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Primary focal length measurements
Hi, I am sure this is a very basic simple question, but i would like to ask just to be sure.
I am trying to find out what the true f number of my scope is, ( sorry, not quiet sure what the correct term is, like f5, f.4.5 )
It is a Skywatcher 16 inch Dobsonian.
I know it says diameter 400 mm ( 16 inches ) Length 1800 mm ( 70 Inches )
But i would like to find out what it truly is.
I have read that you simply divide the Primary focal length by the primary mirror diameter.
My question is, how do you actually correctly measure the Primary focal length?
1. Is it just simply the full length of the outside tube from top to bottom?
Or 2. is it from the reflective surface of the primary mirror to the centre of the eyepiece?
Primary mirror is simple enough, just run a tape across it to get the true diameter.
If someone could please just correct me on which one is the correct method for finding the true primary focal length it would be greatly appreciated.
Thanks.
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