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Great pictures Bill. The one of AR2546 coming around the limb in particular looks wonderfully three-dimensional. Your brightness plot is interesting and, with the help of Messrs Stefan and Boltzmann (and the back of a convenient envelope!), suggests a temperature at the visible depth limit on the limb of just under 4540K, which is about what you would expect, given the usually quoted figure of 4400K for the top of the photosphere (as you suggest, the number is distorted a bit by the blue filter but the plot does suggest a temperature neatly in between that to be expected in the umbra and penumbra of the sunspot – it might be interesting to check against pixel values there too). Plotting across a range of wavelengths should indeed provide a more accurate result.