PaperlayerView exact revision on arXiv

Source-linked AI summary

AstroImageJ: Image Processing and Photometric Extraction for Ultra-Precise Astronomical Light Curves (Expanded Edition)

Karen A. Collins, John F. Kielkopf, Keivan G. Stassun, Frederic V. Hessman

arXiv:1701.04817v1astro-ph.IMastro-ph.EP

TL;DR

Ultra-precise astronomical photometry needs an integrated environment that combines research-grade capability with broad usability. AstroImageJ extends ImageJ with astronomy-specific calibration, photometry, detrending, fitting, and visualization tools in a GUI-driven, cross-platform package. Its capabilities were demonstrated with sub-300-parts-per-million transit-model residual RMS values and transit-timing residuals below approximately 30 s, while parameter-uncertainty estimates remain unavailable.

  • Problem

    Ultra-precise photometry lacks a sophisticated yet easy-to-use, integrated environment for image processing, light-curve extraction, and analysis.

  • Method

    AIJ extends ImageJ with astronomy-specific image calibration, time-series differential photometry, detrending, fitting, plotting, FITS/WCS utilities, and real-time processing.

  • Results

    183 and 255 parts per million RMS were achieved for combined and five-minute-binned ground-based transit-model residuals of WASP-12b and Qatar-1b, respectively, with transit-timing residuals below approximately 30 s.

  • Takeaways & Limitations

    AIJ provides a tightly integrated, GUI-driven package for processing, modeling, and plotting astronomical image data across professional, student, and amateur use.

  • Takeaways & Limitations

    AIJ currently finds best-fit model parameters but does not provide parameter-uncertainty estimates.

Abstract

from arXiv · show

ImageJ is a graphical user interface (GUI) driven, public domain, Java-based, software package for general image processing traditionally used mainly in life sciences fields. The image processing capabilities of ImageJ are useful and extendable to other scientific fields. Here we present AstroImageJ (AIJ), which provides an astronomy specific image display environment and tools for astronomy specific image calibration and data reduction. Although AIJ maintains the general purpose image processing capabilities of ImageJ, AIJ is streamlined for time-series differential photometry, light curve detrending and fitting, and light curve plotting, especially for applications requiring ultra-precise light curves (e.g., exoplanet transits). AIJ reads and writes standard FITS files, as well as other common image formats, provides FITS header viewing and editing, and is World Coordinate System (WCS) aware, including an automated interface to the astrometry.net web portal for plate solving images. AIJ provides research grade image calibration and analysis tools with a GUI driven approach, and easily installed cross-platform compatibility. It enables new users, even at the level of undergraduate student, high school student, or amateur astronomer, to quickly start processing, modeling, and plotting astronomical image data with one tightly integrated software package.

1. INTRODUCTION

AstroImageJ extends ImageJ into an integrated, astronomy-specific environment for image calibration, time-series differential photometry, light-curve analysis, and plotting. Its GUI-driven, cross-platform design targets ultra-precise research while supporting broad usability.

  • Motivation: AIJ addresses the need for an integrated, sophisticated yet easy-to-use environment for ultra-precise photometry and light-curve extraction.The motivation includes exoplanet transit and microlensing research, as well as citizen-science and professional-amateur collaborations.
  • Software platform: AIJ extends ImageJ with astronomy-specific tools for image display, data reduction, analysis, modeling, and plotting.The package is based on ImageJ and includes customized code and astronomy plugins.
  • Astronomical utilities: AIJ integrates FITS handling, FITS-header editing, WCS display and alignment, Astrometry.net plate solving, coordinate utilities, annotations, and object identification.It also supports common image formats and astronomical coordinate information.
  • Image calibration: AIJ calibrates images with bias, dark, flat, and non-linearity corrections and can process data in real time during observations.Its Data Processor facility is designed for image calibration and user-defined processing sessions.
  • Photometry and light curves: AIJ supports interactive multi-aperture differential photometry, detrending, fitting, plotting, uncertainty propagation, and comparison-star ensemble changes.The workflow couples photometric extraction with detrend-parameter selection and interactive model updates.
  • Use and validation: AIJ has been used by the KELT follow-up team and its accuracy was verified against IRAF, IDL, and MaxIm DL photometric extraction packages.The user base includes amateur astronomers, students, and professional astronomers.
  • Use and validation: AIJ demonstrated transit-model residual RMS values of 183 and 255 parts per million for combined and five-minute-binned ground-based light curves of WASP-12b and Qatar-1b.The same demonstration reported transit-timing residuals below approximately 30 s.

