Terms of Reference#

Background#

  • This document lays out terms of reference for TJPCov developers and contributors in the wider MCP and LSST DESC context.

Scope#

  • TJPCov provides a set of APIs for calculation of covariance (and cross-covariance) matrices (or other descriptions of measurement uncertainty as required) for all main canonical large scale structure cosmological probes in DESC:

    • weak lensing shear

    • galaxy clustering

    • galaxy clusters clustering

    • galaxy clusters number counts

  • It provides flexibility in how its functionality is accessed:

    • it is called from TXPipe and APIs are designed to work with TXPipe

    • it can be called as a standalone library

    • it can be called from the command line through driver routines

  • Interface support is as follows:

    • Public APIs are maintained in Python

    • Documentation should be maintained in the main README, readthedocs, and the benchmarks folder.

    • Command-line driver routines are maintained in the bin folder

    • Examples will be maintained in a separate repository. [AS?]

  • In the basic model, covariance matrices are factorised as follows:

    • Disconnected part of the Gaussian covariance matrix

    • Connected part of the Gaussian covariance matrix arising from mode coupling

    • Additional covariance due to super-survey modes

  • External libraries:

    • TJPCov aims to implement basic functionality internally but supports an external back-end for versatility and cross-checking

    • External back-ends can be linked as external libraries (on top of a thin wrapper inside TJPCov), but must be interfaced via pure python (i.e., the external back-end must implement a python wrapper as a precondition to be integrated into TJPCov)

  • Boundaries of TJPCov:

    • TJPCov does not support the calculation of covariance matrices that are not covariant with the main large-scale structure probes (i.e., supernovae luminosity distance)

    • TJPCov performs theoretical computations of the covariance matrix given survey properties and assumed cosmology. It does not perform calculations on the data, e.g., various bootstrap techniques

    • In general, TJPCov does not support covariance arising from systematic effects although exceptions could be made on an as-needed basis (i.e., we provide a covariance matrix as observed by a perfect instrument with no galactic foregrounds, etc.)