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Old 14-02-2013, 07:52 AM
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Satchmo
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Join Date: Dec 2005
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Quote:
Originally Posted by Peter Ward View Post
You can derive (through RMS data ) a Strehl number of say 0.97 for an obstructed system.
I follow your point - its rteally a semantical one as you understand the pint I am trying to make.

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 ( equivilent to an Strehl of around 0.6 )

So simply comparing Strehl ratio between different kinds of scopes, is a bit of furfy for the unwary. Good interferometry software is perfectly capable of putting out an EER number as well as Strehl but it is not the kind of thing instrument manufacturers want to advertise.

I propose manufacturers who supply optical data on finished instruments include a figure for the Encircled Energy Ratio- lets rename it the OSR or obstructed strehl ratio then the waters will be much clearer for those that want to compare relative performance of optical systems. After all its the ability of the instrument to focus light from an available aperture into the Airy Disc that we are interested in - and I know that pixel size in imaging further complicates these considerations.

Last edited by Satchmo; 14-02-2013 at 09:20 AM.
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