2. AIJ OVERVIEW AND BASIC CAPABILITIES

AIJ organizes astronomy-specific image display, calibration, photometry, plotting, and data-processing functions through an integrated toolbar and interactive interface. Its Data Processor can automate image-sequence calibration and optionally perform differential photometry and light-curve plotting during reduction.

  • Astronomical image display: AIJ’s astronomy image display combines menus, quick-access tools, WCS information, overlays, image scaling, zooming, and interactive pixel and aperture measurements.The display supports FITS and other common image formats, while WCS information can provide plate scale and sky orientation.
  • Astronomical image display: Interactive histogram and scale controls let users adjust image brightness and contrast automatically, manually, with fixed settings, or across the full dynamic range.Automatic scaling maps a default mean−0.5σ through mean+2σ range to 256 grayscale levels.
  • Toolbar and workflow: The AIJ Toolbar provides direct access to astronomy mode, aperture photometry, multi-aperture photometry, plotting, saved measurements tables, data processing, and coordinate conversion.The toolbar also includes an icon for clearing image labels and apertures.
  • Time-series handling: AIJ supports image-stack navigation through scrolling, keyboard arrows, and animated playback for time-series sequences.The sequence playback rate is configurable through the play control.
  • Automated reduction: The Data Processor automates master calibration-image construction and image-sequence calibration, with optional differential photometry and light-curve plotting.Its post-processing options can run Multi-Aperture and Multi-Plot after each calibrated image, supporting real-time reduction at the telescope.

4. ULTRA-PRECISE PHOTOMETRY AND LIGHT

AIJ provides interactive single- and multi-aperture photometry for time-series observations, with configurable background estimation, uncertainty propagation, and real-time operation. Its integrated plotting and fitting tools support differential light-curve analysis, detrending, model assessment, and derived transit quantities.

  • Multi-Aperture Differential Photometry: AIJ’s differential photometry processes time-series images and measures target flux relative to one or more comparison stars.Multi-Aperture places and adjusts target and comparison apertures interactively in the image display.
  • Single-Aperture Photometry: Single-aperture photometry calculates net integrated counts by subtracting estimated background flux from pixel values within circular apertures.The background is estimated from a centered annulus, with optional centroid and sky-background algorithms.
  • Differential Measurements: Target differential flux is calculated by dividing target net counts by the summed net counts of all comparison stars, while comparison-star differential fluxes are also computed.AIJ records these relative or differential flux measurements in the measurements table and requires detector properties for proper error calculation.
  • Background Estimation: AIJ supports configurable sky-background removal, including iterative 2σ cleaning and optional plane fitting across the background region.Without plane fitting, the mean of the remaining background-annulus pixels is subtracted from each aperture pixel.

5. ASTROIMAGEJ UPDATER

AstroImageJ includes an integrated updater that lets users select a release, view release notes, install it, and activate it after restart.

  • Users select the desired release in the Upgrade To field and can view its latest release notes before installation.
  • Clicking OK installs the selected update, after which AIJ automatically closes.
  • Restarting AIJ activates the newly installed version.

A.1. Coordinate Converter

The Coordinate Converter transforms astronomical coordinates and times using observatory and target information, with user-controlled or automated operation in differential photometry and multi-plot workflows.

