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Photometric Software for Transits (PhoS-T)

D. Mislis & R.Heller - Observatory of Hamburg
J. Fernandez & U. Seemann - University of Göttingen

The Photometric Software for Transits (PhoS-T)1 is a software for data reduction and analysis of transiting exoplanets. PhoS-T uses a graphical enviroment and is able to handle fits data for data reduction, photometry and transit light curves correction, including all routines for data reduction, data alignment, differential photometry and raw light curves correction. PhoS-T also includes a Monte Carlo routine that can be applied to fit the light curve of transit candidates. The software comes with functions for data reduction, transit search, and model fitting of CoRoT data. PhoS-T is an open-source tool with a focus on follow-up photometric observations, light curve analysis and CoRoT studies.

More that 10 years passed after the detection of the first transiting exoplanet and many teams these days, restudy known exoplantes for transit timing variation (TTV) 2 and characteristic changes in these systems. Also space missions like CoRoT and Kepler will provide us with thousands of objects for analysis. Transits offer powerful techniques for a complete study of exoplanets. Very few teams are searching for transiting exoplanets but more and more are making follow up observations in known systems. Although there are many tools for data reduction and analysis (e.g. IRAF, DaoPhot, Starlink, Midas etc.), these codes and sub-routines need to be combined very often to yield a full analysis, starting with data reduction and resulting in light curve modeling. The problem is that some times we need to combine softwares, codes and sub-routines to complete a full analysis from data reduction to light curve modeling. PhoS-T combines all techniques necessary for photometric follow-up observations of transiting exoplanets. PhOS is an abbreviation for Photometric Software for Transits, where 'phos' stems from the Greek word for 'light': ΦΩΣ.

Since December 2007, CoRoT data are public. However, these data had some problems. PhoS-T includes sub-routines for CoRoT light curve reduction, detrending and analysis using CoRoT Detrend Algorithm (CDA)3. In principle we could say that PhoS-T has three kind of modes. The first is data reduction and photometry. The software can handle fits input data in order to do the photometry and to extract the transit light curve. The second mode is the analysis in transiting light curves (model fitting, Monte Carlo simulations). The third mode is linked with CoRoT data. PhoS-T reduces CoRoT data, and also applies standard functions for transit detection and modeling. One of the advantages of the software is the graphical environment.

The graphical environment of PhoS-T is set up with the programming languageGTK. For the majority of the sub-routines we have used Python but also AWK, Fortran and Bash-scripting in some parts of the code. PhoS-T' main screen is separated in two windows. The first one is the main window in which the user can select the functions and the routines (data reduction, align photometry etc). Moreover, information for data and the total procedure are given. The second window is the display window. The user there can check the output frames after the reduction, and all the output plots from align plot to the final light curve and transit model fit. Between functions, very often, there are dialog windows, which makes the procedure easier. For a package of 600 frames in total (1.2 Gb) - consisting of 30 bias frames, 70 flat frames, and 500 scientific frames - PhoS-T needs about 1 hour for the full procedure on a 4 GHz processor, from master frames creation until error calculation of the transit model.

PhoS-T is running under Linux Operating System and the first version is written for Ubuntu. A MAC version is planned. The software is free for the astronomical community. Here you can find the PhoS-T manual.

Technical Requirements

PhoS-T requires GTK- dialog version 0.7.20 or higher, Python laguage version 2.6.2 or higher (including PyFits sub-routine) and the Intel Fortran Compiler (ifort). Also it needs AWK version higher than 3.1.6 and Gnuplot version 4.2. Firefox browser is optional.


PhoS-T software is available upon request. Please send us a message (below).

  1. Mislis et al. A&A, 2010 (Photometric Software for Transits)
  2. Kipping, D., Ribas, I., Font-Ribera, A., IAUS, 253, 490 2009
  3. Mislis et al. A&A, 2010 (CoRoT Detrend Algorithm)