Astronomical Magnitudes

[mileslucas]

Hello, I have a somewhat niche unit problem.

Over at UnitfulAstro there's been an implementation of astronomical magnitudes (Lines 89-149). There's a few flavors, but the basic interaction is that it is a logarithmic scale with a conversion like

f * 10 ^ (-0.4 * lambda_m)

where f is a flux value and lambda_m is the magnitude.

What I've noticed is an issue of undefined behavior when using some of the densities of fluxes. For instance, when looking at the brightness of stars, we can measure them and get a power value, or energy per second (energy for us is flux) eg erg/s. Furthermore, we might have a flux density, or a solid angle in which the energy is absorbed (like the aperture of a telescope) eg erg/s/cm^2. Even further, we could disperse the light and obtain a spectrum that is dependent on wavelength eg erg/s/cm^2/cm.

Any one of these values ought to be able to be affected by this magnitude, but only the following cases work:

julia> using Unitful, UnitfulAstro

julia> flux = 10u"erg"
10 erg

julia> λ_m = 1u"mag"
1 mag

julia> flux * λ_m
ERROR: undefined behavior. Please file an issue with the code needed to reproduce.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] isrootpower_dim(::Unitful.Dimensions{(Unitful.Dimension{:Length}(2//1), Unitful.Dimension{:Mass}(1//1), Unitful.Dimension{:Time}(-2//1))}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:167
 [3] isrootpower(::Quantity{Int64,𝐋^2*𝐌*𝐓^-2,Unitful.FreeUnits{(erg,),𝐋^2*𝐌*𝐓^-2,nothing}}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:166
 [4] *(::Quantity{Int64,𝐋^2*𝐌*𝐓^-2,Unitful.FreeUnits{(erg,),𝐋^2*𝐌*𝐓^-2,nothing}}, ::Gain{Unitful.LogInfo{:Magnitude,10,-2.5},:?,Int64}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:213
 [5] top-level scope at none:0

jjulia> flux = 10u"erg/s"
10 erg s^-1

julia> flux * λ_m
3.981071705534972 erg s^-1

julia> flux = 10u"erg/s/cm^2"
10 erg cm^-2 s^-1

julia> flux * λ_m
3.981071705534972 erg cm^-2 s^-1

julia> flux = 10u"erg/s/cm^2/cm"
10 erg cm^-3 s^-1

julia> flux * λ_m
ERROR: undefined behavior. Please file an issue with the code needed to reproduce.
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] isrootpower_dim(::Unitful.Dimensions{(Unitful.Dimension{:Length}(-1//1), Unitful.Dimension{:Mass}(1//1), Unitful.Dimension{:Time}(-3//1))}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:167
 [3] isrootpower(::Quantity{Int64,𝐌*𝐋^-1*𝐓^-3,Unitful.FreeUnits{(erg, cm^-3, s^-1),𝐌*𝐋^-1*𝐓^-3,nothing}}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:166
 [4] *(::Quantity{Int64,𝐌*𝐋^-1*𝐓^-3,Unitful.FreeUnits{(erg, cm^-3, s^-1),𝐌*𝐋^-1*𝐓^-3,nothing}}, ::Gain{Unitful.LogInfo{:Magnitude,10,-2.5},:?,Int64}) at /Users/miles/.julia/packages/Unitful/dPidf/src/logarithm.jl:213
 [5] top-level scope at none:0

Any tips?

[ajkeller34]

Thanks for the report and your interest in the package. I'm pretty sure #267 should resolve your issues. This sort of error happens whenever someone tries to multiply a quantity of some dimension by a gain for the first time; you need to decide if the behavior should be more like watts (10dB = 10x) or volts (20dB = 10x).