Using a Digital Camera with a Telescope

Images can be captured on a digital camera image sensor using a telescope in a number of ways...

Prime Focus Projection (for dSLR) 
Prime Focus Projection is the preferred method for digital imaging and will produce the best results by keeping aberrations to an absolute minimum while allowing the maximum amount of light to reach the sensor. The telescope is used in a similar way as a normal pre-set super telephoto lens. 

A T-adapter and a matching T-Ring is required for connecting the dSLR to the telescope. 

Resultant Focal Length = telescope focal length x crop factor (wrt 35mm full frame) 

e.g, a telescope of 560mm focal length used with 4-thirds dSLR (2x crop factor) will be equivalent to 1120mm telephoto lens on 35mm full frame. 


Eyepiece Projection 
Eyepiece Projection is ideal when high magnifications are required, such as for  imaging planets. In this method an eyepiece is used to magnify the projected image. 

The resultant equivalent focal length of a telescope can be calculated using the following formula. 

Resultant Focal Length = (telescope focal length) * A * (crop factor wrt 35mm full frame) 

where amplification, A = ( S – F2 ) / F2 
S = distance form the last eyepiece lens to the sensor 
F2 = eyepiece focal length 

The smaller the focal length of the eyepiece the larger the projected image will be. The quality of the eyepiece is very important and an eyepiece projection kit will also be required. 

Afocal Projection (usually using a compact digicam) 
Afocal Projection involves the telescope, an eyepiece and a camera with its own photographic lens attached to the camera .

Basically the focal length of the camera is further extended by the telescope’s magnification. 

The resultant focal length can be calculated by the following formula: 

Resultant focal length = (Telescope Magnification) * (camera lens equivalent focal length or efl) 

where Telescope Magnification = (Objective Focal length) / (eyepiece focal length) 

For example a telescope with a 560mm focal length with 10mm focal length eyepiece will have a magnification power of 56x. When used with Lumix LX-3 having equivalent focal length of 60mm the resultant equivalent focal length (telescope x camera lens) will be 56 x 60mm = 3360mm

The picture below was taken with Afocal projection method.
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