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VaST homepage

VaST [Variability Search Toolkit]

A software for variable star detection
on a series of astronomical images

Contents
What is VaST?
Screenshots
Download
Usage
Output files
VaST mailing list
News/Release notes
Frequently Asked Questions
Publications using VaST
Other useful tools

VaST is a software tool for finding variable objects on a series of astronomical images. The images (CCD frames or digitized photographic plates) must be taken with the same instrument using the same filter and saved in the FITS format. CCD images should be calibrated (dark-subtracted and flat-fielded). The input images may be shifted and rotated with respect to each other, but they have to have the same scale (arcsec/pix). The images in a series should overlap by at least ~40% to ensure successful cross-identification. VaST performs object detection and aperture photometry using SExtractor on each image, cross-matches lists of detected stars, performs magnitude calibration with respect to the first (reference) image and constructs a lightcurve for each object. The sigma-magnitude, Stetson's L variability index, Robust Median Statistic (RoMS) and other plots may be used to visually identify variable star candidates. If data permit, period-search techniques may be employed to identify periodic variable stars among the candidates. All lightcurves are constructed in arbitrary magnitudes with zero magnitude corresponding to the background level on the reference image. If there are reference stars with known magnitudes in the field of view, all magnitudes can be later converted to the standard system.

Unlike software based on the image subtraction method, VaST can be used in case of unstable PSF (e.g., bad guiding or with digitized wide-field photographic images). VaST is not tied up to any external catalog and WCS (if operated in its main "variable star search" mode), so it can be used on images taken with a telephoto lens as well as with a 2.6-m class telescope. If needed, absolute astrometric calibration may be performed through an interface to the Astrometry.net code allowing one to perform automatic magnitude scale calibration and variable star identification (however, this will work only if the input images are large enough to perform blind plate solution).

VaST is written in C (and partly in BASH scripting language) for GNU/Linux operating system. The latest versions are also tested on MacOS X and FreeBSD. The best practical way to run VaST under Windows is through Linux installed in a virtual machine (e.g. VirtualBox, see also a collection of pre-built VirtualBox images). The AAVSO forum thread mentions attempts to run VaST under Windows with Cygwin, but the results seem to be inconclusive. On all the supported systems VaST is able to utilize multiple processing cores to perform most of its computations in parallel.

VaST is free software: you can redistribute and/or modify it under the terms of the GNU General Public License. The program is in a continual, albeit slow, state of development. You are welcome to contribute bug reports, patches and feature requests: please write me at kirx[at]scan.sai.msu.ru

Screenshots

Various VaST subroutines (lightcurve plotter, image viewer) may be started automatically from the main program or manually by a user from a terminal. Names of the corresponding executable files are given under each screenshot. Click on images to see them in high resolution:

VaST running in a terminal animation
VaST (the main program) running in a terminal
./vast
RMS-mag plot
The variability search window
./find_candidates

Lightcurve plot
The lightcurve inspection program
./lc
FITS image viewer
VaST FITS image viewer
./pgfv

Video (screencast)
voice comments are in Russian only, sorry... I hope to prepare an English version for quite a few years already...
Download, compile and test VaST
How to identify variable stars with VaST

How to remove trend from a lightcurve

Getting VaST

To compile and use VaST you'll need:
  • GNU/Linux or MacOS X or FreeBSD and some basic skills in using one of these operating systems through a command line interface (terminal)
  • GCC (including Fortran and C++ compilers), comes with your GNU/Linux distribution
  • BC calculator (can be found in your GNU/Linux distribution)
  • cURL (can be found in your GNU/Linux distribution)
  • SExtractor. If SExtractor is not installed system-wide, VaST will use built-in SExtractor v2.5.

You can find at this page the latest version of VaST (download via ftp or http, ~9.5M) along with the sample data set (ftp or http, 24M).

Files described here as well as older versions of VaST are available via our anonymous FTP.

You may download VaST with the command

wget -c ftp://scan.sai.msu.ru/pub/software/vast/vast-latest.tar.bz2
unpack the archive containing the program
tar -xvjf vast-latest.tar.bz2
cd vast-1.0rc75
and compile it by running
make
(or 'gmake' if you are not on Linux). At this point VaST should be ready for work.

