with 3 additional collimation springs"

 

Not sure I understand that.  Normally, you wouldn't want springs on the "locking bolts".

 

Mike R.

[Salmja79]

"1) Replace the locking bolts with 3 additional collimation springs"

 

Not sure I understand that.  Normally, you wouldn't want springs on the "locking bolts".

 

Mike R.

There are a couple of threads on this spring thing with some saying 

 

https://stargazerslo...y-duty-springs/

 

and

 

https://www.cloudyni...locking-screws/

 

 

I'd never heard of this being done before either so wanted to see if it is indeed a "thing" and if so, whether anyone has had any success with it (or failures)

[Salmja79]

I'm assuming the mirror cell has additional screws to lock the mirror down when collimated. Instead of more springs you could add washers to the spring assembly to compress the springs a little more perhaps. Or add one extra turn to each screw. Pictures could help.

Thanks for your reply, yes, it's a  standard mirror cell with 3x collimation bolts (with springs) and 3x locking bolts. I'll see if I can add a picture later.  The collimation bolts are tightened almost all the way down (new springs are harder to compress) and the locking bolts are left loose and scope holds collimation better that it did before. Since upgrading the springs the collimation shifts by the same amount when slewing  whether I have the locking bolts tightened or not (this was not the case on the stock springs)

[Jim T]

Mirror cells basically 'float' the primary. If you transport your scope, especially horizontally, the cell and mirror may not be properly aligned. It could be that your mirror is shifting in the cell as you use it.

Before you make major modifications I'd advise the following. When setting up your scope, point it to the zenith and give it a little shake or two to settle the primary and center it in the mirror cell. Do this before collimation.

If you continue to have problems, my next step would be to examine the mirror cell closely where it engages the primary, in case something is missing or jammed. I've had to replace bits of cork (spacers) on cheaper cells, and pressboard cells can warp with moisture/humidity.

[Salmja79]

Thanks for your reply, yes, it's a  standard mirror cell with 3x collimation bolts (with springs) and 3x locking bolts. I'll see if I can add a picture later.  The collimation bolts are tightened almost all the way down (new springs are harder to compress) and the locking bolts are left loose and scope holds collimation better that it did before. Since upgrading the springs the collimation shifts by the same amount when slewing  whether I have the locking bolts tightened or not (this was not the case on the stock springs)

Attached Thumbnails

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[Salmja79]

Above and below pics collimation and locking bolts

Attached Thumbnails

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[TayM57]

Hi,

 

The scope loses collimation when slewing in RA. 

First question: 

 

How are you determining this? 

 

If it is with a glatter laser, what I would do is point the scope 45 degrees and press on the glatter/drawtube of the focuser. Does the dot on the primary move? Then the flexure is coming from the focuser. 

[Salmja79]

First question: 

 

How are you determining this? 

 

If it is with a glatter laser, what I would do is point the scope 45 degrees and press on the glatter/drawtube of the focuser. Does the dot on the primary move? Then the flexure is coming from the focuser. 

Thanks for your reply. I'm collimating using the laser, locking everything down and rechecking the scope is still collimated and then slewing the scope with the laser in the focuser and the laser moves outside the centre circle on the primary.  For collimation i'm using a very light-weight Baader laser (and Cheshire backup) and the scope has a 3 inch focuser with little to no play in it even when I apply pressure so I don't believe the focuser is the issue (but could be wrong here). I've also tested with the focuser in different orientations which is why I thought it could be the Spider or the Primary Mirror cell. I've modded the cell a bit (heavier duty springs and adding cardboard between the side of the mirror and mirror cell) and the issue has improved a bit so I'm now thinking it could be the secondary as it's holding a hefty 88mm mirror

[Jon Isaacs]

Have you tightened the spider? 

 

Have you tested the collimation of the laser? Are you removing the laser when measuring the shift? 

 

Are you using a Barlowed laser to collimate the primary?

 

Jon

[Salmja79]

Have you tightened the spider? 

 

Have you tested the collimation of the laser? Are you removing the laser when measuring the shift? 

 

Are you using a Barlowed laser to collimate the primary?

 

Jon

Hi Jon, thanks for your reply. The laser remains in the focuser when i'm slewing in RA and this why i can see the movement. The  laser is itself well collimated (believe me. I went through 4 of them, all of which needed collimating ....fun.....before settling on the Baader Laser Collimator [none of the others i tried  under £120 could provide reproduceable results). I also have a Cheshire backup. I have not used a barlowed laser at this time. The focuser is a beefy 3" one with little to no play and I've  tried in different focuser orientations. Given the collimator is so light weight, I'm kind of ruling focuser sag out at this time.

 

I've also applied some mods to the primary cell as mentioned above, and  replaced the secondary's collimation bolts with Bob's knobs. The secondary mirror is large at 88mm so I'm starting to think it's the spider........

