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I'm trying to make light curves for Pluto using old data. Most papers give magnitudes "reduced to standard viewing geometry" of r = 39.5 AU, Δ = 38.5 AU, and α = 1 degree, where r is heliocentric distance, Δ is geocentric distance, and α is phase angle.

When papers give apparent magnitudes, I'm left to perform the reductions myself. I'm fine with the adjustment for r and Δ, but how do I reduce to phase angle of 1 degree?

I would also really appreciate a good source if you have it on hand.

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Phase functions are nonlinear in general, but Pluto presents such a small range of phases to Earth that a linear approximation will do: about 0.03 mag/deg for Pluto and 0.08 mag/deg for Charon, or 0.04 mag/deg for the two together. The Stern and Tholen book summarizes the estimates up to 1997. A search for Pluto phase coefficient yields some of the papers behind those and more recent estimates.

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