Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Test of a Slow Off-Axis Parabola at it



R E. Parks, Christopher J. Evans, Lianzhen Shao


We describe the interferometric testing of a slow (f/16 at the center of curvature) off-axis parabola, intended for use in an x-ray spectrometer, that uses a spherical wave front matched to the mean radius of the asphere. We find the figure error in the off-axis mirror by removing the theoretical difference between the off-axis segment and the spherical reference form the measured wave-front error. This center of curvature test is easy to perform because the spherical reference wave front has no axis and thus alignment is trivial. We confirm that the test results are the same as the double-pass null test for a parabola that uses a plane autocollimating mirror. We also determine that the off-axis section apparently warped as the result of being cut from the symmetric parent part.
Applied Optics


off-axis aspherics, optical testing, Zernike polynomials


Parks, R. , Evans, C. and Shao, L. (1995), Test of a Slow Off-Axis Parabola at it, Applied Optics (Accessed June 16, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created October 31, 1995, Updated October 12, 2021