lightcurvelynx.astro_utils.zeropoint

Functions

sky_bg_adu_to_electrons(sky_bg_adu, gain)

Convert sky background from ADU/pixel to electrons/pixel.

magnitude_electron_zeropoint(→ numpy.typing.ArrayLike)

Photometric zeropoint (magnitude that produces 1 electron) for

flux_electron_zeropoint(→ numpy.typing.ArrayLike)

Flux (nJy) producing 1 electron.

calculate_zp_from_maglim([maglim, sky_bg_electrons, ...])

Calculate zero points based on the 5-sigma mag limit.

Module Contents

sky_bg_adu_to_electrons(sky_bg_adu, gain)[source]

Convert sky background from ADU/pixel to electrons/pixel.

Parameters:

  • sky_bg_adu (float or ndarray of float) – Sky background in ADU/pixel.

  • gain (float or ndarray of float) – The CCD gain (in e-/ADU).

Returns:

Sky background in electrons/pixel.

Return type:

float or ndarray of float

magnitude_electron_zeropoint(*, filter: numpy.typing.ArrayLike, airmass: numpy.typing.ArrayLike, exptime: numpy.typing.ArrayLike, instr_zp_mag: dict[str, float] | float | numpy.typing.ArrayLike, ext_coeff: dict[str, float] | float | numpy.typing.ArrayLike) → numpy.typing.ArrayLike[source]

Photometric zeropoint (magnitude that produces 1 electron) for LSST bandpasses (v1.9), using a standard atmosphere scaled for different airmasses and scaled for exposure times.

Parameters:

  • filter (ndarray of str) – The filter for which to return the photometric zeropoint.

  • airmass (ndarray of float) – The airmass at which to return the photometric zeropoint.

  • exptime (ndarray of float) – The exposure time for which to return the photometric zeropoint.

  • instr_zp_mag (dict[str, float], float, or ndarray of float) – The instrumental zeropoint for each bandpass, i.e. AB-magnitude that produces 1 electron in a 1-second exposure. Keys are the bandpass names, values are the zeropoints.

  • ext_coeff (dict[str, float], float, or ndarray of float) – Atmospheric extinction coefficient for each bandpass. Keys are the bandpass names, values are the coefficients.

Returns:

AB mags that produces 1 electron.

Return type:

ndarray of float

Note

Typically, zeropoints are defined as the magnitude of a source which would produce 1 count in a 1 second exposure - here we use electron counts, not ADU counts.

References

Lynne Jones - https://community.lsst.org/t/release-of-v3-4-simulations/8548/12

flux_electron_zeropoint(*, instr_zp_mag: dict[str, float] | float | numpy.typing.ArrayLike, ext_coeff: dict[str, float] | float | numpy.typing.ArrayLike, filter: numpy.typing.ArrayLike, airmass: numpy.typing.ArrayLike, exptime: numpy.typing.ArrayLike) → numpy.typing.ArrayLike[source]

Flux (nJy) producing 1 electron.

Parameters:

  • filter (npt.ArrayLike) – The filter for which to return the photometric zeropoint.

  • airmass (npt.ArrayLike) – The airmass at which to return the photometric zeropoint.

  • exptime (npt.ArrayLike) – The exposure time for which to return the photometric zeropoint.

  • instr_zp_mag (dict[str, float], float, or ndarray of float) – The instrumental zeropoint for each bandpass in AB magnitudes, i.e. the magnitude that produces 1 electron in a 1-second exposure. Keys are the bandpass names, values are the zeropoints.

  • ext_coeff (dict[str, float], float, or ndarray of float) – Atmospheric extinction coefficient for each bandpass. Keys are the bandpass names, values are the coefficients.

Returns:

Flux (nJy) per electron.

Return type:

ndarray of float

calculate_zp_from_maglim(maglim=None, sky_bg_electrons=None, fwhm_px=None, read_noise=None, dark_current=None, exptime=None, nexposure=1)[source]

Calculate zero points based on the 5-sigma mag limit.

Calculated according to formulas:

snr = flux/fluxerr
fluxerr = sqrt( flux + sky*npix*gain + readnoise**2*nexposure*npix
                + darkcurrent*npix*exptime*nexposure)
5 = flux/fluxerr
25 = flux**2/( flux + sky*npix*Gain + readnoise**2*nexposure*npix
               + darkcurrent*npix*exptime*nexposure)
flux**2 - 25*flux -25*( sky*npix*Gain
                        + readnoise**2*nexposure*npix
                        + darkcurrent*npix*exptime*nexposure)
                    = 0
flux = 12.5 + 0.5*sqrt(625
                        + 100( sky*npix*Gain
                               + readnoise**2*nexposure*npix
                               + darkcurrent*npix*exptime*nexposure) )
zp = 2.5*log10(flux) + maglim
Parameters:

  • maglim (float or ndarray) – Five-sigma magnitude limit.

  • sky_bg_electrons (float or ndarray) – Sky background in electrons/pixel.

  • fwhm_px (float or ndarray) – PSF in pixels.

  • read_noise (float or ndarray) – Read noise (in e-/pixel).

  • dark_current (float or ndarray) – Dark current (in e-/pixel/second).

  • exptime (float or ndarray) – Exposure time (in seconds).

  • nexposure (int or ndarray) – Number of exposure. Default is 1.

Returns:

zp – Instrument zero point (that converts 1 e- to magnitude).

Return type:

float or ndarray