  • Core capabilities: The Coordinate Converter converts astronomical coordinates and times into alternative formats using observatory location and target coordinates.It integrates AstroImageJ with SIMBAD and sky-map.org services and can operate under user, DP, or MP control.
  • Core capabilities: User-entered coordinate or time values can become the active source for automatically calculating other formats and related astronomical quantities.The panel also calculates solar-system object proximities, altitudes, moon phase, and nautical twilight times.
  • Core capabilities: BJDTDB conversion requires an up-to-date leap-second table because dynamical time accounts for changes in Earth’s rotational speed.The table should be updated through the Coordinate Converter’s Update button after announced leap-second changes.
  • Automation: DP can extract observation time, target, and observatory information from FITS headers or accept manual input to add calculated astronomical values to calibrated-image headers.When required FITS metadata is present, DPCC calculations can run without user input.
  • Automation: MPCC calculates astronomical values such as TDB and adds them as new measurements-table columns for plotting, detrending, and saving.The workflow allows time-format and column-name selection, with target and observatory information entered according to the selected source mode.

A.2. FITS Header Editor

The FITS Header Editor displays and edits image-header keywords and values while validating edited value formats against FITS data-type specifications.

  • Editor capabilities: The FITS Header Editor displays information from an open image’s FITS header and optionally permits editing.FITS headers consist of keywords with associated value and comment fields.
  • Caution: FITS header keywords and values should not be edited without understanding how changes may affect downstream interpretation of the image data.This warning defines the practical boundary for using the editor safely.
  • Editor capabilities: AIJ validates edited header values to ensure they conform to allowed FITS formats, including strings, integers, real numbers, and booleans.Keyword values are locked by default and require disabling the lock before editing.
  • File operations: The editor supports deleting or inserting rows, exporting headers to text, saving modified images, and canceling changes.Save options include retaining the filename or writing the image under a new filename.

A.3. Astrometry/Plate Solving

AstroImageJ plate-solves images through the astrometry.net web portal, adds returned WCS information to FITS headers, and supports single-image or stack processing.

  • Plate-solving workflow: The astrometry feature sends coordinates from selected bright sources to astrometry.net, which returns an astrometric solution and enables automatic WCS-header updates.Users can optionally resave the FITS image with the new headers.
  • Plate-solving workflow: A free nova.astrometry.net user key is required, and DP can invoke plate solving for each image during calibration.The Astrometry Settings panel controls the process and can be opened from the toolbar.
  • Plate-solving workflow: Images can be blindly solved without prior sky coordinates or plate scale, although entering known scale or approximate center coordinates may improve solve time.The search radius must cover the image’s field of view.
  • Performance and outputs: 10–20 seconds per image is the default duration for the complete plate-solving process.A stack can be processed by selecting the Process Stack option, with progress shown on the AIJ Toolbar.
  • Performance and outputs: After a successful solve, returned source names can be displayed on the image or saved into the FITS header.These outputs are controlled by the Annotate and Add to Header options.

A.4. Image Alignment

AstroImageJ aligns image stacks through translation, using WCS headers or aperture-identified stars, and also offers stabilization for moving or non-stellar targets. Its alignment implementation does not currently include rotation or scaling.

  • Alignment methods: Stack Aligner provides image-translation alignment for image stacks.At the time described, rotation and scaling are not implemented.
  • Astronomical data: The Add astronomical data to table panel can calculate and add TDB data after target and observatory coordinates are entered through MPCC.The described manual mode uses the RA/Dec Source Manual setting and the Update Table button.
  • Alignment methods: Plate-solved stacks can be aligned from WCS headers, with every image registered to the first image.Users enable the WCS-only option before starting alignment.
  • Alignment methods: Unsolved images can use aperture alignment based on average centroid offsets between consecutive images.The method works best with approximately 3–5 isolated alignment stars and fails when shifts exceed the aperture radius.
  • Stabilization: Image Stabilizer removes atmospheric jitter from rapid planetary or lucky-image sequences and tracks comets across a star field.This functionality is available from the Process menu above an image display.

A.5. Radial Profile

AstroImageJ generates azimuthally averaged radial profiles for image objects and reports characteristic widths and suggested aperture radii. The profile can be centered on the object when centroiding is enabled.

