The shape of the secondary in a Newtonian design is elliptical. Its size is mostly given as the length of its minor axis, i.e. the shortest possible distance between two opposite points on its circumference. For a secondary placed at 45 degrees w.r.t. the optical axis of the primary the difference between minor and major axes is sqrt(2). I.e. Major axis = Minor axis * sqrt(2).
The size of the secondary is very important. To large and it will block to much of the incoming light, to small and it will not catch all the light reflected by the primary.
The following figure shows the relation between the secondary size and the other scope parameters:
In this figure :
- R = Radius of primary
- s = Sagitta
- h = Distance between primary and secondary on the optical axis
- f = Focal length
- n = Secondary minor axis
- k = Radius of the fully illuminated area
The blue lines enclose the cone that is used for the fully illuminated area. They are NOT light ray's coming from a star located on the optical axis !.
Of the secondary, only the minor axis is shown.
The formula to calculate the size of the secondary minor axis:
For the formula to calculate the fully illuminated area, see the corresponding page.