Reducing Light Curves with AstroImageJ

12. Reducing Light Curves with AstroImageJ#

This guide shows how to use AstroImageJ (AIJ) to turn a sequence of astronomical FITS images into a differential light curve. The workflow is useful for eclipsing binary stars, exoplanet transits, and asteroids, but the details are not identical for every target. A fixed star can usually be measured with the same aperture pattern from image to image. A moving asteroid requires extra care because the target moves relative to the comparison stars.

The goal is not to describe every AIJ menu item. The goal is to give you a practical reduction path: organize the data, inspect the FITS headers, calibrate or confirm calibration, plate solve when needed, choose apertures, run differential photometry, inspect the light curve, and save enough information that the result can be checked later.

What AIJ is good for

AstroImageJ is especially useful for time-series differential photometry because it combines image display, FITS handling, calibration, aperture photometry, plotting, detrending, and exoplanet transit modeling in one graphical interface.

12.1. The Big Picture Workflow#

A light-curve reduction is a chain of steps. If one early step is wrong, the final light curve can look noisy, shifted, or physically misleading. Work slowly the first few times and write down what settings you used.

A typical AIJ workflow is:

  1. Organize the raw images and calibration frames.

  2. Check a few FITS headers for target name, exposure time, filter, date, time, and observatory information.

  3. Calibrate the images with bias, dark, and flat frames, or confirm that the images were already calibrated.

  4. Load the science images as an image stack.

  5. Plate solve the images if you need WCS coordinates, BJD_TDB, target identification, or astrometry.

  6. Choose the target aperture and comparison-star apertures.

  7. Run multi-aperture differential photometry across the image stack.

  8. Inspect the measurements table and light curve.

  9. Remove or flag bad frames only when there is a defensible reason.

  10. Normalize, detrend, phase fold, or model the light curve as appropriate for the target.

  11. Save the measurements table, plots, settings, and a short research-log entry.

Do not skip inspection

A light curve can look plausible even when the reduction is wrong. Always inspect the images, aperture placement, comparison stars, measurements table, and final plot.

12.2. Setting Up the Project Folder#

Keep the AIJ work in a project folder with a predictable structure. This prevents the common problem where raw images, calibrated images, plots, exported tables, and notes all end up in the same directory.

AIJ-project/
├── raw/
├── calibration/
│   ├── bias/
│   ├── darks/
│   └── flats/
├── calibrated/
├── plate_solved/
├── results/
├── settings/
└── logs/

Use raw/ for the original images, calibration/ for bias, dark, and flat frames, calibrated/ for reduced images, plate_solved/ for images with WCS information, results/ for exported tables and plots, settings/ for saved AIJ photometry settings, and logs/ for notes about what you did. Do not edit or overwrite the raw images unless your mentor explicitly tells you to.

File naming tip

Use names that include the date, target, and processing stage, such as 2026-05-18_TIC123456789_calibrated_001.fits or 2026-05-18_TIC123456789_AIJ-measurements.csv.

12.3. FITS Headers Before AIJ#

Before you start clicking through AIJ, inspect at least a few FITS headers. The header is the metadata record attached to the image. It usually contains the exposure time, filter, observation time, object name, telescope, camera, image type, detector temperature, binning, and sometimes RA/Dec or WCS information.

Important keywords to check include OBJECT, DATE-OBS, EXPTIME or EXPOSURE, FILTER, IMAGETYP, XBINNING, YBINNING, GAIN, RDNOISE, AIRMASS, RA, DEC, SITELAT, SITELONG, and CCD-TEMP. Different cameras and observatory pipelines may use different keyword names, so do not assume every file uses exactly the same convention.

Header sanity check

Before using AIJ, confirm that the science images have the expected target, exposure time, filter, and observation time. Also confirm that calibration frames are labeled as bias, dark, or flat frames and that their exposure times and filters make sense.

AIJ can display FITS header information from the image window. If you plate solve images, the solution may add WCS keywords such as CRVAL1, CRVAL2, CRPIX1, CRPIX2, CD1_1, CD1_2, CD2_1, and CD2_2. Those keywords connect image pixels to sky coordinates.

12.4. Loading Images into AIJ#

Start AIJ and load the science images as a stack. A stack lets AIJ move through the time series one frame at a time while keeping the same reduction and photometry workflow.