[Salmja79]

Update, I took the mirror cell off again and reset mirror clips etc to make sure not too tight or loose (etc etc). I also placed the stock collimation springs in place of the lock bolts (as they were lying around), collimated, slewed but issue remained. I added the lock bolts back leaving the additional springs in situ,  locked mirror in position and then started to slew the scope in RA but changed the focuser orientation. I usually have the focuser pointing straight down when collimating (as this is it's position I use for imaging), issue remained. When the focuser is rotated 90 degrees so it's pointing horizontally out to one side (doesn't matter which) the scope holds collimation when slewing. With focuser oriented directly upwards, collimation doesn't quite hold but it's not as severe an issue as it is with the focuser pointing downwards. I think the offset of the large 88mm secondary mirror may be the culprit and perhaps the stock spider twists a little due to this when in one orientation but not the other. My wife is going to kill me but i'm going to bite the bullet and buy the CNC machined  spider. if it solves the issue, i'll pop an update here.

[Tom Stock]

If you use a barlow in combimation with the laser collimator you should be able to determine if the primary is actually shifting.  Of it doesn't then you have flex in the focuser or secondary holder.

[ccate]

the secondary could be made tighter but left in the same position by adjusting all 3 collimation screws as well as the center screw I believe.

[Ranger Tim]

With AP as your hobby at least you don’t have any extra cash for other vices. I tell my wife I have telescopes instead of another woman. She doesn’t think that is funny.

[TayM57]

With AP as your hobby at least you don’t have any extra cash for other vices. I tell my wife I have telescopes instead of another woman. She doesn’t think that is funny.

My scopes are my mistresses, is what my wife says.

[Asbytec]

I had a similar problem with a cheap thin beam laser moving away from the primary center mark as I slewed my 12" Dob after collimation at 45 degrees toward the horizon and I believe toward the zenith, as well. 

 

I replaced the primary end with a piece of paper and noticed the laser still drifted. The movement was in the direction of gravity. I recall tugging on the focuser but could not replicate the direction of the drift. The laser was not heavy.

 

It turns out, in my case, the reason seemed to be the heavy diagonal mirror drooping a tiny bit even with the spider vanes tight. The spider was just not robust enough to hold it sitting away from the spider hub and under the focuser. 

Edited by Asbytec, 23 April 2025 - 05:37 AM.

[Salmja79]

With AP as your hobby at least you don’t have any extra cash for other vices. I tell my wife I have telescopes instead of another woman. She doesn’t think that is funny.

[Salmja79]

the secondary could be made tighter but left in the same position by adjusting all 3 collimation screws as well as the center screw 

 

 

With AP as your hobby at least you don’t have any extra cash for other vices. I tell my wife I have telescopes instead of another woman. She doesn’t think that is funny.

 

This is so true. Literally last week I told her...."i'm not going to spend any more money on this"....hit a snag and 24 hours later my mindset became "I need to spend more money on this".

[Salmja79]

I had a similar problem with a cheap thin beam laser moving away from the primary center mark as I slewed my 12" Dob after collimation at 45 degrees toward the horizon and I believe toward the zenith, as well. 

 

I replaced the primary end with a piece of paper and noticed the laser still drifted. The movement was in the direction of gravity. I recall tugging on the focuser but could not replicate the direction of the drift. The laser was not heavy.

 

It turns out, in my case, the reason seemed to be the heavy diagonal mirror drooping a tiny bit even with the spider vanes tight. The spider was just not robust enough to hold it sitting away from the spider hub and under the focuser. 

Brilliant as I think this is exactly where I am.  I think the Stock GSO secondary spider is ok and works well for visual but not up to the task for AP. 

Did you upgrade your spider and if so, did it solve your issue?

[ccate]

"torsion compensator": https://www.cloudyni...dary-stability/

[Asbytec]

Brilliant as I think this is exactly where I am.  I think the Stock GSO secondary spider is ok and works well for visual but not up to the task for AP. 

Did you upgrade your spider and if so, did it solve your issue?

No. I didn't upgrade my spider. You can check the spider by gently pushing it against gravity and see if the laser returns to top dead center on the primary mirror. I had too many issues the 12" mass produced Dob, including the spider and weight and balance, so I just sold it. I am not recommending you do the same.

[Salmja79]

My scopes are my mistresses, is what my wife says.

This is the way!

[Salmja79]

No. I didn't upgrade my spider. You can check the spider by gently pushing it against gravity and see if the laser returns to top dead center on the primary mirror. I had too many issues the 12" mass produced Dob, including the spider and weight and balance, so I just sold it. I am not recommending you do the same.

No way I can sell it as I flocked it last night. Bought it new so don't think it had ever been flocked before, we're kind of committed now and definitely bad form to flock and go! 

 

Ordered the spider (ouch) but will take 3 weeks to get here, Hopefully UK clouds should have cleared up by then!, update will follow

 

Clear skies to you