Looking for guidance on collimation

Equipment Collimation Troubleshooting
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Andy Wray avatar
Andy Wray avatar
I think I'm slowly getting there with collimation, but could use some guidance.

* I use a laser collimator that I have made sure is collimated itself.  That means I align the secondary to the primary by centering the laser on the primary centre spot.  I then align the primary so that the laser is dead down the centre of the collimator.  All that is relatively easy.

* I do, however struggle with getting the secondary in exactly the right position and angle on the focus tube.  That means that I have uneven vignetting and also weird shaped stars I think due to uneven diffration spikes.

Is it possible that my bad alignment of the secondary would cause uneven spikes and star shapes?  It's almost like two out of three of my mirror clamps are causing spikes.

See the below (particularly visible in the larger stars):


M51 with a cheap telescope on a hot summer's night
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Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar
Your stars have a very slight trefoil (triangle) shape to them.  If you reach in behind the primary and try to gently move or rotate it is it fixed hard or does it move slightly in the cell?  You want the latter.  The clips need to be tight enough to keep the mirror from flopping, but loose enough that they dont cause strain on the mirror. 

I fyou ar egetting some "crunchy" spikes on stars that are not caused by your secondary, that can definitely be caused by the mirror clips on the primary.  You can mask these with a 3D printed ring to cover them up.  It should help clean up star edges.

Finally, as far as collimation is concerned… I recommend the catseye collimation system.  It consists of tools that amplify collimation error and allow you to ally dial things in.  They are not cheap but can sometimes be found on classifieds… they are absolutely worth it and the gold standard in my opinion.  

Vic Menards excellent "book" on collimation is also a must have for a newtonian imager.
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Dark Matters Astrophotography avatar
Dark Matters Astrophotography avatar
Laser collimators aren't very good in my experience. But I would just get Vic's book and read that. It teaches you everything you need to know.

https://www.catseyecollimation.com/perspectives.html
Arun H avatar
Arun H avatar
Agreed. I cannot recommend the Catseye tools highly enough. Jason Khadder and Vic Menard came up with the definitive way to collimate fast Newtonians - an iterative process where the primary is collimated with a Cheshire or barlowed laser and the secondary with offset pupil collimator. The Catseye website has instructions on how to use these in conjunction. Get the right tools, do it right.
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Jacob Heppell avatar
Jacob Heppell avatar
Catseye all the way!! It's what I've been using for about two years with excellent results. Round stars in every corner on the 268M.
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