Re: ATM Advice w/ specs on 12.5" Newt

["Dan Chaffee" <dchaffee@gvi.net>]

Hi Jay,
 
> determined this by taking the ID of the OTA of 15.4 and subtracting the
front
> aperature diameter of 13.7" to get a space of 1.7".  Half of 1.7" is .85"
and
> so that is the space between the OTA inside wall and the light cone. 

I don't understand why the front aperture is 13.7" I.D. and the tube's
I.D. is 15.4".  If this is a baffle, why use a nice, oversized tube
only to choke it at the end? The generous extension of your tube's
length beyond the focuser should easily ward off bad light.

>The
> focuser protrudes to about 1.1" so the draw tube can come into the light
cone
> when fully racked-in by .25".   Is this bad? 

Well, it's an obstruction and that means diffraction, but I've heard
of medium size mirrors with a quarter size chip on their surfaces 
delivering brilliant images. Also, the differing eyepieces will 
determine whether  you cross the line or not. I suppose the most 
noticeable effect might be an additional spike on very bright objects. 
 
> some ideas for improvement.  I am especially concerned about the diameter
of
> the 100% illumination focal point.

The diagonal must be good to the edge and planets will have
to remain near dead center of the FOV to take full advantage
of the aperture. Not a problem if you have a good drive.
Still, as I recall from your previous posts, you're going all 
out with premium materials and components with this scope. Moving 
from a 17% to a 20% obstruction would relax things a bit and  
you might even  gain more than you'd lose.


D Chaffee