The method: transit photometry

When a planet passes in front of its host star, it blocks a small fraction of the star's light. This produces a measurable dip in brightness — typically between 0.5% and 2% for hot Jupiters, less for smaller planets. With careful differential photometry, an amateur setup can detect these events in real time.

The data needed to predict transits is publicly available. The EPIC catalog (from the K2 mission) and the TOI (TESS Objects of Interest) catalog list candidate systems with predicted transit times, durations, and depths. You look up an event that falls during your observing window, point your telescope at the right star, and take a continuous series of short exposures across the predicted window.

Exoplanet transit light curve — EPIC 210957318b brightness dip
A detected transit of EPIC 210957318b — the characteristic U-shaped dip in the host star's brightness.

Equipment requirements

The good news: you do not need specialised hardware. The basic requirements are:

  • Telescope: Any telescope with sufficient aperture to collect good signal from a 10th–13th magnitude star — typically 100mm or larger
  • Mount: Equatorial, with reliable tracking. An autoguider helps, but the transit windows are often short enough that unguided tracking is acceptable
  • Camera: A monochrome astronomy camera gives the best signal-to-noise. Colour cameras work, but you lose sensitivity
  • Software: AstroImageJ is the standard tool for exoplanet transit reduction — free, and specifically designed for this measurement

Differential photometry

The key technique is differential photometry: instead of measuring the absolute brightness of the target star, you measure it relative to one or more comparison stars in the same field. This cancels out atmospheric effects, thin cloud, and seeing variations — all of which affect all stars in the field equally. The result is a light curve that shows the true variation in the target's brightness.

AstroImageJ automates most of this process. You define your target star and comparison stars, set aperture sizes, and the software produces the light curve from a batch of calibrated images.

Sample field image used for exoplanet transit photometry
Sample imaging field — target star and comparison stars used in differential photometry.

Amateur observations of exoplanet transits have real scientific value. The AAVSO Exoplanet Section and the ETD (Exoplanet Transit Database) accept and publish amateur observations. A well-reduced transit light curve from a modest setup can help refine the orbital period or confirm a candidate system.

Between transits

While you are waiting for the next transit event, the same setup that you use for transit photometry is the same setup you use for deep-sky imaging. The only difference is cadence: exoplanet work needs a continuous series of short exposures rather than longer guided subs. Your regular imaging session and your transit observation share the same hardware.

Bubble nebula HOO narrowband — deep-sky imaging with the same setup
In between transits — the same setup works beautifully for deep-sky narrowband imaging.