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location of secondary and focuser on a dob build.


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I am going to have a go at building another Dobsonian and this will be the biggest with a 16" F5 mirror.

Now, because it is getting near a very tall scope, I am keen to keep the eyepiece as low as possible. If the focal length is 78" I assume if I draw a triangle with a line 90 degrees from the centre of a 16" base 78" high and then from there to each side of the base side, the secondary would have to fit on the triangle at the point it's width is that of the drawn triangle?

And is there anything else I can do. I am going to angle the base at the point of the collimation base to save height there but, can I move the focuser out further.

Finally, would the benefit of a bigger secondary and ease of use outweigh the larger obstruction of light?

Good luck with my explanation.

Kind regards,


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