Re: ATM stopping a newtonian.

["Arjan te Marvelde (ELN)" <Arjan.te.Marvelde@eln.ericsson.se>]

Frank e.a.,

If i understand what you write, you say that the triangle is related to the F ratio. However, the top of the triangle is on the primary, and is determined by the diameter of the desired field in the focal plane, and the focal length. extending the reflected lines to the front aperture gives you the additional width over the primary diameter.
For the focal field diameter you may for example take the drawtube size. 

So let
 D=primary diameter, 
 f=focal length, 
 d=field size, 
 T=tube length measured from primary surface
then the front aperture diameter = d*T/f + D

I'd say, for most practical newton designs, just make the aperture as big as the primary plus eyepiece diameter. If you make it too small, you will cut of light from off-axis objects, i.e. vignet the image.

/Arjan te Marvelde

> surface of the mirror is not a "column" equal to the diameter of the
> mirror, but that the "circle of least confusion" at focus is 
> actually the
> tip of a triangle that has been folded by the secondary and the mirror
> but that continues out into space. The aperture constriction 
> at the entrance
> to the tube or secondary ring needs to be larger than the 
> mirror surface
> to encompass this "triangle". Now I make mine about 10% larger, but