collimation

[sunshine185]

I thought I had collimation down to a fine art but now I'm not sure. I use a Cheshire and it looks bang on, but tonight I centred polaris in the ep and racked out the focusser. I got a perfect circle as I thought it should be but the black circle in the middle wasn't centred, it was slightly toward the 11 o'clock position, so I moved the focusser all the way in and again got a perfect circle but this time the black circle was slightly toward the 5 o'clock position. I put the Cheshire in and as usual it looked perfect, what's the deal here? I'm confused.

[Moonshane]

when you say racked out I presume you mean by a few mm of a turn from focused? you literally tweak it just out of focus for a star test. the star must be dead centre too.

[Astro_Baby]

It could be two things,

The mirror spot is not dead centre OR is physically centred but not the centre lf the mirror. The measured dead centre of the mirror may not be exactly where the mirrors focal centre is. That would be fairly unusual by the way.

The focuser is not square or has excess slop....slop is more likely I would think.

The real test of collimation is always a star test, a cheshire will get you close but a star test is the acid test of it.

[sunshine185]

Yes Shane, just a few mm either side of focus. I will take out the primary and check the centre spot is exact, I will also somehow check the squareness of the focusser. I bought an adapter that screws onto the drawtube, then I bought a self centering 1.25mm ep holder so I hope its either the centre spot or the unsquare focusser that's causing the issue. The thing is, when I was viewing Jupiter tonight it looked magnificent, really crisp with loads of detail, so do you think its worth tinkering about with the mirror and focusser or should I leave well alone?

[Moonshane]

ps

I never bother with star tests. as I can never be sure to within a few mm / degrees of accuracy whether the star is centred in the field, nor can the magnifications needed to get an accurate star test be achieved that often

[sunshine185]

Now you mention it, I'm not 100% sure I had polaris dead centre. One thing I did do was cut the little crosshairs out of the Cheshire, I always thought they were more of a hindrance than a help.

[Moonshane]

if you have no cross hairs how do you check the tilt of the secondary?

[Mr Fiery Jack]

Get it as near as you can but don't turn it into an obsession, I have seen it drive men mad...

[Moonshane]

+1 Fiery Jack. Seeing will make more of a mess than bad collimation usually but do get it spot on in the Cheshire and you'll have no issues.

[Mr Fiery Jack]

+1 Fiery Jack. Seeing will make more of a mess than bad collimation usually but do get it spot on in the Cheshire and you'll have no issues.

Here, here, here.

[sunshine185]

I use a webcam and that mir di collimation thingy to get the secondary set up.

[Moonshane]

ahhh fair point. I don't really do gadgets.

[Astro_Baby]

To be honest I wouldnt rush into assuming problems especially if the star wasnt centred. I'd take a check on a different night.

If the stars look sharp at the centre of the field with no obvious distortion when in focus I would leave well alone. Make a mental note to check the spot at some future point when you take the scope to bits for other work.

Dont let collimation become an obsession or a paranoia, if the views look clean and views are sharp leave it alone.

[sunshine185]

I'm gonna leave it alone and I'm gonna concentrate on the far more serious issue of how to afford some new ep's. Many thanks for the advice.

[Damo636]

I have to confess I never bother with a star test either. A cheshire & the barlowed laser technique on the primary does the job well enough for me.