Fringe Calculation

[The Timns]

I have the sme problem.
According to Yoder's Opto-Mechanics book on pg 22 He states "One
fringe=0.5wavelength @ 546nm (mercury green). Fringes are specified
over the maximum dimention of the clear aperture. Hope this helps.

[EarlJ]

Power fringes or Newtonian Fringes gives an information about the
accuracy of the radius of curvature. These fringes are exactly
circular if the real radius is exactly spherical but not of exactly
the same value as the target radius.
That means the real surface is not spherical but rather toric.
Accordingly the shape of the fringes change from a circle to an
ellipse. The difference of fringes in two perpendicular sections is
the number of "irregular fringes".