Star Test image - Good enough collimation?

[bus_ter]

Had to stack in RS6 and use wavelets to get a clear pattern.

This is Mirach (mag 2) star with c9.25 and x3 barlow (so F30?) to SPC900 webcam.

Good enough collimation?

[bus_ter]

Here's the video, going in and out of focus.

[JamesF]

I'd say that's a little off-centre to be honest.

James

[JamesF]

This is how my 127 Mak looked after I'd finished with it. I think it's still a tiny bit out, but it was the closest I could get without an artificial star.

James

[Cath]

When I've collimated my scope I get 'very' slightly different collimation result depending on which side of focus it's on (intra/extra), ie, if I collimate on one side of focus then move to the other side it shows up as being as slightly out. What would be the likely cause of this I wonder James? .. not that I can fix it

Their doesn't appear to be any backlash or flop in the focusing, it's nice and tight and smooth, turning the focus knob either way doesn't change anything.

So what I do now is adjust collimation so that it's similar on both sides of focus - hoping that it's correct when in focus.

I'll take some video of it if I get chance to get the scope out again.

[Cath]

But yes, bus_ters collimation does look a bit out to me, the rings don't look central.

[bus_ter]

I'd say that's a little off-centre to be honest.

Needs the screw at 6 o'clock tightened slightly?

[Stu_2011]

When I've collimated my scope I get 'very' slightly different collimation result depending on which side of focus it's on (intra/extra), ie, if I collimate on one side of focus then move to the other side it shows up as being as slightly out. What would be the likely cause of this I wonder James? .. not that I can fix it

Their doesn't appear to be any backlash or flop in the focusing, it's nice and tight and smooth, turning the focus knob either way doesn't change anything.

So what I do now is adjust collimation so that it's similar on both sides of focus - hoping that it's correct when in focus.

I'll take some video of it if I get chance to get the scope out again.

Hi

This is related to mirror flop, when focusing one way the mirror is pushed and tilted one way slightly but when focusing the other way it is pulled and tilts the other way. The mirror flops (i.e. changes angle) slightly around and hence collimation changes

I have a secondary focuser with my SCT, I do a rough focus with the primary mirror focus, collimate, then don't touch the primary focus and just use the secondary focus on the back of the SCT. I think the primary focus should always be such that the mirror moves towards the back of the scope, gravity helps keep things stable this way.

Thanks

Sent from my iPad using Tapatalk

[JamesF]

Needs the screw at 6 o'clock tightened slightly?

I'm honestly not sure. I usually have to play with the screws to see what the effect is which is why it's much easier to do with an artificial star. Wobbly images from a real star make it quite tricky. The one advantage with a real star is that whichever source you use it needs to be quite some distance from the scope.

If you stand a couple of metres in front of the OTA and look into it square on, can you tell that the concentric reflections don't drift off to one side?

James

[bus_ter]

Looks okay looking into it. Also star tests right down to my smallest FL Ep (5mm) seem to produce nice round circles.

It's only when the webcam and the 3x barlow comes out, and I focus right down to one or two diffractions rings (just off focus) that I see a problem. It's so hard to see at that level I need to stack video just to properly see the circles.

If I goto a focal point and use metaguide I can see a coma going upwards (thus putting the red box at 12 o'clock') but I can't seem to adjust this out with the collimation screws.

[JamesF]

In that case I think if you can't actually tell the difference when using it in the way that you usually will, it's probably good enough.

James