Fast computation of broadband fluxes and magnitudes

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Project description

Broadband fluxes (`bbf`)

Table of contents

Installation
[Getting started](#getting started)
License

A module to evaluate the broadband fluxes and magnitudes of spectrophotometric standards.

Installation

The bbf package works on python>=3.9<3.13 for Linux and macos. It should work on Windows too, but it has not been tested.

:warning: `bbf` relies for the moment on a modified version of `sncosmo` that must be installed separatly

Install using conda (recommended)

conda install -c conda-forge bbf
pip install git+https://github.com/nregnault/sncosmo

Install using pip

Installing bbf from pip requires to install SuiteSparse which is not installable from pip. On Debian/Ubuntu systems, the command sudo apt install libsuitesparse-dev suffices. Or you can compile SuiteSparse from sources. Or a simpler alternative is to install scikit-sparse from conda:
```
conda install -c conda-forge scikit-sparse
```
cmake and a C++ compiler with OpenMP support are required. On Linux, gcc supports OpenMP out of the box, and cmake is installed as needed during bbf installation. On macos an extra installation is required:
```
# for macos only
conda install cmake llvm-openmp
```

sncosmo fork must be installed separatly with:

pip install bbf git+https://github.com/nregnault/sncosmo

Finally install bbf:
```
pip install bbf
```

Install from sources

If you prefer installing from sources, ensure git lfs is installed (have a git lfs, if the command is missing install it with conda install git-lfs; git lfs install).
Then follow the steps from the [Install using pip](#install using pip) section, and replace the last step with:
```
git clone clone git@gitlab.in2p3.fr:lemaitre/bbf.git
cd bbf
pip install -e .
```
To run the tests suite, install the required dependencies, then run pytest:
```
pip install -e .[test]
pytest
```

If you are a developper and want to work on the bbf C/C++ extension, insall the package with:

pip install nanobind ninja scikit-build-core[pyproject]
VERBOSE=1 pip install --no-build-isolation -Ceditable.rebuild=true -ve .

Installing the Lemaitre bandpasses

If you plan to use the latest version of the megacam6*, ztf and hsc passbands, install the lemaitre.bandpasses package:

pip install lemaitre-bandpasses

git clone https://gitlab.in2p3.fr/lemaitre/lemaitre/bandpasses
cd bandpasses
git lfs pull
pip install .

Getting started

The goal of bbf is to efficiently compute broadband fluxes and magnitudes, i.e. quantities of the form:

$$f_{xys} = \int S(\lambda) \lambda T_{xys}(\lambda) d\lambda$$

where $\lambda$ is the SED of an object, $T_{xyz}(\lambda)$ is the bandpass of the instrument used to observe it. $T$ may depend on the focal plane position of the object and, if the focal plane is a mosaic of sensors, on the specific sensor $s$ where the observation is made. In practice, $x,y$ are coordinates, in pixels, in the sensor frame, and $s$ is a unique sensor index (or amplifier index).

Computing magnitudes requires an additional ingredient: the flux of a reference spectrum $S_{ref}(\lambda)$, usually the AB spectrum, integrated in the same passband (same sensor, same position).

$$m = -2.5 \log_{10} \left(\frac{\int S(\lambda) \lambda T_{xyz}(\lambda) d\lambda}{\int S_{ref}(\lambda) \lambda T_{xyz}(\lambda) d\lambda}\right)$$

To compute these integrales, bbf uses the technique implemented in nacl, which consists in projecting the bandpasses and SED on spline bases:

$$S(\lambda) = \sum_i \theta_i {\cal B}_i(\lambda)$$

and

$$T(\lambda) = \sum_j t_j {\cal B}_j(\lambda)$$

If we precompute the products $G_{ij} = \int \lambda {\cal B}_i(\lambda) {\cal B}_j(\lambda) d\lambda$ the integrals above can be expressed as a simple contraction:

$$f = \theta_i G_{ij} t_j$$

where $G$ is very sparse, since the B-Splines ${\cal B}_i$ have a compact support. If the bandpass $T$ is spatially variable, the $t_j$ coefficients are themselves developped on a spatial spline basis.

$$t_j = \sum_{kj} \tau_{kj} {\cal K}(x,y)$$

The contraction above is then of the form: ...

FilterSets and StellarLibs

bbf implements two main kind of objects: FilterLib, which holds a set of band passes, projected on spline bases (${\cal K_j(x,y)}$ and ${\cal B}i(\lambda)$), and StellarLib which manages a set of spectra, also projected on a spline basis (not necessily the splines used for the filters).

Loading a filter lib

Building a complete version of a FilterLib requires some care. The standard FilterLib used in the Lemaître analysis is build and maintained within the package lemaitre.bandpasses. To access it:

from lemaitre import bandpasses

flib = bandpasses.get_filterlib()

The first time this function is called, the `FilterLib`` is built and cached. The subsequent calls access the cached version, and never take more than a few milliseconds.

Loading Stellar Libraries

As of today, bbf implements two kinds of StellarLibs: pickles and Calspec. An interface to gaiaXP is in development.

To load the pickles library:

import bbf.stellarlib.pickles
pickles = bbf.stellarlib.pickles.fetch()

To load the most recent version of Calspec:

import bbf.stellarlib.calspec
calspec = bbf.stellarlib.calspec.fetch()

Computing Broadband fluxes

With a FilterSet and a StellarLib in hand, one can compute broadband fluxes and broadband mags.