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Programmatic Replacements for NICMOS Units Conversion Form

Programmatic Replacements for NICMOS Units Conversion Form#

This notebook illustrates programmatic ways to perform unit conversions as provided by NICMOS Units Conversion Form. The examples here are not exhaustive and will never be. They are meant to give users enough basic knowledge to then calculate what they really want.

Imports#

  • All the examples require astropy.units. See Units and Quantities in Astropy for more information.

  • Some examples require the package snyphot.

  • Some examples require the math library.

from astropy import units as u

from synphot import SourceSpectrum
from synphot import units as syn_u
from synphot.models import BlackBodyNorm1D

import math

Example 1: Simple Jy to AB mag#

INPUT FROM THE FORM IS 
Input units = Jy             
Output units = AB magnitude   
Input flux =    1.00000E-13 Jy             
Temperature of the blackbody =    5500.00
INPUT wavelength =     1.00000 micron
OUTPUT wavelength =     1.00000 micron
AB magnitude   = 41.43

In plain English, user wants to know what is 1e-13 Jy converted to AB magnitude at 1 micron. Blackbody is not needed for this conversion.

input_flux_ex1 = 1e-13 * u.Jy
input_wave_ex1 = 1 * u.micron
abmag_ex1 = input_flux_ex1.to(u.ABmag, u.spectral_density(input_wave_ex1))
print('{:.2f}'.format(abmag_ex1))

41.40 mag(AB)

Example 2: PHOTLAM to FNU for Blackbody#

INPUT FROM THE FORM IS 
Input units = photons/cm2/s/A
Output units = erg/cm2/s/Hz   
Input flux =    1.00000 photons/cm2/s/A
Temperature of the blackbody =    5500.00
INPUT wavelength =    0.500000 micron
OUTPUT wavelength =    0.600000 micron
Flux=4.62E-23 erg/cm2/s/Hz

This example is more complicated in that it requires assumption of a blackbody and has different input and output units.

First, we create a source spectrum with a blackbody model with the given temperature of 5500 K.

bb_ex2 = SourceSpectrum(BlackBodyNorm1D, temperature=5500*u.K)

Then, we calculate the normalization factor required for the blackbody to have the given input flux at the given input wavelength.

input_flux_ex2 = 1 * syn_u.PHOTLAM
input_wave_ex2 = 0.5 * u.micron
factor_ex2 = input_flux_ex2 / bb_ex2(input_wave_ex2)

We apply this factor to our blackbody source.

input_ex2 = bb_ex2 * factor_ex2

Finally, we calculate the desired flux in given output unit and wavelength.

output_wave_ex2 = 0.6 * u.micron
flux_ex2 = input_ex2(output_wave_ex2, flux_unit=syn_u.FNU)
print('{:.2e}'.format(flux_ex2))

4.62e-23 FNU

Example 3: W/m2/Hz to I-mag for Power-Law#

INPUT FROM THE FORM IS 
Input units = W/m2/Hz        
Output units = magnitude I    
Input flux =    1.00000E-23 W/m2/Hz        
Index of the power-law spectrum as a function of frequency =   0.250000
INPUT wavelength =     1.00000 micron
OUTPUT wavelength =    0.900000 micron
I =  0.85

First, we define the input flux and wavelength.

input_flux_ex3 = 1e-23 * (u.W / (u.m * u.m * u.Hz))
input_wave_ex3 = 1 * u.micron

Then, we define a power-law function.

def powerlaw_ex3(nu):
    """F(nu)=nu**(spectral index)
    
    nu is a Quantity.
    """
    spectral_index = 0.25
    return (nu.to(u.Hz, u.spectral()) ** spectral_index).value

We use this power-law and a normalization factor based on input flux and wavelength to calculate output flux at output wavelength in input flux unit.

output_wave_ex3 = 0.9 * u.micron
flux_ex3 = powerlaw_ex3(output_wave_ex3) * input_flux_ex3 / powerlaw_ex3(input_wave_ex3)

Finally, we convert the flux to I magnitude, as defined by the converter as:

magnitude I: zero-flux=2250 Jy; central wavelength=0.90 microns;

imag_zpt = 2250 * u.Jy
i = -2.5 * math.log10(flux_ex3 / imag_zpt)
print('{:.2f}'.format(i))

0.85
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