Re: ATM (no subject)

[Brad Davy <bdavy@bigdog.fred.net>]

At 11:01 AM 6/13/97 -0400, Fred Maxwell - No spam e-mail accepted. wrote:
>David Eric Allen wrote:
>
>telescope manufactuers produce egg-shaped diagonals.  When you think
>about it, that's the shape you get when you cut a cone at a 45 degree
>angle.
>

Just as another aside, the correct cross section of a cone is an ELLIPSE,
hence the term "elliptical secondary."  Since the light coming from the
primary mirror is in fact conical, the ellipse is the most efficient shape
for the secondary.  

I believe all the questions about offsets arise from a misunderstanding of
where the center of the secondary mirror actually is.  The center of the
ellipse from one end to the other does not coincide with the center of the
cone it sections.  Hence if you align the geometric center of the ellipse
with the geometric center of the cone, and the secondary is precisely sized
for the cone, some light will miss the secondary.  If the secondary is
slightly oversized, the offset becomes less critical.


Brad
bdavy@fred.net               http://www.fred.net/bdavy/home.html
bdavy@nox.cs.du.edu