a little help with coma,tilt and collimation please ?

[glowingturnip]

Hi all,

So I've managed to get some decent time under the skies with my new kit now (Moravian G2-8300 with OAG and internal filter wheel on a Skywatcher 200PDS, focuser is a Lacerta Octo60 rack&pinion).  However despite my best efforts I'm still getting little bits of coma in the corners - strongest in the bottom right, weakest/none in top-left, same across a few hundred lights, all filters.  It's only small coma, compared to what I've had in the past, but still, annoying.

I'm using a Baader MPCC MkIII which I believe to be spaced at the correct 55mm distance.

I laser-collimate both secondary and primary before every session.

 

I ran a random one of my subs through a trial verson of CCDInspector, and got the following:

 

So as far as I understand those, it raises a few questions:

- I assume my collimation is good enough, those circles are nearly concentric ?

- I seem to have some tilt reported, 0.3" in each axis - should I worry about that and what can I do to fix it ?

- I notice my centre of vignetting, which nearly coincides with my centre of coma, seems to be quite far off centre towards the top-left - should I worry about that and what can I do about it ?  Anything to do with the placement of my focuser vs secondary mirror ?

- should I be getting better coma correction than that, and if so what can I do about it ?

 

Would appreciate any help,

 

[glowingturnip]

polite bump - could anyone help me on this please ?

cheers

[Marci]

If still getting coma anywhere other than bottom right I’d guess your spacing is every so slightly (fractions of mm) out. 

If you can rotate your coma corrector alone, does this affect the centre of vignetting? If you rotate the coma corrector by 90 degrees does the centre of vignetting move also? If it DOESN’T...

Your offcenter-ness I *think* should be correctable by adjusting the bearing blocks (tiny black allen heads on the main focuser body [1 in pic]), compensating adjustment in one direction on one side by equal adjustment at opposite side. Correct this before trying to correct your coma.

 

[glowingturnip]

ahh, makes perfect sense, thanks very much.

I don't know if you know this focuser, but do you know which black allen head screws am I looking at ? 

When I installed the focuser, I put the base plate on first, and then the main focuser body is attached to the baseplate with those two allen screws at the bottom of the pic (where it says '5mm' and '82mm', and there are 2 more on the other side).  Is it those I'm adjusting, or is it the screws in the groups of 4 slightly higher up ? (the groupings with the large silver screwhead, the two medium ones and the small black one - there are 4 groupings altogether, 2 on each side) 

Sadly there are no instructions for this focuser and the only German translation I can find for those grouped screws are 'to the bearing set, tension and pressure screws'

Appreciate your help, tvm

[Marci]

Nope, no direct experience with it, just what I can suss out from the pictures... 

These:

But if they don’t have any impact then there’s no harm in trying the baseplate clamp screws (by the 5mm & 82mm labels) also...

It could even be as simple as needing to nip up one corner of the main plate mount bolts (the thin blue line).

If none of that has any impact, and the centre of vignetting doesn’t rotate with your imaging equipment if you rotate it all in the focuser, then it’s mirrors / collimation.

If you can rotate your imaging gear and the resultant vignetting is still in the same place in every frame, then it’s something between focuser and ccd.

[glowingturnip]

perfect, thanks vm for your help, I shall experiment next time I'm in the field

[Marci]

Technically should be able to experiment in the kitchen (for instance), just point it all straight up at the ceiling (assuming it’s white). No point wasting starlight time if you can do it indoors whilst the clouds are playing out...

[glowingturnip]

it's in a different country unfortunately !  but yes, will experiment with the flat panel over the end - will just have to make sure the camera doesn't fall out when I'm unscrewing the focuser...