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Collimation process still not quite right


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I am still having trouble getting my skywatcher 10inch dob accurately collimated. I can align the primary to the secondary and the secondary to the primary fairly accurately and repeatably using the hotech laser collimator, but the final setup is not centred in the collicap - it has moved up a bit giving a gap at the bottom. The starting point is fine and I can get the image in the colli cap fairly central by adjusting the central screw in the spider as per Astro baby's guide, but as soon as I align the secondary with the primary centre spot with the hotech collimator - which entails moving the 3 hex bolts - than as soon as I am centred, the image in the collicap has moved upwards. What am I doing wrong, or should I just ignore the error. The whole process seems to be very repeatable nomatter how much I deliberately disturb the secondary mirror, the same ending situation alway occurs so maybe the scope is deliberately offset in some way.

Thanks in anticipation.

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I had this problem. Take your laser collimator... and either A) throw it in the trash, or :eek: (assuming your draw tube is square to your secondary) shine the laser through a Barlow and use the centre ring shadow to collimate, or C) use a decent Cheshire.

B and C result in perfect star tests for me, which is unarguably the ultimate test of collimation... Option A would just be gratifying. :rolleyes:

I've used both these methods and achieved perfect collimation, whereas the laser alone always gave me the same results you get.

Edited by pook
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To the OP, see if the first few page (only the first few pages) of this link helps

Telescope Reviews: Useful info about secondary mirror alignment

Few things to keep in mind:

1- Laser collimators are NOT designed to ensure the secondary mirror is centered under the focuser -- unless you have Glatter's laser with holographic attachment which is not the case here.

2- You can't accurately evaluate the centerness/roundness of the secondary mirror unless your laser beam strikes the primary center.

3- Fixing your issue might require touching the spider vanes thumb knobs in addition to the central bolt.

Again, refer to that link and see if it would help you.

Jason

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Thanks for that Jason. A fascinating article but it is starting to get rather complicated - I shall try again tomorow. At the end of the day is it worth getting the collimation absolutely spot on? When do diminishing returns set in? I suppose that the ultimate test is with a star but there seems no chance of that at the moment with the current weather.

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... At the end of the day is it worth getting the collimation absolutely spot on? When do diminishing returns set in?....

With an F/4.7 newtonian the collimation "sweet spot" is pretty tight - it's one of the downsides about having a fast scope :rolleyes:

I find final tweaking under the stars the best way to achieve the final collimation adjustments.

With the cloudy weather we have had lately that seems a bit academic at the moment though :eek:

Edited by John
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Thanks for that Jason. A fascinating article but it is starting to get rather complicated - I shall try again tomorow. At the end of the day is it worth getting the collimation absolutely spot on? When do diminishing returns set in? I suppose that the ultimate test is with a star but there seems no chance of that at the moment with the current weather.

Collimation consists of 3 alignments:

1- Optimizing illumination field within the FOV. Centering/rounding the secondary mirror is how you do it.

2- Eliminating the tilt between the primary mirror and the eyepiece focal planes. Getting the forward laser beam to strike the primary mirror center by adjusting the secondary mirror is how you do it.

3- Ensuring the primary mirror focal point is positioned along the focuser axis. Getting the returned laser beam to retrace the forward beam per step2 or using the collimation cap or star testing is how you do it.

The following post has more info

Telescope Reviews: Re: How important is secondary alignment?

I do not want to complicate things but the point here is that star testing can only be used effectively for the 3rd alignment – which happened to be the most important alignment. It can't be used to ensure the secondary mirror is well-positioned under the focuser as much as the laser collimator can't be used for the same purpose.

You could leave things as is. Assuming the 2nd and 3rd alignments are OK, then your views should be OK. The first alignment is more critical for imaging.

Jason

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