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 (0.001°<α<1.5°) 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 (α⊙=0.266°, 0.051°, 0.028°). 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 α⊙ before finally flattening at 0.4α⊙. 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].

► We use planetary phase curves which have the highest sampling at phase angles <α⊙. ► Our analysis of the phase curves shows that there is a flattening below 0.4α⊙. ► We convolve a morphological model with limb darkening functions to fit the flattening. ► The convolved models yield a better fit of phase curves data over wider phase angles. ► Our deconvolution method consists in a logarithmic increase below α⊙ down to 10−3 degree.