How do you measure 'Strehl ratio' ?

[skysurfer]

In many discussions or specifications I read 'Strehl ratio' which should be the amount of light exiting the open eyepiece end of the telescope tube (so exclusive eyepiece losses) divided by the amount entering the objective or primary mirror.

EDIT: It is not a measure for amount of light but point source spreading. http://en.wikipedia.org/wiki/Strehl_ratio

How can I measure this ?

[bratislav]

look up under Roddier test. Join the group, read manuals and discussions, grab yourself a webcam and you are ready to go.

http://www.astrosurf.com/tests/roddier/roddier.htm
(use web translator if your French is not that good)

http://tech.groups.yahoo.com/group/roddier/
(join in and start reading - there is also a hacked version of software with English interface instead of French)

If you want to get serious :

François Roddier, Claude Roddier "Wavefront reconstruction using iterative Fourier transforms" Applied Optics (April 1991)

Claude Roddier, François Roddier "Wave-front reconstruction from defocused images and the testing of ground-based optical telescopes". Journal of the Optical Society of America (November 1993)

[Shiraz]

Question.. why do you want to measure the Strehl ratio of your scope(s)?
if the Strehl is not what you expect, there is not really much you can do about it anyway. May be better to let sleeping dogs lie and enjoy what you have?