AN INTERFEROMETRIC APPROACH TO THE MEASUREMENT OF THE DIFFUSE LIGHT FROM OPTICAL-SURFACES AND SYSTEMS

Images of astronomical sources are affected by stray light produced in the Earth atmosphere and in the telescope optics. The stray light cau ses a cross-talk phenomenon between intensity and spectral distributio n at the image plane. As far as telescope stray light is concerned, it s amount largely depends on the finish grade of telescope optics. Avai lable techniques to measure the finish grade of optical components inc lude the accurate profilometry of the surfaces and the usage of star s imulators. Referring to digital phase-shift interferometry, a novel te chnique is presented where the diffuse light is evaluated after speckl e field statistics. This technique may be easily used to give an a pri ori estimate of telescope stray light. At the exit pupil of a measurin g interferometer, partially developed speckle patterns are produced by reflection or transmission of the probe beam on the optics under test . The standard deviation of the field distribution is shown to account for the stray light at the image plane by Fourier transform. The tech nique is demonstrated on a set of selected samples of different surfac e finish. Results prove the approach suited to qualify single optical components and to properly model their imaging properties. The applica tion of the speckle technique to functioning reflecting telescopes is also discussed.