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Old 14-02-2013, 10:23 AM
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Satchmo
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Quote:
Originally Posted by Peter Ward View Post
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.

Indeed 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.
From memory that resolution increase with obstruction is only for sources fairly similar in brightness..its a small anomaly rather than a "feature"

I did specify effect on MTF in my earlier post ( Contrast Transfer Function ) . The effect on it by obstruction or spherical aberration is similar, so optical quality and obstructiuon are both important influences on the MTF.

I think supplying MTF curves would be the ultimate piece of documentation but how many punters would understand it? People seem to have grasped the Strehl Ratio, so I think furthering our understanding of what an instrument is capable of through an EER or Obstructed Strehl Ratio would be a positive step.

I don't think its accurate to say that an instrument with 1/2 wave spherical aberration does not focus. Anyone owning a high quality optic with a 50% obstruction is seeing an in focus Airy Pattern similar to an unobstructed telescope with 1/2 a wave of spherical aberration.

The Hubble Space Telescope had a 1/2 wave RMS figure ( around 2 waves P-V) . Official report on the whole debarcle here...

Last edited by Satchmo; 14-02-2013 at 10:38 AM.
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