Sunspot photometry with phase diversity - I. Methods and global sunspot parameters

The global brightness structure of a small sunspot is investigated. Seeing and instrumental effects are compensated by application of the phase-diversity technique and a conventional deconvolution method. We calculate brightness temperatures for the reconstructed data in three simultaneously observed continuum bands of the solar spectrum (402.1 nm, 569.5 nm, 709.1 nm). The darkest umbral regions are on average 0.17I(.), 0.27I(.) and 0.32I(.) bright. The corresponding temperatures lie in the range around 4790 K, 4600 K and 4460 K. The spatially averaged penumbral brightness amounts to 0.72I(.), 0.81I(.) and 0.85I(.), which corresponds to 5910 K, 5750 K and 5640 K, respectively. Although the spectral distribution of the umbral and penumbral intensities is consistent with former measurements, the derived values support the idea that there exist a real difference in the thermal properties between the umbrae of small and large sunspots.