Photometry

Measuring the light of stars, frame after frame, to see what changes: a pulsating star, or a planet passing in front of its star.

What is photometry?

You take a long series of exposures of the same field. On each frame, you measure the light of the star under study (the target) inside a small circle, the aperture, minus the sky background around it.

That measure alone is worthless: the atmosphere keeps changing (altitude, haze, clouds). So you also measure stable comparison stars in the same field. The target / comparison ratio, or the magnitude difference (V − C), cancels what the atmosphere does to all stars at once. Only what the target does remains.

Plot this ratio against time and you get the light curve.

A variable star: AE UMa

AE UMa is a high-amplitude Delta Scuti pulsating star. It swells and shrinks in just over two hours.

Amateur photometry: light curve of the HADS variable star AE UMa measured with Siril and plotted with gnuplot — Light curve of AE UMa (Siril, relative magnitude against Julian date). Higher = brighter.

The curve shows a period of about 0.086 day (≈ 2 h 04 min) and an amplitude of about 0.5 magnitude. The maxima do not all reach the same height: AE UMa pulsates in two modes at once, which add up.

An exoplanet: the transit of WASP-52 b

WASP-52 b is a gas giant orbiting its star in 1.75 days. When it passes in front, it hides a little over 2 % of the star's light for about 1 h 50 min. That small dip is what we are after.

The raw measure

Raw light curve of the exoplanet WASP-52 b transit measured in Siril with a 76 mm refractor — Night of 12 to 13 October 2026: V − C measured by Michel with the Siril photometry tool, frame by frame. The dip is there, but noisy.

Fitting a model

Exoplanet WASP-52 b transit light curve fitted with a batman MCMC model, depth 2.36 % — Relative flux, transit model (batman, MCMC fit), residuals and sky transparency (labels in French).

The model gives a planet radius of 0.154 ± 0.013 times the star's, i.e. a depth of 2.36 ± 0.40 %, and mid-transit at 23:46 UTC (± 4 min). Published values (about 0.164 and 2.7 %) are within about one standard deviation: consistent.

The bottom panel shows why some points stray: around 23:30 UTC, clouds pulled the transparency below 60 %. Those frames are shown as open circles.

Equipment and processing

TS 76 EDPH refractor with reducer (345 mm focal length), ZWO ASI2600MC camera.
Acquisition: N.I.N.A. For WASP-52 b: 45 exposures of 300 s, gain 0.
Photometry: done by Michel with the Siril photometry tool (6 px aperture, RGGB superpixel); this gives the raw curve.
Transit model: batman, MCMC fit done by Claude (Anthropic) from the Siril measures.