The overall wavefront error of a diagonal has less meaning than that of a primary mirror. In a primary mirror every image must pass thru the entire aperture, therefore the entire aperture must have high wavefront error rating in order for the image to not be distorted at the focal plane. In the case of a diagonal, the overall wavefront error can be much lower, yet the effect on the image as far as distorting it, will be negligeable. That is because the diagonal mirror is so close to the focus point that only a very small portion of the surface is used to form the image. In the case of a 2" diagonal, only about 1/10 inch to 1/4 inch diameter is used to form the actual image of any star or planet. Therefore, a diagonal with as low as 1/4 wave surface can have an effective 1/40 wave error at the focus point.
Rolando
Rolando
