Aperture and PSF Photometry

Aperture and point-spread-function photometry are the two principal methods for extracting an object's brightness from a digital image, by summing flux in a defined aperture or by fitting a model of the stellar profile.

Definition

Aperture photometry measures source brightness by integrating counts inside a fixed region and subtracting an estimated sky level, while PSF photometry derives brightness from the amplitude of a fitted point-spread-function model.

Scope

This topic covers measurement of source flux on detector images: aperture photometry, in which counts are summed within a chosen radius and a local sky background is subtracted, and PSF-fitting photometry, in which an empirical or analytic model of the point-spread function is fit to one or many overlapping sources. It addresses background estimation, aperture corrections, and the handling of crowded fields.

Core questions

• How is the sky background estimated and subtracted when measuring a source's flux?
• When is aperture photometry preferable to PSF fitting, and vice versa?
• How are overlapping or blended stars deconvolved in crowded fields by simultaneous PSF fitting?
• What is an aperture correction and why is it needed?

Key theories

Point-spread-function fitting

The brightness of a star is recovered by scaling a model of the instrumental profile to match the observed image, enabling accurate photometry even where stellar images overlap.

Optimal aperture and sky subtraction

Choosing an aperture radius and an annular sky region trades enclosed signal against added noise, and the resulting flux is corrected to a total magnitude using a measured curve of growth.

Clinical relevance

These techniques enable photometry of globular clusters, resolved stellar populations in nearby galaxies, and faint sources near bright neighbors, which are the empirical basis for stellar evolution studies and distance measurement.

History

The advent of CCDs in the 1980s made digital pixel-level photometry routine, and Stetson's DAOPHOT package established PSF-fitting as the standard approach for crowded-field stellar photometry.

Seminal works

• stetson1987
• howell2006

Frequently asked questions

Why is PSF fitting better in crowded fields?

When stars overlap, a single aperture captures light from neighbors; fitting PSF models to all sources at once separates each star's contribution, giving accurate individual magnitudes.

What is a curve of growth?

It is the measured total flux as a function of aperture radius; it shows how much light a finite aperture misses and provides the aperture correction to a total magnitude.