Re: ATM 100% illumination and secondary sizing

[Gary Seronik <gseronik@netway.com>]

Hi David:

At 02:30 PM 3/16/2001 +1300, David Moorhouse wrote:
>   I'm wanting to know a bit more detail about  secondary mirror sizing,
>100% illumination area and what difference it all  really makes.     I'm
>only doing visual work, not  photography.   Punching these numbers into
>Newt    2.6" secondary gives a 0.6" 100% illuminated  feild. but with a 21%
>obstruction..   2.14 secondary gives an almost 0" 100% illuminated  feild.
>with a 17% obstruction. 

The trouble with looking at this parameter from the perspective of the size
of the 100%-illuminated field is that it gives a poor indication as to what
you'll see at the eyepiece.  For example, a fast f/ratio scope can tolerate
a very small 100% field and still look very good, while a slower f/ratio
scope cannot.  Personally, I find it far more useful to look at the problem
from the perspective of edge-of-field illumination.  What is the biggest
drop you're willing to tolerate at the edge of field?

Alan Adler and I wrote a pair of articles that appeared in the August 2000
issue of Sky & Telescope.  If you review those (keeping in mind the
comments I have made above), I think you'll be well equipped to make a
decision and be fully aware of what you give and what you get with various
diagonals.  I would further recommend that you download Adler's software
"Sec" (described in his article) too -- it's a wonderful program
specifically designed to answer the questions you have just posed.

Good luck,
Gary Seronik