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kniclander

ten years i resisted... : (

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been happily observing for ten years plus with nothing more than an occasional tweak to the primary.  yesterday i took out the primary and gave it a little clean (it was totally covered in gunk as the scope's been semi exposed to the elements for a few years) and that seemed to go ok.  today i took out the secondary just to see if it was clean (it was).  taking it out was easy but putting it back - woah!  every time i got it properly centred and nice and round I then tried to tighten up the big screw and the damn thing moved again.   the only way i could do it was trial and error by getting the big screw semi tight and then progressively tightening the little hex bolts and essentially doing the centring and adjusting the tilt as a single exercise.  Is that how you're supposed to do it?

thanks

 

 

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I had th same prob myself th other week n thts Wat I had to do just tinker with it until everything was centred in th focus tube few people on SGL says a collimation cap and chesire are best to sort this out if u hav any of those hth 

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2 hours ago, kniclander said:

 Is that how you're supposed to do it?

Yes - that's how it's done. Whilst it can be argued that the whole assembly could be much better designed, removal is far less frequent than minor collimation tweaks so I guess it's a cost-cutting thing.

I sometimes wonder if those who say that collimation is easy fully understand the subject. A full secondary collimation can be a very tricky exercise because the geometry isn't entirely straightforward and the tweaks interact. If the mirror is physically centred on the spider vanes (as it tends to be on modern scopes) then you need to offset the secondary towards the primary somewhat, which means it's not quite at 45 degrees and introduces some amount of cone error that you just have to live with. You also need to ensure that the secondary assembly doesn't get twisted relative to the focuser which is tricky since it does tend to shift as you tighten everything up - and of course, the simplistic mechanism means that all these adjustments will interact to some degree, so the process becomes iterative and time-consuming.

Makes it all the more satisfying when it all comes together though! :-)
- and of course subsequent adjustments are trivial in comparison.

Edited by great_bear

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Thanks GB. Yes, the guides tend to give the impression that its simply sequential rather than iterative. 

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Ten years sounds about the right time to wait before removing mirrors ;)

Secondary holders are notoriously flimsy but it sounds like you're maybe doing things the wrong way round. Secure the big bolt first - tighten until the secondary is central with respect to the horizontal axis of the focuser (the holder may be floppy and you may need to support it with your hand). Then tighten the small hex bolts to fix the secondary in place. Make the secondary central with respect to both horizontal and vertical axes of the focuser (don't sweat if it's not exact - there may not be enough travel on the big bolt, for example). Then collimate your scope. The easiest way is to use a laser to adjust the secondary - adjust the hex bolts until the dot is centred on the primary. Then adjust the primary using a Cheshire.

With the secondary, the principle to bear in mind is that the central bolt is what stops it falling on your primary (which is why you do all this with the tube horizontal), the small hex bolts are for fine adjustment. Tension between the big bolt and the three little ones is what keeps the secondary in position. To make the whole thing easier do two things. First replace the hex bolts with knobs (e.g. from Bob's Knobs). Then cut out a little piece from a plastic milk bottle, make a central hole in it to accommodate the big secondary bolt, and position it at the top of the secondary holder. This will stop the small bolts skidding and make collimation a lot easier.

 

Edited by acey

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OK but the secondary can be roughly centred horizontally and vertically but still not be round as seen through the collicap so it still needs to be twiddled with the main bolt loose?  But you need to secure it with the hex screws not the main bolt.  And then you collimate it against the primary with the hex screws but to do that you need to loosen off the main bolt and how do you stop it twisting again?

PS don't rush to answer as I'm going to leave it at least another ten years until I fiddle with it again... 

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Hav you previously collimated your scope before started having th prob with th secondary mirror 

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I found tht wen I lossen tht primary screws thn tryd again it help wen setting th secondary mirror because th primary was still in same position wen it was collimated at 1st try this see is it any help

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Ultimately it's just a game of balancing the "push" of the hex screws against the "pull" of the centre-bolt. As you've (no doubt) discovered, you have to keep - shall we say - "re-asserting" the correct degree of twist of the secondary assembly in order to keep it looking circular as seen through the finder. If you simply tighten everything without doing this, the secondary can tend to do its own thing. It's a bit of a fight really - especially since the hex screws may have already ground slight dimples into the back of the mirror holder, thus influencing how it settles.

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On 13/04/2016 at 19:57, kniclander said:

....   And then you collimate it against the primary with the hex screws but to do that you need to loosen off the main bolt and how do you stop it twisting again?

 

You don't need to loosen the central screw to adjust the three tilt-adjustment screws.  When the circular outline view of the secondary is centred in the drawtube, you don't want to be moving the central screw.  When everything is snug, adjust the tilt screws by loosening one of the three by a very small amount and immediately take up the slack by tightening one or both of the other two; that will keep everything tight and stop the secondary from rotating. Do it in small steps and always balance the loosening of one with the tightening of the other ones. The central screw should remain snug throughout and not need re-tightened.

One potential problem if you have loosened off the tilt adjustment screws a long way is if you have not kept them roughly in step and inadvertently introduced a big tilt error.  If it's a long way out, it's possible then that the outline does not appear centred or circular, and you start to 'correct' the problem by rotating or moving the secondary.  So without realising, you are trying to fix a severe tilt error error by introducing a rotation error!  If the three adjustment screws are still close to their original positions before you removed the secondary, you should be OK.  But if you turned any of the screws in or out a few turns and did not turn the other two by about the same amount to keep them roughly in step, it could be that you introduced a big tilt error and should correct that first.  A rough check is to measure, or just eyeball, the gap between the bottom of the spider central boss where the points of the adjustment screws emerge, and the back of the secondary mount where the screws press down.  The gap should look pretty even all round, so the back of the secondary mount is approximately parallel to the spider boss.  In other words, the three screws are all screwed down about the same amount and there's no obvious big tilt on the secondary mount at the outset.  If it looks badly out, correct that first before making centring and rotation adjustments.

Adrian

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All good stuff above - and the definitive check as to whether you're compensating tilt error with rotation is to examine the positioning of the *shadow* of the secondary when viewed through the focuser. If your mirror is mounted centrally to the vanes (like most mass-produced scopes, and certainly the SkyWatcher ones) then offset of the secondary shadow should be in the direction of the primary like this:

Central.PNG

 

If on the other hand, that shadow is veering more to the top or bottom, like this:

Upper.PNG

- then you've got a tilt/rotation mismatch.

 

Hope that helps...

Edited by great_bear

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3 hours ago, great_bear said:

All good stuff above - and the definitive check as to whether you're compensating tilt error with rotation is to examine the positioning of the *shadow* of the secondary when viewed through the focuser. If your mirror is mounted centrally to the vanes (like most mass-produced scopes, and certainly the SkyWatcher ones) then offset of the secondary shadow should be in the direction of the primary like this:

Central.PNG

 

If on the other hand, that shadow is veering more to the top or bottom, like this:

Upper.PNG

- then you've got a tilt/rotation mismatch.

 

Hope that helps...

Good idea!   I had not thought of checking that way.

Adrian

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