A common path is File > Import > Image Sequence or one of the AIJ image-series options, depending on your version. Select the science images in chronological order. If the images are large or numerous, use virtual-stack options when available so the entire sequence is not loaded into memory at once.

After the stack opens, step through several frames. Check that the field is the expected target, stars are not badly trailed, the background is not saturated, the target is visible, and the images are in the correct time order. Adjust brightness and contrast only for display. Display settings do not change the underlying pixel values.

Common first check

Blink through the first few frames. The target field should look stable. Clouds, tracking jumps, focus changes, or a meridian flip can be easier to see by stepping through the images than by looking at a final light curve.

12.5. Calibration in AIJ#

Raw images contain detector and optical-system signatures. Bias frames remove the electronic offset, dark frames remove thermal signal, and flat fields correct pixel-to-pixel sensitivity differences, dust shadows, and vignetting. For precise photometry, calibration is not optional.

If your images were already calibrated by an observatory pipeline, record that fact in your log and do not calibrate them a second time. If you have raw images and calibration frames, use AIJ’s Data Processor to create or apply master calibration frames. The exact menu labels vary by AIJ version, but the workflow is the same: identify the calibration frames, create master calibration frames if needed, apply them to the science images, and save the calibrated images to a new folder.

Calibration checklist

Use bias frames with the same camera readout mode and binning as the science frames. Use dark frames with the same exposure time, temperature, binning, and readout mode when possible. Use flats taken with the same filter and optical configuration as the science images. If the camera was rotated, focus changed substantially, or filters were moved, old flats may not correct the images well.

After calibration, inspect a calibrated image. Dust donuts should be reduced, the background should look smoother, and the image should not show obvious overcorrection. If the calibrated image looks worse than the raw image, stop and check whether the wrong darks, flats, or filters were applied.

12.6. Plate Solving and Astrometry.net#

Plate solving determines how image pixels map onto sky coordinates. In a solved FITS image, the header contains WCS information that lets software connect each pixel to RA and Dec. Plate solving is useful for identifying the target, importing aperture positions by coordinate, checking field orientation, calculating some time and airmass quantities, and documenting exactly which field was measured.

AIJ can interface with Astrometry.net for plate solving. Astrometry.net is useful when the pointing is uncertain because it can solve an astronomical image and return astrometric calibration metadata. In practice, plate solving is much faster when you provide a good initial estimate of the image center, image scale, and search radius.

Basic plate-solving workflow:

  1. Confirm the FITS header has a reasonable DATE-OBS and exposure time.

  2. Estimate the image scale in arcsec/pixel from the telescope and camera.

  3. Provide a target name or approximate RA/Dec.

  4. Run the plate solve on one image first.

  5. Check that catalog-star overlays match real stars.

  6. If the first image solves, solve the full stack or save the solved images.

  7. Record whether WCS information was added to the FITS headers.

When plate solving matters most

Plate solving is especially useful for asteroid work, crowded fields, target identification, and any project where apertures may need to be placed by coordinates instead of by eye.

Astrometry.net troubleshooting

If a solve fails, check the image scale, field center, search radius, image orientation, and whether the frame has enough stars. Cropped images, very small fields of view, saturated stars, clouds, or wrong scale estimates can all cause plate-solving failures.

12.7. Choosing Target and Comparison Stars#

Differential photometry compares the target brightness to one or more comparison stars in the same images. A good comparison star is stable, unsaturated, isolated, visible in every frame, and reasonably similar in brightness and color to the target when possible.

Use more than one comparison star when the field allows it. A single comparison star can quietly ruin the light curve if it is variable, blended, saturated, or affected by bad pixels. Also choose at least one check star if the project requires a formal comparison-check plot.

Comparison-star checklist

Good comparison stars are not saturated, not near the edge of the image, not blended with nearby stars, not on bad columns or dust artifacts, and not so faint that their noise dominates the result. They should also stay inside the image for the entire run.

For time-series projects, avoid comparison stars that are crossed by an asteroid, affected by a diffraction spike, or near the sky annulus of the target aperture. If the target is an asteroid, check the path of the asteroid across the field before choosing comparison stars.

12.8. Setting Apertures and Sky Annuli#

AIJ aperture photometry uses an object aperture and a sky annulus. The object aperture sums the target’s light. The sky annulus estimates the local background that should be subtracted from the object aperture.