If compilation fails, read the output carefully. Most probably it can't find some necessary libraries, header files or external programs. Please install the required programs and try to compile VaST again. If you are sure that the needed libraries and headers are installed in the system but the compiler cannot find them, try to open Makefile with a text editor and change the libraries locations to match your environment. Very old versions of VaST require PGPLOT, CFITSIO and GSL libraries to be installed in your system, but most likely you don't want to use such an old VaST version.

Special note for Ubuntu users! The following installation procedure is recommended:
  • Install these libraries from pre-compiled Ubuntu packages:
    sudo apt-get install build-essential gfortran g++ libX11-dev libXi-dev libXmu-dev libpng-dev curl wget
    If you want to install a newer SExtractor and PSFEx in order to perform PSF-fitting photometry with VaST, you'll need to install also
    sudo apt-get install libfftw3-dev libatlas-base-dev
    When compiling SExtractor and PSFEx from the source code, specify explicitly the location of ATLAS include files:
    ./configure --with-atlas-incdir=/usr/include/atlas && make && sudo make install
  • Compile VaST as described above
For Red Hat based Linux distributions (Fedora, CentOS, Scientific Linux):
  • Install some basic development tools (login as root or use sudo):
    yum install gcc gcc-c++ gcc-gfortran libX11-devel libpng-devel
  • Compile VaST as described above
Special note for MacOS X users: VaST is tested on MacOS X 10.8.2 'Mountain Lion' with XQuartz, Xcode, MacPorts installed in the system and gcc, gfortran, g++, and wget installed through MacPorts. Please use gmake instead of make to compile VaST.

Special note for FreeBSD users: VaST scripts may fail to compile the internal copy of PGPLOT library. If that happens, please install PGPLOT system-wide with the command (as root):
pkg install pgplot
then try to compile VaST again. Please use gmake instead of make to compile VaST.

Using VaST

To try VaST, download the sample data set, unpack it
wget -c ftp://scan.sai.msu.ru/pub/vast/sample_data.tar.bz2
tar -xvjf sample_data.tar.bz2
change to the VaST installation directory and run the program
cd vast-1.0rc75
./vast ../sample_data/*fit
After a brief computation the variability search window will be opened. Click on any star on the sigma-magnitude plot to inspect its lightcurve. Click on any point on the lightcurve plot to see an image from which this point comes. The star on the image will be marked with a red cross, a red circle around the star corresponds to the aperture used to measure this image. You may switch from sigma to other variability indexes by pressing 'M' or 'N' on the keyboard in the sigma-magnitude plot window.

If you quit the program without deleting any data, you can restart it without doing all the computations again by running:
./find_candidates aa

To calibrate the magnitude scale using comparison stars with known magnitudes within the field of view use this script:

util/magnitude_calibration.sh
It will ask you to specify one or more comparison stars and their magnitudes. For each comparison star, click on this star on the displayed image then enter its catalog magnitude in the terminal window.

After the comparison stars have been specified, close the image window with the right mouse click and inspect the relation between instrumental and catalog magnitudes. This relation may be fitted by a linear function with the slope fixed to 1, linear function with a free slope, second degree polynome (parabola) or the Bacher et al. (2005, MNRAS, 362, 542) magnitude calibration relation a.k.a "photocurve". This option is suitable for processing digitized photographic plates. Use (P) key on your keyboard to change the fitting function. For most CCD data sets, linear function with the fixed slope provides a good fit. The data points may be weighted according to their estimated errors or weighted equally. Use (W) key to change weighting. Close the dialog with the right mouse click to apply the calibration to all stars when you are satisfied with the fit.

