Newtonian Offset

[beachbum]

Okay so I'm very confused about whether or not an offset on the secondary is necessary for collimation and if it is required for each scope. I have an 8 inch dobsonian from Skywatcher with a F/ratio of 5.9 and I'm not sure if I need an offset or not and if so, how exactly does it work and by how much would the offset be?

Thank you

[Sam]

I take it the scope is already built and you're putting in new secondary vanes? or something that would require you to want to modify the secondary? On most reflectors there is a small offset on the secondary but when you're collimating the scope you wouldn't change this as its fixed and usually its barely perceptable anyway. So you just collimate a per normal and not worry about it - I hope that answers your question.

Sam

[beachbum]

Oh okay so you suggest just centering the spider vanes?