Quote:
Originally Posted by Satchmo
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.
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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.