Re: ATM stopping a newtonian.

["Frank Ward" <thewards@mindspring.com>]

Hi Sam,

>This would seem to allow off-axis light to enter the tube
> and hit the mirror...


I use to think the same thing....until I too realized that the light hitting
the
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
I'm sure someone here on the List has the mathmetical formula for the
correct amount.

Check out the diagrams in NEWT.

Frank Ward
Atlanta, GA






----- Original Message -----
From: "Sam Michael" <samm@idcomm.com>
To: "atm LIST" <atm@shore.net>
Sent: Sunday, January 13, 2002 8:11 PM
Subject: ATM stopping a newtonian.


>
> Why is it that we do not stop the aperture of a newtonian to be the same
> size as the mirror.  If one has a 6" mirror in an 8" tube.  Shouldn't one
> stop the aperature at something much closer to 6"?
>
> I am looking at baffeling a newtonian and am confused as to why the tube
end
> is left large.  This would seem to allow off-axis light to enter the tube
> and hit the mirror...
>
> I am sure I am going to kick myself when I hear the answer.
>
> Sam.
>
>