Question: Optical Geometry - Eyepiece with DSLR

[Kev11 (Kevin)]

Is there anything on-line which will let me calculate the spacers necessary to focus images onto the DSLR with an eyepiece in the train (my high school optics was 53 years ago!)? I have a standard SW 8" and have successfully used a 25MM EP (Celestron ordinary) in the standard Bintel nosepiece/T ring with my Canon. However no other EP, longer or shorter FL, will come to focus.

[bojan]

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

[Kev11 (Kevin)]

Thanks, Bojan for those links, I am starting to recover my rusty optics knowledge. Googling around I did come across a post on a non-IIS forum which stated the the eyepiece in an eyepiece projection set up had to have a focal length close to the diagonal dimension of the DSLR sensor. That is consistent with my finding that the 25mm EP works. The post didn't explain the reason for this rule of thumb. I also now understand that varying the position of the eyepiece in a projection alters the magnification rather than the focus.
Cheers
Kevin

[bojan]

You are most welcome, I'm glad I helped