  • Profile generation: An azimuthally averaged radial profile is generated by selecting an object and using Analyze→Plot seeing profile or Alt-left-clicking.The profile is centered on the object when centroiding is enabled.
  • Profile outputs: The radial profile plot reports half-width at half-maximum, FWHM, and suggested aperture radii.

A.6. Photometry Settings

AstroImageJ’s photometry settings control aperture geometry, centroiding, background handling, measurement-table contents, and detector parameters used for error calculations. The FITS Header Editor separately supports direct header-row editing and saving.

  • Access and controls: Photometry settings are accessible through single-aperture, multi-aperture, image, and differential-photometry controls.The More Aperture Photometry Settings panel is opened from the main aperture settings panel.
  • Measurement settings: The Aperture Photometry Settings panel controls aperture radii, centroiding, background settings, and FITS keywords added to the measurements table.CCD gain, readout noise, and dark current should be entered for photometric error calculations.
  • Centroiding: Howell centroiding provides highly repeatable x,y results, while center-of-mass centroiding performs better for apertures around defocused stars.
  • Measurement-table contents: AIJ recommends enabling all photometric data items because some functionality requires specific measurements-table fields.Users can also change the maximum number of apertures and control aperture-overlay display behavior.
  • FITS header editing: The FITS Header Editor allows most fields to be edited directly and supports deleting, inserting, and saving header rows.Headers can be saved to text, memory, or disk, while some fields cannot be directly edited.

A.7. Data Processor FITS Header Updates

Data Processor FITS Header Updates calculate new astronomical quantities from existing FITS-header inputs and write them to calibrated-image headers. Coordinate inputs can be entered manually or supplied automatically from image headers.

  • Header updates: Enabling the General FITS Header Updates option lets Data Processor calculate new astronomical data for calibrated images.
  • Header updates: Input settings identify existing raw-image FITS keywords, while output settings specify calculated values and their keyword names.The resulting data are added to calibrated-image headers.
  • Coordinate inputs: The calculations require target sky coordinates and observatory geographical coordinates supplied through DP Coordinate Converter.Both coordinate sets may be entered manually or obtained automatically from FITS headers.

A.8. Save All

AIJ’s Save All feature consolidates typical image, plot, configuration, table, subset, aperture, and fit-panel outputs into one configurable save operation, while related panels support astrometric alignment and aperture setup.

  • A.8. Save All: Astrometry.net plate solving adds WCS headers to FITS images and can optionally resave the solved files.The Astrometry Settings panel provides the internet-connected plate-solving interface.
  • A.8. Save All: Save All writes selected AIJ data and image products using a shared base path with configurable suffixes.Individual products are enabled through checkboxes, and images may include the current non-destructive overlay.
  • A.8. Save All: The process can save plot configurations, full measurements tables, tab-delimited data subsets, aperture settings, and fit-panel images.Data subsets allow selected measurement columns and formatting options; other products use dedicated suffixes and file types.
  • A.8. Save All: Images can be aligned from WCS information or, without plate solving, from centroid offsets measured around selected alignment stars.The Stack Aligner uses consecutive-image centroid offsets when WCS headers are unavailable.
  • A.8. Save All: The radial profile tool reports HWHM, FWHM, and suggested aperture and sky-annulus radii in pixels.Its suggested settings are aperture radius 1.7×FWHM, inner annulus radius 1.9×FWHM, and outer annulus radius 2.55×FWHM.

B. PHOTOMETRIC ERROR CALCULATION

AIJ estimates aperture-photometry uncertainties from CCD noise terms and propagates them through differential flux calculations. The estimate excludes several additional observational and instrumental noise sources.

