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Author Topic: collimation issue with focusser?  (Read 454 times)

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Offline Ian Straton

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collimation issue with focusser?
« on: 23:21:25, 24 September, 2017 »
I was collimating my newt this afternoon and I noticed once I was done that if I rotated the laser collimator in the focuser the dot drifted around (a lot!) but returned to the centre mark when it returned to the original position.

Does this imply that the focuser is out of position?  If so is there a good method of adjusting it to resolve the problem?

Offline swashy

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Re: collimation issue with focusser?
« Reply #1 on: 08:38:07, 25 September, 2017 »
Check that your colimator is itself colimated first, if that is ok then do as you were doing and adjust the focusser to bring the laser halfway between the error, and repeat until the error is corrected
Ade

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Offline chris.bailey

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Re: collimation issue with focusser?
« Reply #2 on: 08:51:45, 25 September, 2017 »
Focus draw tubes themselves are often not properly orthogonal, even expensive ones. We tried collimating a dob at Kelling over the weekend that had just that problem. If the laser itself is collimated (you need a V-Block and distant target to check it) then as Ade says the only thing you can do is rotate it and take out half the error and repeat. At then end the dot should rotate around the centre spot evenly. It is then worth trying to adjust the focuser to bring it into line but that in itself is a tricky adjustment. I did my Tak focuser by putting three or four lengths of fine fishing line over the dew shield so they gave a perfect centre and then putting a laser in a self centring adapter and adjusting the focuser until the laser spot hit the crossing point in the lines...not perfect but a lot better than it was.

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Offline mechanoid

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Re: collimation issue with focusser?
« Reply #3 on: 15:19:33, 25 September, 2017 »
The orthogonality in the draw tube won't have any effect on the spot position when the collimator laser is rotated. However, there are many sources of error when using these things ...

- The collimator is not a sufficiently close fit in the draw tube to be repeatable when rotated.  This is a common fault, not entirely fixed by the so-called "self centering" versions.
- The laser beam is not co-axial. Its not enough that it is just parallel to the mechanical outside diameter, it must be coincident with the axis too. i.e. Not offset to one side. If checking with a vee block then test both near and far positions.
- The draw tube isn't sufficiently rigidly mounted to the tube, so that it moves when the collimator is rotated.

I ended up making an adaptor with the outside diameter custom machined to a very close sliding fit to the focusser tube.  It has six adjustable screws in two planes in order to make the laser co-axial.  The collimator itself was a half degree misaligned to the outside diameter when I got it. No amount of self centering will fix that !
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Offline Ian Straton

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Re: collimation issue with focusser?
« Reply #4 on: 20:50:54, 25 September, 2017 »
ok, so I checked my collimator with a v block and it described a significant circle of about 5cm at about 5m, I have corrected that as much as I can (I was unable to improve on about 1.5cm circle) however I am now worried about whether it is off axis... How do I test for that?

I think the aberration I observed at the scope was larger than can be explained by the error in the collimator itself so there is an issue with the focuser position.  This wouldn't be entirely surprising as it is a replacement focuser that I fitted myself..

Offline mechanoid

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Re: collimation issue with focusser?
« Reply #5 on: 17:13:39, 27 September, 2017 »
Ian

You can test if the beam is co-axial by moving the test screen close to the laser.  If it descibes a significant circle when rotated, but doesn't do so at a greater distance, then it is off axis.

I think that a significant off axis error is unlikely, and there isn't much you can do about it anyway.  If it is co-axial then the beam alignment is only off by about 5 arc mins, which I would think is plenty good enough.
It means that your image plane will be "inclined" by this amount, which I doubt will have any detrimental effect even with a large CCD.

Don't worry about getting the focusser accurately perpendicular to the mirror axis - it doesn't matter.  Bolt it to the tube and forget it. But you must of course adjust the secondary so that the spot hits the mirror centre.  Personally I always find that getting the secondary in the right place is by far the most difficult bit of scope collimation. The primary is easy.

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Offline Ian Straton

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Re: collimation issue with focusser?
« Reply #6 on: 18:22:11, 30 September, 2017 »
I just refitted the focuser, it is now rather more securely attached (I put rivnuts in to provide a secure thread), now when I rotate the collimator in the draw tube it describes a circle about 3cm in diameter on the primary mirror.   Obviously I can place the centre dot of the primary anywhere I like along that circle but I am really not sure what is out of alignment here or how to properly correct it :(

The focal length of the scope is 900mm if that is of any consequence here..

Offline swashy

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Re: collimation issue with focusser?
« Reply #7 on: 19:53:25, 30 September, 2017 »
if everything else is concentric and it is still descibing a circle, it has to be the colimator slightly off kilter then Ian
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Offline Ian Straton

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Re: collimation issue with focusser?
« Reply #8 on: 20:04:52, 01 October, 2017 »
Yup, collimator was off... turned out one of the adjustment screws was stuck!  I have found it much easier to adjust with it mounted in the scope than in the v block..  What I did was this:
1. Mount the collimator in the draw tube with the grip screw done up tight enough to hold it in place but still allow deliberate rotation.
2. Rotate the collimator 360 degree whilst watching the laser spot on the primary mirror.
3. adjust the secondary mirror so that the laser describes a circle centred on the centre dot on the primary
4. adjust the collimator until the laser dot is on the primary centre dot
5. repeat steps 2-4 until no visible circle is described.

I'll let you know what the result of all this is the next clear night we get (which won't be for sometime as I now have a properly setup and collimated imaging scope!)

 

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