A practical starting point is to measure the FWHM of several stars and choose an object aperture radius roughly 1.5 to 2.5 times the FWHM. The inner sky radius should be far enough from the star that it does not include much stellar light, and the outer sky radius should include enough sky pixels to estimate the background robustly.

Aperture-size rule

Start with one aperture size for all stars. Then inspect the light curve and aperture overlays. A larger aperture includes more of the star but also more sky noise. A smaller aperture reduces sky noise but can lose stellar flux if seeing or guiding changes.

Use centroiding for stars when the target and comparison stars are fixed on the detector from frame to frame. For fast-moving asteroids or trailed objects, centroiding can fail or follow the wrong object, so check aperture placement carefully.

12.9. Running Multi-Aperture Photometry#

The Multi-Aperture tool is the normal AIJ tool for time-series differential photometry. The typical order is to click the target first, then comparison stars. AIJ labels the target and comparison apertures in the measurements table. Check the labels carefully so you know which aperture is the target and which are comparisons.

After starting the run, watch several frames. Confirm that apertures remain centered, sky annuli do not include nearby stars, the target is not saturated, and comparison stars remain inside the image. If the field jumps, the apertures may fail to recenter correctly.

Important output columns often include the frame number, filename, time column, airmass, source-minus-sky fluxes, relative flux, relative flux error, FWHM, sky background, and aperture positions. The exact column names depend on your AIJ settings.

Save the table

The measurements table is your main science product. Save it even if you also save a plot. A plot is useful for presentation, but the table is what you need for checking, replotting, and later analysis.

12.10. Plotting, Normalizing, and Detrending#

Use AIJ’s Multi-Plot tool to plot relative flux versus time. For time, use BJD_TDB when it is available and trustworthy. If BJD_TDB is not available, use JD_UTC or another documented time column, but write down exactly which one you used.

Normalize the light curve so that the baseline is near 1.0. For eclipsing binaries and exoplanet transits, normalize using out-of-eclipse or out-of-transit data. Do not include eclipse or transit points in the baseline estimate. For asteroids, normalization depends on the project goal; you may want relative changes over time rather than a transit-like baseline.

Detrending can remove slow systematic trends caused by airmass, seeing, sky background, or image motion. Use detrending carefully. A detrending model should remove an instrumental or atmospheric trend, not erase real astrophysical variability.

Detrending warning

Never apply a trend correction just because it makes the light curve look prettier. Record which detrending variables were used and check that the correction does not remove the signal you are trying to measure.

12.11. Target-Specific Tips#

The same AIJ tools are used for different targets, but the reduction strategy changes with the science goal.

12.11.1. Eclipsing Binary Stars#

For eclipsing binaries, the target is fixed relative to the comparison stars, so the standard multi-aperture workflow usually works well. The main goal is to measure changes in brightness over one or more orbital cycles. Use a reliable period and epoch if you phase fold the data. If the period or epoch is uncertain, save the time-series light curve before doing any phase folding.

Tips for eclipsing binaries:

  • Use comparison stars that are stable and not much brighter or fainter than the target.

  • Preserve both primary and secondary eclipses; do not detrend through eclipse features.

  • Check whether the out-of-eclipse baseline changes from night to night.

  • Record the period and epoch used for phase folding.

  • If combining multiple nights, check whether the same comparison stars and filters were used.

12.11.2. Exoplanet Transits#

For exoplanet transits, timing and systematics matter. Use accurate site information, accurate FITS times, and BJD_TDB when possible. Observe baseline before and after the transit so the out-of-transit level is well defined. A transit is often shallow, so comparison-star choice and detrending can strongly affect the result.

Tips for exoplanets:

  • Use an ensemble of comparison stars when available.

  • Avoid saturated stars and stars near detector defects.

  • Keep exposure times long enough to reduce scintillation but short enough to avoid saturation and excessive smearing.

  • Record ingress, egress, predicted mid-transit time, exposure time, filter, and detrending variables.

  • Use detrending only when there is a physical or instrumental reason, such as airmass, FWHM, sky background, or centroid motion.

