<p>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 <a href="https://books.google.com/books?id=VcY7iYJwJZoC&amp;pg=PA211&amp;lpg=PA211&amp;dq=Pluto+phase+coefficient&amp;source=bl&amp;ots=YDfEZAxWP3&amp;sig=5nEneBnP92ozpOp2Ia-I1E3EFoo&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwiD4ZvMi9XNAhXCJB4KHURdCQIQ6AEITDAG#v=onepage&amp;q=Pluto%20phase%20coefficient&amp;f=false" rel="nofollow">Stern and Tholen book</a> summarizes the estimates up to 1997. 
A search for <a href="https://www.google.com/search?q=Pluto+phase+coefficient" rel="nofollow">Pluto phase coefficient</a>
yields some of the papers behind those and more recent estimates.</p>

CC BY-SA 3.0
2016-07-02T15:33:17.497Z
2016-07-02T15:33:17.497Z
Mike G
<p>I'm trying to make light curves for Pluto using old data. Most papers give magnitudes "reduced to standard viewing geometry" of <em>r</em> = 39.5 AU, <em>&Delta;</em> = 38.5 AU, and <em>&alpha;</em> = 1 degree, where <em>r</em> is heliocentric distance, <em>&Delta;</em> is geocentric distance, and <em>&alpha;</em> is phase angle. </p>

<p>When papers give apparent magnitudes, I'm left to perform the reductions myself. I'm fine with the adjustment for <em>r</em> and <em>&Delta;</em>, but how do I reduce to phase angle of 1 degree?</p>

<p>I would also really appreciate a good source if you have it on hand.</p>

<pluto><photometry>
How do I reduce planetary photometry to standard viewing geometry using phase angle?
Confused Cosmologist
# How do I reduce planetary photometry to standard viewing geometry using phase angle?

(Asked by: username_0 on 2016-06-16)

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.

**Tags:** pluto, photometry

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## 1 answers

### Answer by username_1 on 2016-07-02. Score: 1

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](https://books.google.com/books?id=VcY7iYJwJZoC&pg=PA211&lpg=PA211&dq=Pluto+phase+coefficient&source=bl&ots=YDfEZAxWP3&sig=5nEneBnP92ozpOp2Ia-I1E3EFoo&hl=en&sa=X&ved=0ahUKEwiD4ZvMi9XNAhXCJB4KHURdCQIQ6AEITDAG#v=onepage&q=Pluto%20phase%20coefficient&f=false) summarizes the estimates up to 1997.
A search for [Pluto phase coefficient](https://www.google.com/search?q=Pluto+phase+coefficient)
yields some of the papers behind those and more recent estimates.


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