Collimation query

[Spec-Chum]

Can someone shed any light on this collimation issue? It is to do with having the centre spot aligned and the whole primary mirror in view.

I've read pretty much every collimation guide out there (including the excellent Astro-baby one) and thought I had it down!

I will again astound you with my artistic skills to (hopefully) illustrate my point:

Try as I might, if I align the cross-hairs of the Cheshire to the primary centre I get the image on the left (clipped mirror) and if I have all 3 clips in view I got the view on the right (off-centre). It's almost like the centre spot is not in the centre, but I doubt that to be fair.

If I unscrew the centre screw on the secondary it obviously moves the mirror further down the OTA which seems to help, but once I start to adjust/tighten the tilt screws it seems to tighten the secondary right up against the holder again, thus taking it back towards the opening and leaving me back to square1 in regards to having the centre and whole mirror in the view. As per the bold bit, I feel I need to keep the secondary further down the OTA after the tilt screws have been tightened. Is this possible or do they always tighten the mirror against the holder, by design? Am I using the centre screw and tilt screws wrong? The guides state you use the centre screw to move the holder up and down the OTA, all good so far, but once I start to use the tilt screws it goes back towards to opening again! Argh lol

Just to clarify, this has happened since new and I've not altered the focuser in any way (I was going to last night but didn't get round to it). The only thing I have done is to adjust a spider vane to lower the secondary mirror in the focuser, but this issue was there before that, and that action didn't change it.

Last point is that image is a little exaggerated and I've been pleased with my images so far (I use the image on the right, so all primary mirror in view) but I can't help thinking my collimation is by no means optimal.

Any help would be appreciated.

Yours Frustratedly lol

[andrew s]

I would look at three possible issues. Firstly, is the secondary correctly centered under the focuser tube? You need a sight tube to determine this. Secondly, is the spider mount centered in the tube? Thirdly, is the focuser tube square on to the tube?

I would check these and correct if necessary. If none apply I don't know what to propose.

Regards Andrew

[Spec-Chum]

I would look at three possible issues. Firstly, is the secondary correctly centered under the focuser tube? You need a sight tube to determine this. Secondly, is the spider mount centered in the tube? Thirdly, is the focuser tube square on to the tube?

I would check these and correct if necessary. If none apply I don't know what to propose.

Regards Andrew

Righteo

"Firstly, is the secondary correctly centered under the focuser tube?" - By sight yes, but I had to adjust the spider to achieve this, however this issue was there before this step.

"Secondly, is the spider mount centered in the tube?" - No, see above.

"Thirdly, is the focuser tube square on to the tube?" - I honestly have no idea, but by sight it seems to be off as per this thread: http://stargazerslounge.com/topic/154993-adjusting-the-focuser/

I was going to have a go at the focuser last night but chickened out as I read it could cause more issue if I bodged it lol

[Spec-Chum]

Oh, and I have a sight tube on my Cheshire. One of them all in 1 jobbies...

[Spec-Chum]

Quick question, how long is the central screw? As in, how far can I loosen it before it falls off and ruins my day? I've only been turning it very slightly.

[andrew s]

Sorry I don't know the details of your Newt so I can't help with how to adjust your model. With the sight tube set so the out ring at the end of it is just outside the edge of the secondary they should be concentric. You may need to put some white card opposite it to see it clearly.

Maybe an experienced member close to you could help practically?

Andrew

[umadog]

Ok. I think this is related to the order in which you're doing things. You need to adjust the secondary tilt with the sight-tube cross hairs BEFORE you adjust the secondary position. Otherwise you will end up with the problems you are having. See here: http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Board=reflectors&Number=3033065 Particularly note the blue text in post #3033074

This is why I originally suggested you:

1) Re-center the secondary in the tube. (I will now add that you should make the three secondary tilt screws of roughly equal length at this point).

2) Axially align the scope

3) Investigate what you have to do center the primary in the secondary. That could be a combination of altering the mirror rotation, the spider, or (if you have to) the focuser tilt.

Having completed step three you will have to re-align axially and check that everything is still good.

[Spec-Chum]

Done. And it's dead on!

All vanes central, crosshair bang in middle and all 3 clips visible.

I had to loosen the tilt screws as much as they'd go then tighten the central screw until bang in centre, which was really helped but using the paper in the tube method. After that I twiddles the tilt screws as before, got the crosshair dead on the primary centre spot and had to very slightly adjust a vane to make all 3 clips equally visible, but that was probably more my measuring then the alignment itself. No focuser adjustment needed.

Well happy! Apart from these darn clouds, that is!

Thanks everyone!

[umadog]

Good stuff! Glad it worked out: it's unusual for the focuser to need adjusting.

[knobby]

Now you just have to adjust the clouds ...