Constructing a Bandpass¶
Bandpass objects represent the transmission fraction of an astronomical filter as a function of dispersion (photon wavelength, frequency or energy). They are basically simple containers for arrays of these values, with a couple special features. To get a bandpass that is in the registry (built-in):
>>> import sncosmo >>> band = sncosmo.get_bandpass('sdssi') >>> band <Bandpass 'sdssi' at 0x...>
To create a Bandpass directly, you can supply arrays of wavelength and transmission values:
>>> wavelength = [4000., 5000.] >>> transmission = [1., 1.] >>> sncosmo.Bandpass(wavelength, transmission, name='tophatg') <Bandpass 'tophatg' at 0x...>
By default, the first argument is assumed to be wavelength in Angstroms.
To specify a different dispersion unit, use a unit from the
>>> import astropy.units as u >>> wavelength = [400., 500.] >>> transmission = [1., 1.] >>> Bandpass(wavelength, transmission, wave_unit=u.nm) <Bandpass 'tophatg' at 0x...>
Using a Bandpass¶
A Bandpass acts like a continuous 1-d function, returning the transmission at supplied wavelengths (always in Angstroms):
>>> band([4100., 4250., 4300.]) array([ 0., 1., 1.])
Note that the transmission is zero outside the defined wavelength range. Linear interpolation is used between the defined wavelengths.
Bnadpasses have a few other useful properties. You can get the range of wavelengths where the transmission is non-zero:
>>> band.minwave(), band.maxwave() (4000.0, 5000.0)
Or the transmission-weighted effective wavelength:
>>> band.wave_eff 4500.0
Or the name:
>>> band.name 'tophatg'
Adding Bandpasses to the Registry¶
You can create your own bandpasses and use them like built-ins by adding them to the registry. Suppose we want to register the ‘tophatg’ bandpass we created:
>>> sncosmo.register(band, 'tophatg')
band.name has been set:
>>> sncosmo.register(band) # registers band under band.name
After doing this, we can get the bandpass object by doing
>>> band = sncosmo.get_bandpass('tophatg')
Also, we can pass the string
'tophatg' to any function that
Bandpass object. This means that you can create
and register bandpasses at the top of a script, then just keep track
of string identifiers throughout the rest of the script.