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Old 14-02-2013, 09:23 AM
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Peter Ward
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Quote:
Originally Posted by Satchmo View Post
I follow your point -

I did talk about the Encircled Energy Ratio- this is the performance of the system taking into account the central obstruction (Essentially a modified Strehl ratio of the system including negative effects of central obstruction.). The owner of an 0.999 Strehl ratio scope with 50% obstruction would find his airy pattern and contrast transfer function similar to the owner of an unobstructed scope with 1/2 a wave or more of spherical aberration ( a Strehl of around 0.6 )
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Indeed but the elephant in the room is MTF gives a handle on a resolution limit due diffraction and should not be confused with an optical error.

A larger aperture obstructed system can produce higher resolution than an smaller unobstructed one.

A 1/2 wave spherical error does more than just lower contrast. A telescope with this sort of error simply doesn't focus, and produces caustic images full of halos. The most famous example being Hubble pre-servicing mission.

Last edited by Peter Ward; 14-02-2013 at 10:01 AM.
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