[AstroPy] Blackbody fit and Wien's displacement law

[Derek Homeier]

Hi Rudolf,
> 
> Where does the  500 (micron * K) come from?
> 
> What would be the correct way to determine the position of  lambda Wien for my plot?
> 
the 5099 micron * K result from substituting the (energy) value of the B_lambda maximum, 4.965 k_B T,
in hc / E_max [hc / 4.965 k_B = 2898 micron * K] with the corresponding one for B_nu, 2.821 k_B T:
hc / 2.821 k_B ~ 5099 micron * K

To find the equivalent in (lambda B_lambda) ~~ (nu B_nu), you’d have to solve for the derivative of that function
analogously to the derivation of Wien’s law for the other two functional forms, e.g. in
https://en.wikipedia.org/wiki/Wien%27s_displacement_law#Derivation_from_Planck's_law

If I am not mistaken the numerical value is also given in
https://en.wikipedia.org/wiki/Planck%27s_law#Percentiles 
for the 41.8 percentile (maximum in logarithmic spectral axis), i.e. 3670 (μm·K).

Cheers,
				Derek

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