# 6. Examples¶

In the following, examples of use of PSFEx are given, together with commented command lines.

## 6.1. Hands-on example 1¶

Let us consider a V band FITS image RX_J2202-19_V.fits and its weight map RX_J2202-19_V.weight.fits. We wish to fit all the galaxies of the image with a galaxy model using SExtractor, which requires computing a model of the PSF first.

We must first run SExtractor on this image to obtain a temporary catalogue in FITS_LDAC format that contains small sub-images from which the PSF model will be extracted. For this, we define in a SExtractor parameter file — let us call it prepsfex.param — the parameters required for the use of PSFEx:

X_IMAGE
Y_IMAGE
FLUX_APER(1)
FLUXERR_APER(1)
ELONGATION
FLAGS
SNR_WIN
VIGNET(35,35)


TBW

TBW

## 6.3. Example 3: unfocused instrument¶

TBW

Skeptical Sam doesn’t have time to test software extensively but is always keen on asking aggressive questions to the author to find out if a program could fit his needs.

PSFEx represents PSFs as an array of tabulated values! Can it really deal with undersampled images? Isn’t it too noisy?

PSFEx was designed from the ground up to deal with undersampled images and arbitrary PSFs. Although the PSF “model” in PSFEx is actually a small image, it is sampled at a different step than the original pixels: more finely for undersampled observations, and more coarsely for oversampled observations, to avoid any loss and redundancy of information. Despite built-in regularisation, PSF models reconstructed on the pixel basis can indeed be noisy if the number of selected stars is small. This can be circumvented to some extent by using ad hoc basis to solve for the PSF model coefficients.

I heard that PSFEx has been developed almost 12 years ago, and has been used for production at TERAPIX for many years. Why have you waited until 2010 for releasing it to the general community?

PSFEx was originally developed for doing PSF-fitting crowded-field photometry with SExtractor. However I was not very happy with the way it worked, as SExtractor’s detection and deblending engine is not meant to deal with crowded star fields. The current release of PSFEx is made in the framework of the EFIGI [2]_ and DES [3]_ projects, as a support tool for galaxy model-fitting.

I would like to use PSFEx to generate PSF models for weak-lensing analyses. Is it the right tool for that?

Simulations of 1h exposures with a 4m optical telescope and sub-arcsecond seeing show that ellipticities of galaxies with a Signal-to-Noise Ratio SNR$$>20$$ can be recovered with a level of systematics below $$10^{-3}$$ using PSFEx models, even in the presence of significant amounts of coma and astigmatism. This is for constant PSFs. Tests with variable PSFs are ongoing.

TBW