Photometers and Imaging Cameras

Photometers and imaging cameras measure the brightness of astronomical sources and record their images through selected filters, providing the foundational data of position, flux, and variability.

Definition

A photometer or imaging camera is a focal-plane instrument that forms an image of the sky through one or more filters and measures the brightness of sources within it, calibrated onto a standard magnitude and flux scale.

Scope

This topic covers filter systems and photometric bands, aperture and point-spread-function photometry, wide-field and mosaic imaging cameras, the magnitude system and standard stars, atmospheric extinction and colour terms, and time-series photometry for variability and transit studies.

Core questions

• How is the brightness of a source measured from an image?
• What do photometric filter systems standardise?
• How are atmospheric extinction and colour effects corrected?
• How does time-series imaging reveal variability and transits?

Key theories

The magnitude system and standard stars

Brightness is expressed on a logarithmic magnitude scale tied to networks of standard stars, allowing measurements from different telescopes and nights to be placed on a common footing.

Aperture and PSF photometry

Source brightness is measured either by summing light within an aperture or by fitting the instrumental point-spread function, the latter being essential in crowded fields.

Atmospheric extinction and colour terms

The atmosphere dims starlight by an amount depending on airmass and wavelength, and transforming instrumental measurements to a standard system requires correcting for extinction and colour-dependent response.

Clinical relevance

Photometry and imaging provide the catalogues of positions, brightnesses, and colours that underpin astronomy, and precise time-series photometry detects exoplanet transits, characterises variable and eclipsing stars, and discovers transient events.

History

The magnitude scale was placed on a logarithmic footing by Pogson in the nineteenth century, and photoelectric photometry in the twentieth century brought precision. The Johnson-Morgan filter system standardised photometric bands, and wide-field CCD cameras now enable the deep imaging surveys that map large parts of the sky.

Key figures

• Norman Pogson
• Harold Johnson
• William Morgan

Seminal works

• budding2007
• howell2006

Frequently asked questions

Why are astronomical brightnesses given in magnitudes rather than ordinary units?

The magnitude system is a logarithmic scale inherited from ancient star catalogues, where brighter objects have smaller magnitudes. It conveniently spans the enormous range of cosmic brightnesses and matches the roughly logarithmic response of the eye, so it remains the standard despite its quirks.

How can imaging detect a planet that is far too small to see?

By measuring a star's brightness very precisely over time, a camera can detect the tiny, periodic dip that occurs when a planet passes in front of the star. The planet itself is never resolved, but its transit shadow is revealed in the star's light curve.