From: "Dwight K. Elvey" 
To: atm{at}shore.net
Reply-To: "Dwight K. Elvey" 


>From: "Stanley A. Gorodenski" 
>
>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?
>
>Stan Gorodenski
>
>
>

HI Stan
 Image size at prime focus is just a simple trig calculation.
It is how big that size angle will be at the focal length of the telescope
in question. As an example:

2000mm focal length
0.25 degree object size

  2*sin(0.25/2)*2000 = 8.7mm

Because of the small angles used you can just use:

 sin(0.25)*2000 = 8,7mm


As for the parameters of your eyepiece, you'll need to measure them
yourself or use reference objects in the sky( known angles ) to adjust your
setup to a specific magnification. Objects on the Moon can be handy here
because they are bright enough for screen projection and can be measured as
prime focus and with the eyepiece projection for comparison.
 As one of the others suggested, Brown's book is a good source
if you want to find the needed parameters of your particular eyepiece.
Dwight

--- BBBS/NT v4.00 MP