This article shows a basic principle of prime focus astrophotography. It can help beginners to find a focusing position of their camera if eyepiece focusing position is known.

In prime focus astrophotography the camera is mounted in the focal plane of the telescope's objective (the objective can be mirror or lens) without any additional lens. The following picture shows a difference between normal watching through scope (using eye) and prime focus:

O - the main objective (primary mirror or lens) F - focus point (in focal plane) E - eyepiece fO - focal length of objective fE - focal length of eyepiece

As you can see in the picture, when you are watching through a scope using eyepiece, the distance between the main objective and eyepiece lens is f_O + f_E . When using a CCD camera at prime focus, the CCD chip plane must be identical with the main objective focal plane - i.e. the CCD chip distance from the main objective is only f_O . This means, that your scope focuser must enable you to move the camera a bit inwards - towards the main objective.

Example

Let's suppose that we have a Newtonian with focal length 1000mm and we use 10mm eyepiece for visual watching. When the telescope is focused on a very far object (all astronomy objects are very far) - the eyepiece lens is at 1000mm + 10mm = 1010mm distance from the main mirror (distance in our case is length of light ray from the objective to the eyepiece lense). When we want to use the CCD camera at prime focus, then the CCD chip must be positioned 10mm inwards in comparison with eyepiece position - i.e. the distance of CCD from the main mirror is 1000mm.

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Last Update: 10.06.2006