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Collimation Of Fast Newtonians?

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Posted by Roger King on March 1, 2004 19:50:39 UTC

I am amazed by the conflicting information out there about collimation of fast Newtonians.

I have just completed a rebuild of my 350mm f4.6 and have reduced the size of the secondary to 106mm major axis. This is fine according to one half of the collimation community because I have offset the flat to take into account the fact that the light cone is broader at the base of the flat than at the top. This procedure entails either offsetting the whole secondary assembly away from the focus mount, or (as I did in this case) offset the secondary on its holder.

Now the confusion. Popular collimation sites/gurus still talk about the 'concentric view'. If you've offset the flat, the view can only be concentric if collimation is incorrect and the light path is not orthogonal. Imagine looking at that circle of the flat and instead of aiming dead centre, aim a little higher (because that's where the optical axis is now. So if that point is supposed to be dead centre in your focusser, or though a draw tube, the flat projection can't be can it?

Any comments, definitive or otherwise greatly appreciated. I really ned to get consensus on what should be seen when correctly collimated.

Regards

Roger

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