Re: collimation query

Ian Newham <ian@xxxxxxxxxxxx> wrote:
Should I care?

If I am correctly picturing what you describe, then you probably should (but see later).


The only reason I can think of would be if the focusser wasnt perpendicular to the tube. Does that sound feasible. 

Yes.

What is important is that the optical axes are co-linear. If the axis of the focuser (and hence the eyepiece) is not co-linear with the optical axis of the OTA, you will introduce aberrations, notably astigmatism and coma.

The only simple way to be certain is to perform a star test. If the diffraction rings of a defocused on-axis star are nice and concentric, your collimation is fine. Chuck & Nils Olof (for whom I have the greatest respect) notwithstanding, I still think it is good practice to check/refine collimation by star test, not least because it is (a) simple and (b) things can shift between collimating indoors and taking the scope outside (I only ever use a punctured 35mm film can and star test to collimate -- the digital photographic age is not friendly!).


Best,
Stephen

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