If the image field of view is large enough to be blindly solved with Astrometry.net code (as described below), one can automatically match stars detected on the image to the UCAC4/APASS catalog by running the magnitude calibration script with a command line argument specifying the observing band:

util/magnitude_calibration.sh V
The acceptable bands are: "C" - UCAC4 unfiltered red band, "B" - APASS B band, "V" - APASS V band, "R" - APASS R band, "r" - APASS r band, "i" - APASS i band. Magnitude calibration against APASS can be performed only if the field of view overlaps with the APASS survey footprint. Note that the automated magnitude calibration will not work with the sample data set due to its small field of view.

In order to save the current work, use the script

util/save.sh my_favorite_field_name
If the field name was not provided, the script will ask for it. To restore previous work, use
util/load.sh my_favorite_field_name
or just copy all saved files from directory [my_favorite_field_name] back to the VaST directory.

To delete all files from the previous work, you can use:
util/clean_data

There is a tool to automatically identify variable stars detected with VaST. It requires a working Internet connection at runtime. The tool may or may not work for your field depending on luck. If the field of view is greater than, say, 30' and the image quality is fine - the odds that the automatic plate solution will work for your field are pretty good. Here is how to use it... To identify the star number 00190 in the current dataset use the following command:

util/identify.sh out00190.dat 25
where "out00190.dat" is the lightcurve file of the star and "25" is the estimated field of view size in arcminutes. It is wise to set this value a little bit smaller than your best-guess field of view. If no field of value is given, the default value of 40' will be assumed. Note, that the field of view identification may take really long time (up to ~30 minutes), especially for small fields. Please, do not interrupt the process - just wait for a result. If the field identification failed, try to restart it with smaller estimated field of view size. On successful identification, the script will print the equatorial coordinates (J2000) of the star and will attempt to query popular databases to check if this star is already known as a variable. Note, that the coordinates returned by the identification script may be up to 10" off. We strongly advise to use the Aladin interactive sky atlas to verify the identification and obtain more precise star coordinates from some astrometric catalog (e.g., USNO-B1.0). The automatic identification function relies on tools provided by Astrometry.net. For more details visit http://astrometry.net/. Note, that according to the use conditions of the index files required to run the Astrometry.net code, you need to be ready to share the images solved using this software with any other interested party including the Astrometry.net team. According to the same use conditions: "If the indexes are used in any scholarly work, research grant proposals or other scientific or academic publications, the Astrometry.net project must be properly cited and acknowledged. The paper to cite is the Lang et al paper on our bibliography page http://astrometry.net/biblio.html. The acknowledgment text is "This research made use of tools provided by Astrometry.net." The VaST software developers assume, that this statement concerns the work which uses "util/identify.sh" script from the VaST package.

A note on the graphical user interface

The VaST GUI is based on the PGPLOT library. The main reasons are that this library is perfectly suited for displaying and editing data and image plots and is so easy to use from programmer's standpoint. The problem is that the resulting interface may look counterintuitive. The unusual thing about the VaST interface is that it has no buttons. (There is a way to program a GUI with buttons and graphical input fields in PGPLOT, but, trust me, you'll not be impressed by its look and usability.) So, whenever a user has a choice between multiple possible actions, instead of clicking a button to execute the desired action, a user will have to physically press a key on a keyboard. The list of possible keys is always displayed by VaST at the terminal. Please have a look there to know your options. Some actions are common to all VaST windows:
  • To select a data point on a plot (or a given pixel of an image) - click on it with the left mouse button.
  • To close a window - click the right mouse button anywhere in the window.
  • To zoom in - use exactly the following sequence of actions: 1) put the cursor in one corner of the area you want to zoom on, 2) press 'Z' on keyboard and release the key, 3) specify the size of the area you want to zoom on by moving the mouse, 4) click the left mouse button to confirm your selection.
  • To zoom out - press 'Z' key on a keyboard twice.
Before pressing a keyboard key, make sure you have focus set on the VaST graphical window you want to interact with. Naturally, if focus is set on another window (like a terminal), the character you type will affect that other window (like appear in the terminal) instead of performing an action in the VaST window you think you are aiming at.