  • B. PHOTOMETRIC ERROR CALCULATION: AIJ’s CCD noise calculation includes source and sky Poisson noise, dark current, readout noise, and quantization noise.The equation uses gain, source counts, aperture and background pixel counts, sky counts, dark counts, read noise, and digitization variance.
  • B. PHOTOMETRIC ERROR CALCULATION: The total noise approaches the Poisson limit of √F∗ when source counts dominate and gain G = 1.Increasing aperture size adds secondary noise terms, while non-ideal detector inter-pixel variation can decrease with larger apertures.
  • B. PHOTOMETRIC ERROR CALCULATION: AIJ combines comparison-star aperture noises in quadrature and propagates them through the relative flux quotient to obtain differential-flux errors.The ensemble noise uses each comparison star’s equation-B1 noise and the number of comparison stars.
  • B. PHOTOMETRIC ERROR CALCULATION: The resulting relative-flux error uses target-aperture counts and noise together with summed comparison-star counts and ensemble noise.AIJ labels the resulting relative-flux error columns as rel... values in the measurements table.
  • B. PHOTOMETRIC ERROR CALCULATION: AIJ’s uncertainty estimate excludes atmospheric scintillation, neighboring-star flux leakage, calibration-image noise, cosmic rays, shutter variations, and sky-background errors.Scintillation is specifically omitted even though it can dominate for short exposures or small-aperture telescopes.

C. APPARENT MAGNITUDE CALCULATION

AIJ calculates apparent magnitudes for target and comparison apertures from user-entered comparison magnitudes and measured net counts, updating target values as inputs change. The reported magnitude uncertainties exclude uncertainty in those user-entered reference magnitudes.

  • C. APPARENT MAGNITUDE CALCULATION: AIJ calculates target apparent magnitudes from the net counts of target and comparison apertures with entered comparison-star magnitudes.The calculation can run during multi-aperture processing or after photometry through the reference-star settings panel.
  • C. APPARENT MAGNITUDE CALCULATION: Adding or changing a comparison-star magnitude recalculates target magnitudes from all entered comparison magnitudes and net integrated counts.Updates can occur during setup or after differential photometry, with Enter required in the reference-star settings panel.
  • C. APPARENT MAGNITUDE CALCULATION: Comparison magnitudes remain constant across measurements-table rows because comparison sources are assumed to have constant brightness.Their values are user-entered and appear in dedicated comparison-source columns.
  • C. APPARENT MAGNITUDE CALCULATION: Apparent-magnitude uncertainties do not include uncertainty in the user-entered comparison-source magnitudes.This limitation applies to both target and comparison magnitude uncertainty values.
  • C. APPARENT MAGNITUDE CALCULATION: AIJ reports target and comparison apparent-magnitude uncertainties by converting flux-based photometric errors to the magnitude scale.Target uncertainties use net-count uncertainties from the target and entered comparison apertures.

D. MEASUREMENTS TABLES

AIJ stores single-aperture and multi-aperture photometry in measurements tables organized by image rows and measurement columns. The tables expose configurable photometric, positional, timing, noise, aperture, and magnitude fields for plotting and export.

  • D. MEASUREMENTS TABLES: Multi-aperture tables contain one row per processed image and one column for each measurement tracked across the time series.Column names are selectable in Multi-plot panels and the table can be saved or reopened in several delimited formats.
  • D. MEASUREMENTS TABLES: Users can clear rows or all table data and save either complete measurements tables or selected data-column subsets.Subset files are tab-delimited and can include configurable headings, row numbers, and labels.
  • D. MEASUREMENTS TABLES: Each measurement identifier uses T or C for target or comparison aperture followed by the aperture number.The available measurement items can be selected in the More Aperture Photometry Settings panel.
  • D. MEASUREMENTS TABLES: The table records observation timing, aperture radius and background-annulus radii, image and FITS coordinates, FWHM estimates, saturation status, and photometric quantities.Timing fields depend on FITS-header UTC and exposure-time information; variable-aperture radius uses an image FWHM multiplier.
  • D. MEASUREMENTS TABLES: Photometric fields include target and comparison relative fluxes, propagated errors, signal-to-noise values, net counts, sky background, and apparent magnitudes with uncertainties.Target magnitudes are calculated from comparison apertures with entered apparent magnitudes, while saturation records peaks exceeding the configured warning level.
Loading 1701.04817v1…