Old 09-05-2013, 08:41 AM
Kev11 (Kevin)
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Optical Geometry - Eyepiece with DSLR

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.

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Old 15-05-2013, 09:25 PM
Andrew C
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possible alternative


I wonder whether you can find out what is going on by removing the nosepiece/T ring temporarily and focussing by eye on a reasonably bright, crisp image (e.g. the moon, street light) using each eyepiece in turn. In each case, then replace your eye with a sheet of white paper and move it back and forth until the image is clean. That might give you a rough idea of how far the focal plane is behind the EP in each case, and you can go from there.


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Old 16-05-2013, 09:39 AM
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Diagram here:


If the focal length of the telescope objective at A is F and the focal length of the eyepiece is f, then:

1/f = 1/BC + 1/CD

magnification m = CD/BC

Effective focal length of the entire telescope, with eyepiece and camera = mF
Effective focal ratio of the entire telescope = mF/A, where A is the clear aperture of the objective or primary mirror. Note also that F/A is simply the focal ratio of the telescope objective when you are using it visually.

To focus the image on the camera sensor you need to shift the eyepiece out (away from the objective) so that BC will be a bit longer than f.

For a magnification m=1, the distance BC = 2f and CD = 2f, so the distance between the primary focus and the camera focal plane = BC+CD = 4f. For a 25mm eyepiece, this is 100mm. To this you must also add the physical length of the eyepiece optics from the field lens to the eye lens, which - depending on your eyepiece - could add anything from 25 to 100 mm.

If you are using a 25mm eyepiece, the distance CD from the eyepiece to the focal plane in your camera must be > 50mm and if m is say 3, it must be > 75mm, give or take a bit.

So, for a 25mm eyepiece this implies the distance between the primary focal plane at B and the camera sensor at D will be at least 120mm, and could be as much as 200mm or more.

If you are having problems focussing... my guess is that you have the eyepiece too close to the camera.

Last edited by Wavytone; 16-05-2013 at 12:16 PM.
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Old 19-05-2013, 10:27 AM
Kev11 (Kevin)
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Thanks Andrew and Wavy, I will pursue those ideas when I get a chance.
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