Quote:
Originally Posted by Peter Ward
A well made Riccardi-Honders delivers 5 micron stars across around a 70mm flat field.
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Peter, this kind of performance may be possible on a ray trace program but the reality of diffraction theory says its impossible. For a perfect F4 system of any design the diameter of the Airy disc to the first mimima of the airy pattern will be about 6 micron and then the 50% central obstruction will throw light into the first and second diffraction rings making them very bright , probably doubling the best blur spot diameter to twice that in the presence of seeing.