To: atm{at}shore.net
From: MLThiebaux 
Reply-To: MLThiebaux 


>I know how to calculate the image size in inches at the prime focus for
>my telescope. For example for my telescope, 1.459 seconds of arc of the
>sky forms an image at the prime focus of 0.0013 inches. I want to know
>what the image size will be using an eyepiece projection technique. The
>thin lens formula appears to requires information I do not have readily
>on hand for my orthoscopic eyepieces, and in addition the eyepieces are
>not thin lenses. Does anyone know of an approximation formula for
>determining image size within about 10% either way of the actual image
>size?
>


Let y = distance from eyepiece to the final image formed by the eyepiece. 
Let f = focal length of eyepiece.  y is adjustable to any distance you want
and could be difficult to measure accurately because you don't easily know
from what point on the eyepiece assembly to measure from, but this is
unimportant if y/f is a large number.  Then the effective focal length of
the system is (y/f)
-1  times the focal length of the primary.  Equivalently the final image size
is (y/f) - 1 times the primary image size.  You can probably safely ignore the
-1.

Martial Thiebaux
Rawdon Hills, Nova Scotia

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