astropy units for blackbody_lambda does not do proper si conversion

If I run the following code, it correctly converts the unit “bar” to SI:

from astropy import units as u

test = 1 * u.bar
print(test)
print(test.si)
# 1.0 bar
# 100000.0 N / m2

But for blackbody_lambda, it fails to convert the result to correct si unit:

from astropy import units as u
from astropy.modeling.blackbody import blackbody_lambda

testB = blackbody_lambda(1 * u.um, 100 * u.K)
print(testB)
print(testB.si)
print(testB.to('N/s/m^2/sr'))
# 3.8960173143757327e-56 erg / (Angstrom cm2 s sr)
# 3.896017314375732e-54 bar / (rad2 s)
# 3.896017314375732e-49 N / (m2 s sr)

That is, astropy does not convert bar to N/m^2 (which gives factor of 10^5 difference).

I tested with some other complicated combinations, e.g., test = 1 * u.cm * u.bar / u.sr * u.erg, but the SI conversion worked just fine. Maybe the failure is only for blackbody_lambda…?

[pllim]

I don’t think this is a bug in blackbody per se. The SI conversion maps J / m3 to Pa (and bar is 1e5 * Pa). When we’re talking about flux, it makes more sense to have testB.si return something like J / (m3 rad2 s), no? For now, you can force this manually:

>>> testB.to(u.J / (u.m ** 3 * u.s * u.rad ** 2))
<Quantity 3.89601731e-49 J / (m3 rad2 s)>

p.s. Astronomer calculating flux in SI units? That’s crazy talk!

[mhvk]

Very much so. Though bar/rad2 just doesn’t make sense! If someone has the time, I’d do the swap of power and pressure in si.py and see if that helps…