Collimation - help please!

[Jazzy]

I've read and re-read astrobaby and watched youtube videos.  I understand what I'm trying to do, in theory - align secondary to focuser, align secondary to primary, then align primary but I'm having trouble doing it in practice.

I don't understand the adjustment screws on the secondary - I try to loosen one and tighen another and the mirror just seems to move at random, or at least not in the way I want it to, I don't get it.  Then when you think you've got it, you have to adjust them all again to align the seconday with the primary and it knocks it all off... I'm a complete newbie at this sort of thing, so I'd appreciate any advice. 

My scope is the Skywatcher 200p flextube dob and I'm using a cheshire.

[RafalT73]

I hope below picture can help you

CW means ClockWise, CCW - Counter ClockWise

[Scooot]

 Then when you think you've got it, you have to adjust them all again to align the seconday with the primary and it knocks it all off..

I don't follow this. You should use a different set of screws to adjust the primary to the secondary. Or have I miusunderstood you?

[Scooot]

Ah, are you using the secndary adjustment screws to try to align the secondary to the focuser? If so you shouldn't, you only need to make sure they're tight enough that the secondary doesn't flop around once you've correctly positioned it under the focuser and that the secondary appears round when you look through the focuser.

You use the secondary adjustment screws one at a time to tilt the secondary so you can see all the mirror clips. Loosen one slightly, tighten another. Trial and error.

[Night Hawk]

The mistake I made was I unscrewed the secondary(that you use a phillips screw driver on) too much so there was no real way of of tightening the 3 Alan key screws when I first started and the secondary was lose.

Try just unscrewing the phillips screw a fraction then make the adjustments tightening the the 3 Alan key screws. I find this works for me. 

It's just trial and error, but it gets easier, quicker and better the more you do it. My frst time took around 45 mins!!! and looking back I didn't do it correctly, however I'm better at it now and can get the primary/secondary done in around 5 mins but I'm no expert.

[RafalT73]

...Try just unscrewing the phillips screw a fraction then make the adjustments tightening the the 3 Alan key screws. I find this works for me...

Caution: do not unscrew all collimation screw (I am talking about secondary mirror). If you remove it or unsrew too much secondary mirror can fall down and definitely can damage primary mirror!

[damnut]

Hi Jazzy,

Would be good to know how you got on.

[Jazzy]

Thank for the advice guys. My dad had a look at it and he seemed to get it aligned but I'm waiting for my collimation cap to arrive before trying again, because I couldn't see the primary clips with the Cheshire.

So do I just need to use the large Philips screw to align the secondary to the focuser? I don't need to worry about the tilt at the point? Am I just moving the secondary up and down the tube and maybe rotating it to get a perfect circle?

The thing that confused me was it said I had to loosen the adjustment screws before I could turn the Philips screw, which turned out to be the case. The video said turn each one by one full rotation, then adjust the Philips. But it seemed when I did that the secondary flopped downward, so the gap between the secondary and collimation cap was too large at the top and too small at the bottom.

Then I read when you're aligning the secondary to the primary you might have to loosen the Philips. But doesn't that cause the secondary and focuser to move out of alignment again?

[purdo]

Hi Jazzy, is there a Astro Club near you, it would be easier if someone could show you.

[Scooot]

So do I just need to use the large Philips screw to align the secondary to the focuser? I don't need to worry about the tilt at the point? Am I just moving the secondary up and down the tube and maybe rotating it to get a perfect circle?

Yes that's right. Just remember the secondary is only a flat mirror, it's only there so you can see the primary through the focuser. The first stage is making sure it's in the correct position, up and down the tube and round. The next stage is tilting it with the secondary adjustment screws to make sure you can see the whole of the primary.

[Uranium235]

It helps if you keep a hand on the secondary holder when making bold adjustments - that will prevent rotation of the secondary (keep looking at the secondary when making adjustments!). My current method is to use a cheshire sight tube to get the secondary round and centered, then collimate it until its close - then finish with the laser (just minor tweaks).

However, I have seen another method recently where the collimating eyepiece has concentric rings etched into the device, meaning you can get the secondary perfectly centred and round.... a bit pricey, but cheaper than a catseye.

http://www.teleskop-express.de/shop/product_info.php/info/p5506_TS-Concenter-2--colimation-eyepiece-for-Newtonian-Telescopes.html

I guess it depends on what degree of accuracy youre looking for, not such a need if observing - but if youre imaging, its pretty important (ie: for field illumination and proper correction).

[Jazzy]

Well I think I did it, only took 5 minutes this time lol.  I think the problem was a) I was trying to line the secondary up with the doughnut on the primary and I couldn't see the primary clips on the secondary using the Cheshire.  The mirror was already centered on the focuser, so it was just a matter of lining the secondary up so I could view the primary clips, which were visible using my collimation cap, then lining up the primary so it was centered on the Cheshire. 

[Jazzy]

Maybe spoke too soon.  When I adjust the primary mirror by loosening the lock screw and turning the adjustment screws, I can get everything lined up, but when I tighten the lock screws again it seems to make it move out of alignment?

[ghostdance]

Tightening the lock screws can move alignment slightly. Go gently - I tend to only tighten them just finger tight and barring big bumps or knocks, they hold pretty well. I think the trick is in not locking them down very tightly. And small increments!