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Old 13-02-2013, 07:28 PM
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Peter Ward
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Quote:
Originally Posted by Satchmo View Post
To guarantee optical and mechanical quality. 0.5 wave spherical aberration plus a 50% obstruction will probably have an Encircled Energy Ratio ( the Strehl value taking into account obstructive effects ) of less than 0.5... A low Strehl value does not render a telescope useless for imaging. Most of the worlds large RC's probably have 50% obstructions...their resolutions even with large obstruction are way below the seeing limit anyway.
I'd suggest you are mixing your metaphors here.

Strehl is calculated via looking at the RMS values of a * particular system* .

Optical designs vary (eg obstructed vs non-obstructed) hence it makes no sense to compare Airy disk functions between the two.

You can derive (through RMS data ) a Strehl number of say 0.97 for an obstructed system.

What this is saying is: 97% of the flux captured by this system goes where it is expected (ie focus) compared to a theoretically perfect system of the same design.

A system of the *same design* with a Strehl of 0.5 is scattering light all over the shop... clearly rubbish and should be avoided.
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