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Best practice of CW and CCW with collimating and focusing SCT?

collimation SCT
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#1 meegja

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Posted 09 July 2020 - 10:00 AM

So, I have been taking in a lot of info about collimating my Meade LX65 8" scope. It has a fixed primary mirror. Sadly I had to collimate my scope because the factory collimation was done with 1 screw totally over-tightened and the sadly of the secondary mirror was deformed and totally off. Fixed that and collimating it inself goes fine now.

Also the same that is mentioned here more then once: if I collimate in-focus (so CW), things are just fine. Then going to focus, still okay. But then when I continue, so out-focus, the collimation is off. And also the other way around.

And the amount of info about that is a lot but also a lot of ways of approaching: I see mentions of collimating CW as close as possible to focus, then go over the focus and do the finetune collimation CCW. But also info that it doesn't matter as long as I choose either CW or CWW and stick with that. Or choose the in between where the collimation with CW is a bit off and CCW also but the opposite of CW.

And also about focusing: if I collimate with CW, should focusing then also be done CW? Or ....

All in all a typical case of "I can't see the forest for the trees anymore" .... or even better: "I can't see the Milky Way for the stars anymore" :)

 

Thanks in advance

Menno



#2 gezak22

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Posted 09 July 2020 - 10:28 AM

My understanding is to make the final focus correction in such a way that you move the primary against gravity. I have no idea if that requires a CW or a CCW turn of the focuser. And if your last focus corrections is against gravity then the scope should be collimated in that condition as well.


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#3 WadeH237

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Posted 09 July 2020 - 10:57 AM

You  can think of the focus knob on an SCT as a long screw (because that's exactly what it is).  When you turn it clockwise, it tightens, pulling the mirror towards the back of the scope.  Counter-clockwise pushes the mirror away from the back of the scope.

 

When you reach final focus, you want to be pushing the mirror against gravity, so counter-clockwise.  If you finish with a clockwise turn, you are pulling the mirror down.  If there is any slop in the system (which there is, otherwise the mirror carrier would bind against the baffle tube), gravity may not be enough to fully bring the mirror down.  Then, it can - and will - slowly settle for a while after you stop turning the knob clockwise.

 

This is why you should always finish focus on an SCT's primary mirror focuser with a counter-clockwise turn.


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#4 meegja

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Posted 09 July 2020 - 11:14 AM

You  can think of the focus knob on an SCT as a long screw (because that's exactly what it is).  When you turn it clockwise, it tightens, pulling the mirror towards the back of the scope.  Counter-clockwise pushes the mirror away from the back of the scope.

 

When you reach final focus, you want to be pushing the mirror against gravity, so counter-clockwise.  If you finish with a clockwise turn, you are pulling the mirror down.  If there is any slop in the system (which there is, otherwise the mirror carrier would bind against the baffle tube), gravity may not be enough to fully bring the mirror down.  Then, it can - and will - slowly settle for a while after you stop turning the knob clockwise.

 

This is why you should always finish focus on an SCT's primary mirror focuser with a counter-clockwise turn.

Ah, thanks!

So it is best then to always be in (or go to) the intrafocal side and then focus towards extrafocal (CCW) when focusing if I understand correctly?

 

 

My understanding is to make the final focus correction in such a way that you move the primary against gravity. I have no idea if that requires a CW or a CCW turn of the focuser. And if your last focus corrections is against gravity then the scope should be collimated in that condition as well.

Ah, okay! That makes sense indeed :) Thanks!


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