Primary mirror - tension question

[Pixies]

With the bright skies up here now, I'm going to take a few weeks' break from observing and focus on a few more practical and educational things. One thing I fancy is getting the collimation really dialled in. But I had one outstanding question. When adjusting the primary with the 3 screws, am I right in thinking you could evenly tighten or loosen all 3 screws equally and collimation would remain true - but the mirror would just be moved up or down the tube?

Have I understood the mechanics correctly? I'm yet to remove the mirror assembly, so haven't seen it up close. But the impression I get is a spring-loaded assembly that is pushed back against the adjustment screws. So you could tighten them all against the spring force and move the mirror up the tube towards the secondary (very slightly), or loosen them off and the mirror would move back down the tube until you had loosened it enough for the springs to reach their extension limit, or the mirror meet a physical stop?

It's a Bresser 8" dob. Is there a recommended tension/position for the mirror, around which the collimation adjustments should not deviate too much? I could imagine that if one adjusted collimation by only ever tightening the screws, it would eventually reach a limit?

 

Ta

 

[Robindonne]

Im not a pro but when you reach the limit of the screws, just start over and wind them all the way back to the “start”.  Or just stay safe and stay in the middle.

You should be “pulling” the mirror when collimating.  And the “extra” Security screws puts pressure in the same direction as the springs. 
 

but im also curious if this is the correct way to go?!

Edited June 25, 2020 by Robindonne

[DaveL59]

if you alter any of the screws its not easy to get them all tightened/loosened to the exact same degree so you may well alter the alignment a touch. Some systems use springs to keep the cell tension against the adjuster screw, others may use different methods (rubber grommet etc) or even nothing at all as I found on the TAL-M but then that's a small mirror and it does now have springs.

Ideally you shouldn't need to make big adjustments if the mirror is already close to dead-on. You really do not want to unscrew a large amount as you then risk the bolts coming free of the mirror carrier and it floating off by itself.

The other screws are there to lock the adjustment, so you'd loosen them a touch, make the adjustments on the main adjuster/thumb-screws and then tighten to lock the setting. Then re-check as you may have caused a slight drift and tweak as needed.

hope that helps

[NGC 1502]

Some primary mirror collimation screws also have lock screws, easy to tell, if there’s 3 pairs of screws 3 will be for adjustment 3 will be for locking.

Some cells just have 3 spring loaded adjustment screws, the adjustments being held by spring tension, from what you’ve said sounds like yours.  As long as there’s adequate tension on each of the 3 screws then you could raise or lower the primary a bit.   Usually if the collimation screws are tightened this will draw the mirror further from the secondary.

Suggestion - tighten all 3 fully (without over-tightening) you’ll reach a point when no more tightening is possible.  Then collimate by releasing tension as necessary.  That way all 3 springs will be under some tension.  If 1 or more springs have little or no tension then collimation is easier to lose, especially if the whole scope is shifted around or transported.

Ed.

[Pixies]

Hi,

Thanks for the answers. The Bresser has locking screws, and springs as well, by the feel of it. I was just aware that I usually adjusted the primary by tightening. So - instead of loosening one screw, i'd tighten the other 2 (if you see what I mean). I disn;t want to go too far, but nervous about loosening them all too much.

Is there a recommended amount of tension / position when installing the mirrors? ie. a good starting point?

[johninderby]

The Bresser 8” has a six point mirror cell and heavier and longer springs than a SW. so a greater range of adjustment. 

The 10” has a nine point cell. 

[Stardaze]

I’m probably somewhere in the middle at the minute. Still experiencing mine not holding collimation between sessions so my thinking is that I’m going to just increase tension each session ie not loosen any. See whether there’s a point it starts to hold better? I guess you could tighten them all as tight as possible and release gradually until you find collimation. Also stands to reason that as you raise/lower the mirror marginally, the effect will be seen at the eye piece focal point. Albeit must be quite small.

Edited June 25, 2020 by Stardaze

[NGC 1502]

I misunderstood the original post.  If the image above shows the correct scope, it has 3 colliimation and 3 lock screws.

( a 6 or 9 point mirror cell is not relevant here, that refers to how many points the primary mirror sits on in the cell, not how many adjustment and lock screws there are ).

If collimation is frequently lost, there’s several possible reasons.  Not enough tension on 1 or more collimation springs, loose primary cell to tube bolts, loose mirror to cell clips ( mirrors must NOT be clamped, but if you take that too far then the mirror can flop around), something loose at the other end - check spider etc for loose bolts, no tension on spider vanes etc.

Ed.

[Pixies]

Hi again,

Sorry if I didn't make myself clear. I'm not having issues keeping the scope in collimation. Far from it  - it's very stable. Probably because the adjusters are at the tighter end of their range.

I was just wondering if there was a preferred option. Perhaps I have found it more by luck than judgement!

[johninderby]

I am quite well aware that the being a six or nine point cell has nothing to do with collimation.

Edited June 25, 2020 by johninderby

[NGC 1502]

8 hours ago, johninderby said:

I am quite well aware that the being a six or nine point cell has nothing to do with collimation.

 

John, sincere apologies if my comment caused any offence, absolutely none intended, just clarification.

Ed.

[johninderby]

No offence taken.🙂

Edited June 26, 2020 by johninderby