RCA

Resolved Component Analysis

v2.0.2

Before using RCA from this repo, you might want to check out MCCD, a more recent PSF modelling approach. MCCD includes all aspects of RCA, but extends it in several ways. In particular, it can simultaneously fit all detectors of a multi-CCD mosaic camera. It has been shown to outperform RCA. Reference: Liaudat et al., 2020

Description

RCA is a PSF modelling python package. It enforces several constraints, notably some related to sparsity and spatial structure, to build a spatially-varying PSF model from observed, noisy and possibly undersampled star stamps. Some modicum of documentation can be found here - see also quick start below.

Requirements

The following python packages are required:

• numpy
• scipy
• ModOpt

You will also need a compiled version of the sparse2d module of ISAP; alternatively, you should be able to install PySAP and let it handle the compilation and installation of sparse2d.

Installation

After installing all dependencies, RCA just needs to be cloned and python-installed:

git clone https://github.com/CosmoStat/rca.git
cd rca
python setup.py install

References

• Ngolè et al., 2016 - Inverse Problems, 32(12)
• Schmitz et al., 2020 - A&A, 636, A78

Quick start

The basic syntax to run RCA is as follows:

from rca import RCA

# initialize RCA instance:
rca_fitter = RCA(4)

# fit it to stars
rca_fitter.fit(stars, star_positions)

# return PSF model at positions of interest
psfs = rca_fitter.estimate_psf(galaxy_positions)

A complete list of the parameters for RCA and its fit and estimate_psf methods can be found in the documentation. The main ones to take into account are:

• RCA initialization:
  • n_comp, the number of eigenPSFs to learn ("r" in the papers)
  • upfact, the upsampling factor if superresolution is required ("m_d" or "D" in the papers)
• fit:
  • obs_data should contain your observed stars (see note below for formatting conventions)
  • obs_pos, their respective positions
  • either shifts (with their respective centroid shifts wrt. a common arbitrary grid) or, if they are to be estimated from the data, a rough estimation of the psf_size (for the window function - can be given in FWHM, R^2 or Gaussian sigma)
• estimate_psf:
  • test_pos, the positions at which the PSF should be estimated

The rest can largely be left to default values for basic usage.

Note RCA.fit expects the data to be stored in a (p, p, n_stars) array, that is, with the indexing (over objects) in the last axis. You can use rca.utils.rca_format to convert to the proper format from the more conventional (n_stars, p, p).

An example with stars from an HST ACS image can also be found in the example folder.

Changelog

This is "v2" of RCA, which has been very largely overhauled. "v1" can still be accessed here. The most significant changes are:

• Speed of the fitting step has been dramatically increased.
• Interpolation to galaxy (or test star, or any other) positions has been added.
• The source update step is now performed using the Condat (2013 - pdf warning) algorithm, as implemented in ModOpt. In particular, this means the problem solved is now actually in synthesis form.
• A lot of flexibility was added to the graph constraint, and can be accessed through the rca.utils.GraphBuilder arguments (which can be passed on to RCA.fit).
• Simplicity and ease-of-use were favoured, but came at the cost of some flexibility. In particular:
  • "v2" always uses both sparsity in the Starlet domain and the graph constraint (both of which could in principle be turned off in "v1");
  • the package should now be ran from a Python session (whereas it could in principle be launched as a standalone executable before);
  • if shifts are not provided, we now expect a rough estimate of the PSF size to be given as input.

Either of the first two features could easily be re-added to the new version in the future. The latter does not seem too unreasonable, since you likely have a pretty decent idea of the size of your PSF - in fact, in the likely scenario where you obtained star stamps from SExtractor, then you should already have both the shifts and a pretty good estimation of your FWHM.