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Transit Photometry

1. The Concept: Transit Photometry

You are not trying to resolve the planet itself. You are measuring the light curve.

When an exoplanet passes in front of its host star, the star's brightness drops—usually by 0.5% to 2% (or 5–20 millimagnitudes). Your goal is to keep the star on the exact same pixels of your sensor for several hours and measure that brightness change with extreme precision.

2. The Hardware Requirements

You likely already possess much of the necessary gear if you do deep-sky astrophotography, but the requirements prioritize stability over resolution.

The Telescope (Aperture & Optics)

  • Aperture: Generally, 8 inches (200mm) or larger is recommended to gather enough photons to reduce statistical noise (shot noise). However, for bright stars, high-quality 4–6 inch refractors can work.
  • Focal Ratio: Faster scopes (f/4 to f/6) are generally preferred to gather light quickly, allowing for shorter exposures and better time resolution.
  • Type: Schmidt-Cassegrains (SCTs), Newtonians, and Ritchey-Chrétiens (RCs) are popular choices.

The Mount (Tracking is King)

  • Your mount must be an Equatorial (EQ) mount capable of guiding.
  • Why? If the star drifts across the sensor, variations in pixel sensitivity (inter-pixel capacitance) will look like a change in brightness, ruining your data.
  • Guiding: Autoguiding is mandatory. You need the star to stay locked within a few pixels for 3–5 hours.

The Camera (Sensor Linearity)

  • Mono is Best: Monochrome CCD or CMOS cameras are preferred. They are more sensitive and don't have the Bayer matrix (color array) which complicates photometric math.
  • Cooling: A cooled camera is essential to keep thermal noise (dark current) consistent so it can be subtracted later.2
  • 16-bit Depth: You need a high dynamic range (16-bit ADU) to detect subtle brightness changes without saturating the star.

Filters (Standardization)

  • To make your data scientifically useful, it usually needs to match standard photometric systems.
  • Best: Johnson-Cousins V (Visual/Green) or R (Red), or Sloan r' or i' filters.
  • Good: An "Exoplanet" filter (blue-blocking) which cuts out atmospheric scattering.
  • Acceptable: A clear filter (for faint targets), though this makes comparing data with other observatories harder.

3. Critical Techniques for Success

Photometry is the opposite of "pretty picture" imaging.

1. Intentional Defocusing

This is the most counter-intuitive part. You should slightly defocus your telescope.

  • Why? Pinpoint stars saturate pixels quickly (hitting the max numerical value of the sensor), losing data.
  • Benefit: Spreading the star's light over a "donut" of 10–20 pixels allows you to take longer exposures (gathering more photons) without saturating, and averages out pixel-to-pixel variations.

2. Calibration Frames

You cannot skip calibration. The "dip" of a planet is often smaller than the dust motes on your sensor.

  • Flats: Crucial for removing vignetting and dust donuts.
  • Darks/Bias: Essential for removing sensor noise.3

3. Timing Precision

Your computer clock must be synced to UTC. A standard Windows clock drift of 2 minutes can invalidate a scientific observation. Use software like Dimension 4 or similar NTP clients to keep your capture PC synced to the second.