Variations of the apparent angular size of the Sun across the entire Solar System: Implications for planetary opposition surges

We test several convolution and deconvolution models on phase curves at small phase angles ð0:0011oao1:51Þ that have the highest phase angle sampling to date. These curves were provided by cameras onboard several NASA missions (Clementine/UVVIS, Galileo/SSI and Cassini/ISS) when the Sun had different apparent angular radii (a 1⁄4 0:2661, 0.0511, 0.0281). For the smallest phase angles, the brightness of the objects (Moon, Europa and the Saturn’s rings) exhibits a strong round-off below the angular size of the Sun. The brightness continues to increase below a before finally flattening at 0:4a . These behaviors are consistent with the convolution models tested. A simple deconvolution model is also used and agrees with laboratory measurements at extremely small phase angles that do not show any flattening [Psarev V, Ovcharenko A, Shkuratov YG, Belskaya I, Videen G. Photometry of particulate surfaces at extremely small phase angles. J Quant Spectrosc Radiat Transfer 2007;106:455–63]. Published by Elsevier Ltd.