From: "McHatten, Shawn" 
To: 'Dave McCarter' , ATM 
Reply-To: "McHatten, Shawn" 


Hey Dave and all
        I too have struggled with understanding the intricacies of off axis
objects. If we take the Andromeda galaxy for instance it is about 3 degrees
across. I have an 8in f6 newt with a fl of 48in. If I center M31 then the
outer fringes are about 1.5 degrees off axis. If my trig is right that
means that, at the focal plane (forget the secondary for now), the cones
for the fringes would be about 1.25in from my central on-axis cone. Does
this mean that I could never see all of M31 unless I'm
using at least a 2.5in eyepiece (1.25*2).

Thanks
Shawn

-----Original Message-----
From: Dave McCarter [mailto:dmccarter{at}sympatico.ca]
Sent: Friday, March 07, 2003 10:45 AM To: MATTHEW SIMMONS; ATM
Subject: Re: ATM Newbie Q: NEWT Software Q & Secondary Sizing


> understanding of this whole idea is that the primary mirror reflects all of
> the light that it gathers in a cone shaped beam of light that converges(sp)
> to a single point of light, equal in height to the focal length of the
> mirror.

True, You have described the on axis light cone. But our images are made up
of millions of similar cones that surround the one central cone. The larger
the secondary the more those additional cones build our image. At some
point off axis the secondary is too small to reflect the entire cone to the
focal plane, and vignetting occurs, a reduction in the amount of light to
the focal plane. By the time you get to the edge of the field the
illumination will be down by some amount. It is generally considered that
75% illumination is not discernable except by the most picky, and 50% is by
almost everyone.

IMHO a secondary that is 19% defines the border beyond which unacceptable
reduction in image definition and contrast occurs in a visual scope. On the
other hand I have seen astrographs that produce stunning results and have a
40% obstruction. It's all in the application.

Dave

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