What is Aperture Photometry Tool?

Aperture Photometry Tool (APT) is a computer program designed to perform scientific calculations related to aperture photometry[1], a common data-reduction method used in astronomy.  The impetus for the advent of this software was efforts by some scientists at the Infrared Processing and Analysis Center (IPAC) on behalf of the National Aeronautics and Space Administration (NASA) to promote math and science in education, specifically bringing real research into the classroom through a program called NITARP[2], the NASA/IPAC Teacher Archive Research Program and its predecessors [3].   The main feature of the software is that it allows aperture-photometry results to be analyzed via a graphical user interface (GUI), and this provides a convenient platform for the software’s users, both students and astronomy professionals alike, to learn about aperture photometry and gain valuable insights into results particular to their research activities.    A plethora of graphs, displays, statistics, options and results related to aperture photometry in the GUI are available for closely scrutinizing both input and output data in a variety of ways, and this promotes better understanding.  The software is implemented entirely in the Java programming language and, therefore, can be installed and used on any computer system that runs a Java Virtual Machine. APT can be downloaded and used free of charge for astronomical-computing purposes [4]. The basic input to APT is a FITS image, and FITS images from any astronomy project can be processed with APT.

Can Aperture Photometry Tool be used for Professional Work?

Yes! There is nothing “watered down” about APT. It was developed primarily for the classroom, but under the strict standards expected of software used by professional astronomers and NASA personnel. APT is routinely used to evaluate Spitzer Space Telescope data products, as well as other IPAC data products. A study was conducted to see how well APT performs, and APT passed all tests with flying colors![5] Not convinced? You can read the test report for yourself (PDF). Also, below is a short list of real astronomical research projects that benefited from using APT.

Research Projects that utilized APT

  • Forced-photometry computations for the Physics of the Accelerating Universe survey. APT is utilized to run single-thread, batch-mode jobs in the PAU computer farm.
  • Fitzgerald, M. T. et al., “Photometric and proper motion study of neglected open cluster NGC 2215″, The Astronomical Journal 149, p. 190, 2015 (PDF).
  • Rebull, L. M.et al., “New Young Star Candidates in CG4 and Sa101″, The Astronomical Journal 142, p. 25, 2011 (PDF).
  • Souza, S. P. et al., “H-alpha monitoring of early-type emission-line stars”, May 2011 AAS poster, Boston, MA (PDF).
  • Hayes-Gehrke, M. et al., “Lightcurves for 938 Chlosinde and 3408 Shalamov”, Minor Planet Bulletin 38, p. 75, 2011 (PDF).
  • Souza, S. P., G. Beltz-Mohrmann, and M. Sami, “The Light Curve and Period of MT696″, Journal of the American Association of Variable Star Observers (JAAVSO) 42, pp. 154-160, 2014. (PDF).
  • Souza, S. P., “Two New Cool Variable Stars in the Field of NGC 659″, Journal of the American Association of Variable Star Observers (JAAVSO) 41, pp. 92-6, 2013. (PDF).

Who developed Aperture Photometry Tool?

Dr. Russ Laher, a member of the technical staff of IPAC’s Spitzer Science Center at the California Institute of Technology, developed APT. Russ has been creating software for science, engineering, and business over the last 35 years (his first computer was a Radio Shack TRS-80 microcomputer with 4K of memory); he has an MBA from Pepperdine University and a Ph.D. in physics from Utah State University. The first version of APT was released in November of 2007, and, since then, Russ has implemented many software upgrades to fix bugs and augment APT’s capabilities. Beta-testers at the SSC, IPAC, and elsewhere have contributed to testing APT and provided valuable feedback.

How accurate is Aperture Photometry Tool?

Outputs from APT have been compared to similar outputs from SExtractor, a well-respected batch-mode aperture-photometry software program, for sources extracted from R-band optical images acquired by the Palomar Transient Factory (PTF), infrared mosaics constructed from Spitzer Space Telescope images, and a processed visible/near-infrared image from the Hubble Legacy Archive (HLA) [5].  Many more details are given in the referenced paper, and are briefly summarized as follows.  Two large samples from two PTF images, each containing around 3,000 sources from non-crowded fields were studied.  The median values of source-intensity relative percentage differences between the two software programs, computed separately for the two PTF samples, are +0.13% and +0.17%, with corresponding statistical dispersions of 1.43% and 1.84%, respectively. For the Spitzer mosaics, a similar large sample of extracted sources for each of channels 1-4 of Spitzer’s Infrared Array Camera (IRAC) were analyzed with two different sky-annulus sizes, and it was found that the median and modal values of source-intensity relative percentage differences between the two software programs are between -0.5% and +2.0%, and the corresponding statistical dispersions range from 1.4% to 6.7%, depending on the Spitzer IRAC channel and sky annulus.  The results for the HLA image are mixed, but as might be expected for a moderately crowded field.  The comparisons for the three different kinds of images show that there is generally excellent agreement between APT and SExtractor. Differences in source-intensity uncertainty estimates for the PTF images amount to less than 3% for the PTF sources, and these are potentially caused by SExtractor’s omission of the sky-background-uncertainty term in the formula for source-intensity uncertainty, and differing methods of sky-background estimation.


  1. Laher et al., Publications of the Astronomy Society of the Pacific, Vol. 124, No. 917 (July 2012), pp. 737-763 (PDF). This report covers up to v. 2.1.5 of APT, and there have been software upgrades since its publication.
  2. NITARP homepage.
  3. Rebull, Luisa M., V. Gorjian, G. Squires, The NITARP Team, 2011, Bulletin of the American Astronomical Society 43, poster #248.11.
  4. APT-download webpage.
  5. Laher et al., Publications of the Astronomy Society of the Pacific, Vol. 124, No. 917 (July 2012), pp. 764-781 (PDF).