Quote:
Originally Posted by Peter Ward
Spherical aberration is a fabrication error and is only present if the telescope has been made incorrectly.... Checking my RCOS inteferogram is has a 1/25th wave spherical error...i.e. essentially perfect.
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We need to be careful quoting numbers here.
The 30-40% central obstruction throws light out of the airy disc and in to the surrounding fresnel rings reducing an essentially good `theoretical' wavefront to one producing effectively at around 0.8 Strehl , similar to an unobstructed scope with 1/4 wave spherical aberration, so quoting 1/25 wave for the scope is a bit academic. In fact it can't work any better than any 1/4 wave wavefront 'scope with a small obstruction. Certainly an instrument with 1/4 wave spherical and a .3 obstruction will perform worse, but only reduced to around 0.7 Strehl in the presence of obstruction ( what is called the EER or Encircled Energy Ratio which takes in to account the central obstruction) .