Collimation - what am I missing?

[lukebl]

Hi folks,

Having had several Newtonians over the years, you'd think I would know about collimation. I've had issues before, but they've generally been resolved. But something has been making me tear my remaining hair out.

After moving house a few months ago, I've got the new obsy up and running, but I simply cannot get round stars when imaging. It's definitely not a guiding or flexure issue. The distortion occurs at very short exposures and the focuser / cam / coma corrector are all square-on and rigid. I've spent weeks tinkering with the collimation. I've even dismantled and reassembled the whole thing, right down to removing and re-installing both mirrors, secondary vanes and focuser. I've done everything.

Here's a view square-on through the cheshire eyepiece. The secondary is nicely centred, and the primary donut is in the cross-hairs. The reflection of the secondary is slightly off-centre as you'd expect with this type on newtonian. It looks near perfect to my eyes. YET the stars come out eggy-shaped every time, whatever camera I use (CCD, DSLR, webcam). What CAN I be doing wrong?

[Singlin]

How did you polar align your mount?

[Peter Drew]

From your collimation pictures I think your problem is elsewhere with the system.

[lukebl]

23 minutes ago, Singlin said:

How did you polar align your mount?

It's pier-mounted and perfectly aligned. 

Anyway, like I said, the problem is nothing to do with the tracking or polar alignment, as it occurs even with very short exposures (<1 second). 

[Stu]

Perhaps post a few sample images up Luke? Might prompt a few thoughts.

[Peter Drew]

How does a star look like at high magnification visually?

[Singlin]

Pinched Primary mirror?

From tightening too much the pads that hold the primary mirror in situe?

[Moonshane]

Could be. I agree with Peter it's somewhere else. If the primary is not glued in or tight in the clips, then the chances are it's the secondary fixing / holder. Ideally this should be fixed with three blobs of silicone or similar or held safely but loosely in a 'cradle'. My money is on the secondary.

[Alkaid]

I agree with Peter, the collimation pics look good.  I think you need to take some high magnification images of Polaris, in focus, and then de-focused on both sides.  Then you can experimentally eliminate star trailing over a short exposure as Polaris won't move....  Post those pics on here and we'll take a look...

[Dude_with_the_tube]

Luke,

your collimation does look good to me,but i have a few questions:

are the egg shaped stars across all the field or just on the edges?

Reason why i am asking this is that i suspect your mirror might have astigmatism and then collimation will not help(egg shaped stars on the edges of FOV).have you star tested your telescope? If not,then you should do so as then you will see for sure what is happening.

here is a good reading material for you:http://www.loptics.com/articles/starshape/starshape.html

hope you get it sorted.

 

[Moonshane]

That's a great link Gaidis. Not seen that before.

[lukebl]

Thanks folks. I've had the scope a few years, and had good results in the past, so I'm thinking it's not astigmatism. I've also loosened the primary clips, although I don't think pinched optics was the problem as the clips weren't screwed in very tight. 

I'm looking at the coma corrector next. I have been using a Baader one, but will switch to my Skywatcher one and see if there's any difference.