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Introduction Discovering planets outside our solar system no longer requires a multi-billion dollar space telescope. With a modest ground-based setup and the right software, amateur astronomers can detect worlds orbiting distant stars. AstroImageJ (AIJ) is the gold-standard, free software used by both amateurs and professionals to process these discoveries. It streamlines the process of differential photometry, turning raw images into precise light curves.

Here is a step-by-step guide to analyzing exoplanet transits using AstroImageJ. Step 1: Prepare Your Calibration Files

Before analyzing a transit, you must remove instrumental noise from your raw science images.

Bias Frames: Remove the inherent electronic readout noise of your camera sensor.

Dark Frames: Calibrate out the thermal noise that builds up during long exposures.

Flat Fields: Correct for uneven illumination, dust spots on your optics, and vignetting.

In AstroImageJ, use the DPCoordinate (Data Processor) tool to macro-process these files. AIJ will automatically build master calibration frames and apply them to your raw science images, spitting out clean, calibrated “reduced” images. Step 2: Import and Align Your Image Stack

Exoplanet transits take hours. Over that time, your telescope tracking will drift slightly, causing the stars to move across the sensor. You must align (register) the images so the stars stay in the exact same pixel coordinates.

Go to File > Import > Image Sequence and select your calibrated science images.

Open the Align Stack tool (the icon showing two offset stars).

Select two or more bright, non-saturated stars in your field of view to act as alignment anchors.

Click Align. AIJ will shift and rotate every image in the sequence to match the first frame perfectly. Step 3: Set Up Aperture Photometry

AstroImageJ measures starlight using three concentric circles, known as an aperture.

Inner Circle (Target Aperture): Encloses the target star to measure its total brightness.

Middle Ring (Dead Zone): Ensures no starlight spills into the background measurement.

Outer Ring (Sky Background): Measures the ambient sky brightness so it can be subtracted.

To set your aperture sizes, press Alt + Click on a moderately bright star. Look at the radial profile plot. Set your inner aperture radius just wide enough to capture the bulk of the star’s light (often where the profile hits the background baseline), and set your background rings well outside it. Step 4: Select Target and Comparison Stars

Because Earth’s atmosphere constantly changes, you cannot just measure the target star’s absolute brightness. You must use Differential Photometry, which compares your target star to stable neighbor stars in the same field of view. Click the Perform Photometry icon.

Left-click your Target Star (the star hosting the exoplanet). It will be labeled as T1.

Left-click several Comparison Stars (stable stars of similar brightness and color). These will be labeled C2, C3, C4, etc.

Press Enter or right-click to start the measurement loop. AIJ will scan every image in your stack and record the brightness values in a massive data table. Step 5: Plot and Fit the Transit Light Curve

Once the measurements finish, the Multi-Plot window will open. This is where your exoplanet transit comes to life.

Define the Y-Axis: Set your plot to display Relative Flux T1 (T1 divided by the sum of the comparison stars).

Clean the Data: Look at the individual comparison star curves. If one comparison star is drifting or variable, deselect it to clean up your target curve.

Apply a Fit: Check the box for Exoplanet Transit Fit. Input the known orbital parameters if available (such as period and expected duration).

Detrend: Use AIJ’s detrending features to eliminate systematic errors, like changing airmass as the target moves across the sky.

You should now see a distinct, U-shaped dip in your light curve—the unmistakable footprint of an alien world passing in front of its parent star. Step 6: Export Your Findings

Your analyzed light curve holds immense scientific value. You can export your data table as a .txt or .csv file, or save the final plot as a .png for presentation.

Many amateur astronomers submit their finalized transit data to global databases like the Exoplanet Transit Database (ETD) or the Exoplanet Watch program. Your data helps scientists refine the orbital periods of these planets, ensuring space telescopes like James Webb know exactly when to look. If you want to refine your analysis, let me know: What camera type you are using (CMOS, CCD, or DSLR)? The name of the exoplanet target you are analyzing?

If you need help troubleshooting noisy data or bad tracking?

I can provide specific tips tailored to your exact astronomy setup!