
18.2.
Telescope Arrays; Interferometers 451
Computing the rms wavefront error according to Eq. (10.3.8) gives
co
=A. For an
otherwise perfect mirror, the Strehl ratio S = I
—
k^co^.
As an example, if
S = 0.9 at
A
= 500
nm,
then A = 25 nm,
co
= 0.05 waves, and the peak-to-
valley ampHtude is 50 nm.
Thus the effect of the print-through pattern is to remove light from the core of
the PSF and transfer it outward in the PSF. Unlike the radial scattering from
random errors, the regular pattern of the undulations causes the light to appear in
subsidiary peaks around the otherwise radially symmetric PSF. These side peaks
are located at the secondar