12.11.3. Asteroids#

Asteroids move relative to the background stars. This makes asteroid photometry different from eclipsing-binary or exoplanet photometry. The target aperture may need to follow the asteroid, while comparison stars remain fixed to the stellar field. Before running photometry, blink through the image stack to identify the moving object and check whether it crosses comparison stars, bad pixels, or field stars.

Tips for asteroids:

  • Use image blinking, sometimes informally called image blitting, to identify the moving target across the stack.

  • Plate solve the images when possible so the asteroid position and comparison-star positions can be checked against coordinates.

  • Use astrometry.net if the field lacks trustworthy WCS information or if the pointing is uncertain.

  • Avoid comparison stars near the asteroid path.

  • If the asteroid moves significantly, inspect aperture placement throughout the stack instead of trusting the first frame.

  • If the asteroid trails, reduce the exposure time if possible. If the data are already trailed, consider whether a larger, elliptical, or custom aperture is needed.

  • Do not stack images for a time-series light curve unless you preserve correct timing and understand what the stacking does to the moving target.

Asteroid warning

A normal stellar photometry workflow assumes that the target and comparison stars move together on the detector. An asteroid violates that assumption. Always check whether AIJ is following the asteroid or accidentally measuring a nearby field star.

12.12. Exporting Results and Recording Settings#

Save the measurements table, the light-curve plot, the aperture settings, and a short research-log entry. The exported table should go in results/, not in the raw-data folder.

A useful saved-result package includes:

  • the AIJ measurements table,

  • a time-series light-curve plot,

  • a normalized light-curve plot if used,

  • a phase-folded plot if used,

  • the aperture settings or AIJ settings file,

  • a screenshot showing target and comparison apertures,

  • notes about calibration, plate solving, comparison stars, and detrending.

Research-log entry

Write down the target, date, filter, number of images, exposure time, calibration status, plate-solving status, comparison stars used, aperture radii, time column used, and output filenames.

12.13. Common Problems and Fixes#

If the light curve has large scatter, check whether the target or comparison stars are saturated, whether clouds or poor seeing affected some frames, whether the apertures drifted, whether the sky annulus contains field stars, and whether the wrong calibration frames were applied. Also check whether one comparison star is variable by plotting comparison stars against each other.

If the time axis looks wrong, inspect the FITS headers for date, time, exposure, and time zone assumptions. Observation times should generally be recorded in UTC. If AIJ reports BJD_TDB, make sure site coordinates and image times were correct before trusting it.

If plate solving fails, check image scale, field center, search radius, and whether the image has enough unsaturated stars. If an asteroid light curve looks wrong, blink the images and confirm that the aperture follows the asteroid in every frame.

Do not delete data casually

Bad frames can be excluded from analysis, but do not erase them. Keep the original table and make a note explaining which frames were excluded and why.

12.14. Videos and Further Reading#

Use these resources when you need a slower walkthrough or an alternate explanation.

12.14.1. AstroImageJ and differential photometry#

12.14.2. AstroImageJ workflow videos#

Use these when you want to see the software workflow from image sequence to light curve.

12.14.3. AAVSO photometry and variable-star resources#

Use these when students need more background on comparison stars, CCD photometry, and variable-star measurement practices.

12.14.4. Exoplanet transit background#

Use these when students understand the software steps but need help interpreting what the transit light curve means physically.

12.14.5. Astrometry, plate solving, and target identification#

Use these when students are having trouble confirming the target field, matching stars, or understanding why plate solving matters.

12.14.6. Optional comparison tools#

These are not required for this guide, but they show that the same photometry ideas appear in different software tools.

12.14.7. Useful background#

How to use videos

Do not watch a video passively and assume you understand the workflow. Pause the video, write down the settings being used, and compare them with your own data.

12.15. Final Checklist#

Before considering the reduction finished, confirm that the images were calibrated or intentionally left uncalibrated for a documented reason, the FITS times and exposure times were checked, the target and comparison stars were correctly identified, the apertures stayed centered, the sky annuli avoided contaminating stars, the measurements table was saved, the time column was documented, and any excluded frames were recorded.

For eclipsing binaries and exoplanets, save both the time-series light curve and any phase-folded or modeled version. For asteroids, save notes about the asteroid path, aperture tracking, and any frames where the target crossed a star or detector artifact.

Final rule

AIJ can produce a light curve quickly. Your job is to prove that the light curve is trustworthy.

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