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Secondary mirror size


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It depends on the size of the fully illuminated field of view that you want.

The absolute minimum size of flat is easily calculated with trigonometry. The proportion of the primary focal length that the flat is is positioned at times the primary diameter. ( i.e. if the flat is positioned 1/4 of the primary's focal length inside the focal plane then it must be at least 1/4 the diameter of the primary ). That size will fully illuminate only the central point of the field of view and from there to the edge of the field of view there will be a steady fall-off of light making DSO's dimmer towards the edge. Increasing the flat diameter will fully illuminate a larger area of the field of view before the fall-off starts. However, the larger the flat size becomes the more it causes diffraction so the fine detail at high magnifications tends to be lost. In a general purpose telescope therefore it is a balance between full field illumination and definition. A dedicated planetary scope will have the smallest flat while a dedicated DSO scope will generally have a much larger flat.

This site lay's it all out for you: http://www.bbastrode...om/diagonal.htm

Nigel

Edited by Astrobits
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