The calculation may be done using this webpages, or similar:
http://physics.bu.edu/~duffy/java/Opticsa1.html
http://www.netzmedien.de/opticalbench/
http://www.arachnoid.com/OpticalRayTracer/
The object will the star image in your telescope focus point, the image will be formed on the sensor plane.
Lens focal length is your EP FL.
Another way would be to use formula:
1/F = 1/a + 1/b
where:
F = focal length of the EP
a = distance of main focus (object) from optical centre of the EP
b = distance of the sensor plane from optical centre of the EP