pySPT - Source Position Transformation

pySPT is a package dedicated to the Source Position Transformation (SPT). The main goal of pySPT is to provide a tool to quantify the systematic errors that are introduced by the SPT in lens modeling. pySPT is free, open source software compatible with Python 2.7 and distributed under the terms of the MIT license.

Introduction

The modern time-delay cosmography aims to infer the cosmological parameters with a competitive precision from observing a multiply imaged quasar. The success of this technique relies upon a robust modeling of the lens mass distribution. Unfortunately strong degeneracies between density profiles that lead to almost the same lensing observables may bias precise estimate for the Hubble constant. The SPT, which covers the well-known mass sheet transformation (MST) as a special case, defines a new framework to investigate these degeneracies.

Recently, a detailed analysis of how the SPT may affect the time-delay cosmography has been presented in Wertz, Orthen & Schneider (2017). To address this question, we started by developing a flexible numerical framework that encompasses well-tested and efficient implementations of most of the analytical results published in Schneider & Sluse (2014) and Unruh, Schneider & Sluse (2017). Numerous additional features were then added, giving rise to pySPT .

The main repository of pySPT resides on Github, keeping an effective collaboration with Git as a version control system. The code is under continuous development and feedback/suggestions/improvements would be greatly appreciated. To do so, feel free to create an issue to report a bug or send a pull request to submit your contribution.

Documentation

The full documentation for pySPT can be found here: http://pyspt.readthedocs.io.

Jupyter notebooks

Several tutorials in the form of Jupyter notebooks can be found here: https://github.com/owertz/pySPT_tutorials

Installation and dependencies

pySPT relies on packages included in the python standard library and the proven open-source libraries numpy, scipy, matplotlib and numdifftools.

Using the setup.py file

First, you have to download pySPT from its Github repository as a zip file and unzip it on your computer. Enter the pySPT root folder where the setup.py file is located and use the command:

$ python setup.py install

Check the installation

To check that the installation run smoothly, open Python and use the command:

import pySPT

If everything went as planned, you will see a friendly welcome message.

C-shared libraries

For the sake of efficiency, a few methods of the class pySPT.spt.SPT (mainly tilde_alpha_fast and tilde_psi_fast) make use of C shared libraries to compute the closest curl-free approximation of the SPT-modified deflection angle as well as the corresponding deflection potential. These libraries are provided with pySPT and are automatically installed in the sub-repository '.../pySPT/sources_C/' when you install pySPT.

However, you will need a C-compiler pre-installed on your machine to run these methods. I personally recommend gcc. If your machine runs on MacOsX, the gcc compiler is included with 'Xcode', which can be installed for free (see here for info about Xcode). A step-by-step installation guide is given here.

How to deal with pySPT and C-shared libraries are discussed here

Recompile the C files

In the case you want to recompile the C shared libraries on your computer, you need a C-compiler pre-installed on your machine.

How to manually compile a *.c file:

1. Open a terminal and go in the folder '~/pySPT/sources_C' where ~ is the absolute path where pySPT is installed.
2. Run the following command in a terminal

$ gcc -shared -o osx/test.so -fPIC -lm -lgsl sources/test.c

where test is the filename of a C source file, e.g. IntegrandAlphaTilde_NISG_IS1. Note that osx has to be changed depending on your platform: osx for Max OS X and linux for a Linux distribution. Windows is not yet supported. Note that -lgsl is only required for a code (*.c file) which requires the gsl library (see below), and can thus be ignored when gsl is not required.

How to compile all the *.c files:

1. Open a terminal and go in the folder '~/pySPT/sources_C/' where ~ is the absolute path where pySPT is installed.
2. Run the bash script 'compile_shared_libraries.sh':

$ bash compile_shared_libraries.sh

GSL library

A few C shared libraries require the GNU Scientific Library (gsl) to work properly. If you do not have this library pre-installed on your machine, pySPT will raise an exception telling you there is a missing gsl*.so file. An easy way to obtain gsl is to run (in a terminal):

$ brew install gsl

General informations about gsl is given here. Additional informations to install and configure gsl are given here.

Attribution

Most of pySPT capabilities and key features are reported in Wertz & Orthen 2017. Please cite that paper whenever you publish results that made use of pySPT.