Adjusting Secondary Mirror Skywatcher 200P Dobsonian

[mrfishyfingers]

Evening all.

This post probably ties in very closely with parkini's one this morning re their Skywatcher 150P. I've managed to collimate my 200P over the weekend, more by luck than judgement I would say, but the red dots on my laser collimator now hit the donut on the primary mirror and hit the middle of the crosshairs on the collimator itself sitting in the focuser. 

My issue is this: reading the instructions that came with the telescope and looking at numerous articles and videos online re collimation, all state that the adjustment of the secondary mirror is achieved by adjusting the three hex-headed screws at the back of the mirror holder and using the large cross-headed screw to move the mirror position up and down the tube. The Skywatcher instructions, however, then go on to state that all aforementioned screws should be tightened after the adjustment process. Now, if its the action of moving these screws in relation to each other that adjusts the position of the mirror, then surely returning them to their pre-collimation position (ie. tightening them as they were before) completely negates any adjustment carried out?! The instructions explicitly state that leaving these screws untightened will lead to the mirror moving, though. Which baffles me! If you return these screws to their exact position on the threads they were in before you started, surely the mirror will end-up being in the same position as it was before you started!

The primary mirror has adjustment screws to do the actual adjusting and locking screws to hold the mirror in place once the adjustment has been carried out, which seems entirely logical, but the secondary mirror seems to be a different idea entirely. 

Any collimation experts out there who can fill in the blanks between my very basic understanding and the ambiguities of the instructions?

Would be very grateful.

[Stu]

The screws need to be tight, otherwise the secondary will move anyway. When adjusting, I normally loosen one a little then tighten one of the others to make the adjustment. That way they all basically remain tightened as the adjustment takes place. Does that make sense?

[Tinker1947]

You just need to tighten the 3 screws so the laser stays on the primary mirror, i use knurled knob as this allows hand adjustment and no fear of dropping any thing down the tube....

[mrfishyfingers]

Yes, thanks - it does. Just seen a link to a video provided by Dr_Ju_ju on parkini's post which explains how the secondary mirror assembly works. Its the degree by which the screws are tightened in relation to each other that pushes the alignment of the the mirror around. Untightening the screws does nothing - save giving you a wobbly mirror! Great video that fills in the blanks that the Skywatcher instruction manual seems to delight in providing you with!

[spike95609]

I slackened off all of those secondary screws, then played around with them by slight degrees until I could see all of the primary mirror clips evenly spaced around the edge. This means that the secondary is aligned and looking straight down the tube towards the primary. Then I just tightened each up by very small degrees at a time, about an eighth of a turn in succession so that it would maintain the position evenly across the three screws, and carried on until they were all tight.

[OneEyedSam]

take a look at bobs knobs,  I think rother valley optics sell these nowadays...makes secondary collimation a lot easier and reduces the risk of droppng tools on you primary