D4 Astronomical Photometry

Topic

Photometry quantifies the light flux reaching the observer from astronomical objects, and modern detectors achieve such high precision that they can detect the minute dimming caused by an exoplanet transiting its host star.

The quality of a photometric measurement is limited by the same noise sources that affect any astronomical detector: the intrinsic noise of the observed star's light, background sky noise, and the detector's own read noise—sources that combine to determine the measurement's final signal-to-noise ratio.

Light curves, which record an object's flux over time, utilize this photometry to reveal the existence of variable stars, eclipsing binary systems, and planetary transits.

Differential photometry—which continuously compares the object of interest against nearby reference stars within the same field of view—eliminates atmospheric variations that would otherwise be mistaken for the object's own brightness changes. Thus, from the light flux discussed at the beginning of this text to the elimination of atmospheric variations at the end—encompassing combined noise sources and light curves—photometry enables a minuscule change in brightness, capable of revealing an invisible planet, to be confidently distinguished from the simple flickering of Earth's atmosphere.

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