We investigate the effect of atmospheric phase and scintillation anisoplanatism on the measurement of the local gradient of the wavefront using a Hartmann-Shack type wavefront sensor. This is accomplished by simulation of the imaging process, starting with 100 synthetic, anisoplanatic phase and scintillation screens that were computed for several viewing angles and that correspond to Fried parameters of 7 and 12 cm. The screens are calculated using the approximated turbulence profile at the site selected for the ATST, Haleakala on Maui, Hawaii, USA. Phase aberrations are propagated through the wavefront sensor, considering each viewing angle in each subaperture (of adjustable size) separately. The point spread functions (PSF) are calculated for the viewing directions as well as specified (and adjustable) pixel scale in the sensor camera. Subsequently, these PSFs are convolved with a typical wavefront sensor lock structure of solar AO systems, an image of solar granulation. The cross-correlation peak of the thus created anisoplanatic subimages is finally used to find the local gradients of the wavefront. We find that phase anisoplanatism contributes significantly to the measurement error of a Hartmann-Shack type wavefront sensor, whereas we cannot detect a notable increase thereof from scintillation anisoplanatism in the subaperture when using a cross-correlating algorithm to find the gradient of the incident wavefront.

Date Published: 16 July 2008
PDF: 10 pages
Proc. SPIE 7015, Adaptive Optics Systems, 70154X (16 July 2008); doi: 10.1117/12.788074

Published in SPIE Proceedings Vol. 7015:
Adaptive Optics Systems
Norbert Hubin; Claire E. Max; Peter L. Wizinowich, Editor(s)