LSSTDESC.Coord 1.0.5

Python module for handling angles and celestial coordinates

Coord is a Python module that provides basic functionality related to angles and celestial coordinates.

It is similar in functionality to the astropy.coordinates module, but with more of an emphasis on efficiency. Some functions are more than 100 times faster than the corresponding functionality in astropy. On the other hand, the API is somewhat more restrictive than the API used by astropy, so the appropriate module to use will depend on your needs.

Notable functionality:

• Spherical geometric calculations of the distance between two points, angles in spherical triangles, and areas of spherical triangles.

• Tangent-plane projection of a coordinate relative to a given center point of the projection.

• Deprojection back into spherical coordinates.

• Analytic calculation of the jacobian of the tangent projection.

• Precession calculations between different epochs.

• Conversion to galactic and ecliptic coordinate systems (and back).

The code was originally written by Mike Jarvis for TreeCorr and then also used in GalSim. It has been republished here as a stand-alone module to enable others (within the LSST DESC or not) to have easier access to these functions.

One missing feature (for which pull requests would be welcome) is conversion between FK5 and ICRS systems (or even FK4). If you care about the slight differences between these systems, then you should probably stick to astropy, which does handle these distinctions.

Aside: The module was used as a pedagogical tool at the LSST DESC DE School on July 9, 2017. A video of the lesson can be viewed here.

Licence

The code is licensed under the MIT License, which basically means you can use it in any way you want, so long as you keep the copyright notice at the top of each source file and/or include the accompanying LICENSE file with the distribution.

Installation

Installing libffi

This module is mostly pure Python. However, it does have a small bit of C++ code, which is wrapped with cffi. This in turn depends on libffi, which is not itself pip installable. Most modern Python installations will have this already installed, so you probably don’t have to do anything special to use it. However, if not, installing libffi is fairly straightforward:

On a Mac, you should only need to run the command:

$ xcode-select --install

On Linux, if you have root access, then one of the following should work for you:

$ apt-get install libffi-dev
$ yum install libffi-devel

If you don’t have root access (and don’t want to bother your sysadmin), then installing from source into a {prefix} directory where you have write access (e.g. your home directory) is also not very hard:

$ wget ftp://sourceware.org:/pub/libffi/libffi-3.2.1.tar.gz
$ tar xfz libffi-3.2.1.tar.gz
$ cd libffi-3.2.1
$ ./configure --prefix={prefix}
$ make
$ make install
$ cp */include/ffi*.h {prefix}/include
$ cd ..

Installing Coord

Once you have done one of the above (or not if you already have libffi installed), you can install Coord in the usual way with:

$ python setup.py install --prefix={prefix}

or if you have root access, you might prefer:

$ sudo python setup.py install

Or any of the other usual variants of this command as you prefer.