Using VaST with scanned photographic plates

If you have to deal with digitized photographic materials, you'll want first to convert TIFF images (which are typically produced by scanner software) to the FITS format. It can be easily done with the tiff2fits converter. Do not forget to change your images to positive (white stars on black sky). For example:

./tiff2fits -i input.tiff output.fits
Also, you'll have to enter the information about the date and time of the observation in fits header. This can be done, among other ways, using the modhead program from the CFITSIO examples page, MissFITS or edhead from the WCSTools package. You may enter the observation date and time information in the image header by generating the usual "DATE-OBS" and "EXPTIME" header keys, or by simply putting the middle-of-exposure Julian Date into the "JD" key. You will most likely need to modify the default SExtractor settings stored in the "default.sex" file located in the VaST directory. Please use "default.sex.beta_Cas_photoplates" and "default.sex.PHOTO" as working examples (these files are also found in the VaST directory).

Say, you have digitized photographic images in "photo_data" directory. To run VaST on them, change to the VaST directory and run the program

cd vast-1.0rc75
./vast -o ../photo_data/*fit
The "-o" parameter tells VaST to use formulas (1) and (3) from Bacher et al. (2005, MNRAS, 362, 542) a.k.a. "photocurve" for magnitude calibration. This technique provide slightly better results for the aperture photometry on photographic data compared to the second-order polynome used in VaST by default for magnitude calibration. However, the standard calibration technique should also provide reasonably good results (see Figure 2 in this paper). The "-j" parameter that forces VaST to use a linear position-dependent magnitude correction (i.e. stars on one side of the image bing systematically fainter than on the other side) might also be useful for photographic plates photometry.

An example photographic dataset is available here. Beware, it's about 1G in size!

You may find some more details in the ArXiv:1403.5334 e-print "A search for new variable stars using digitized Moscow collection plates".



Search for transients with VaST

Apart from the standard "variable star search mode", where a long series of images is processed and lightcurves of detected stars are explored using the sigma-magnitude plot, VaST can be also used in the specialized "transient detection mode". In this mode VaST will process a series of four images: two first-epoch (reference) images and two second-epoch images. VaST will generate an HTML report containing a list of candidate transients. The transients are defined as either objects that were not visible on the reference images, or where at least 1 mag. fainter compared to the second-epoch images. Two first-epoch images are needed because image artifacts and star blending may cause VaST to miss a star on a single reference image, so it will appear as a false transient search mode is available. If there are two reference images - chances that the program will fail to detect a star at both of them is greatly reduced. Also note that two different reference images are needed: it would be impossible to "cheat" the program by feeding the same image to it twice.

An example dataset that allows one to test the transient search mode is available here.

The unpacked archive will contain two sub-directories with reference and second-epoch images. The search should be conducted in two steps:

# Process images with VaST
./vast -x99 -ukf ../transient_detection_test_Ceres/reference_images/* ../transient_detection_test_Ceres/second_epoch_images/*
# Run script that will generate an HTML report
util/transients/search_for_transients_single_field.sh
Be prepared - the script will ask you to download a few catalogs needed for its work, including Tycho-2, the catalogs are more about 200M in size. The transient search report will be placed in transient_report/index.html that should be opened with your favorite web browser. The report will contain a few false candidates and some good ones including asteroids and variable stars.

Known limitations:

  • The transient detection capability is still in the testing stage and is FAR FROM PERFECT, sorry...
  • You'll need a reasonably high-speed Internet connection with unlimited traffic to efficiently use the transient-detection capability. An effort is made to reduce the Internet traffic to the necessary minimum. However, at the first start, the program will need to download ~200M of catalog data. During the routine work, the program will need to transmit a few 100K of data for every processed image (necessary for astrometric image calibration using the external server). The Internet connection is also needed to access external web-databases that would allow you to check the detected transient candidates.
  • There are two transient-detection scripts for your to consider:
    util/transients/search_for_transients_single_field.sh is the simplified one that searches for transients in a single field.
    util/transients/transient_fatory_test30.sh is a more complex one, that is actually used to process NMW survey data. These scripts cannot cover all possible situations and should serve an example and inspiration for you to write your own script appropriate for your setup.
  • Image field of view should be sufficiently large to allow automatic plate solution with Astrometry.net software. The default value is 180 arcmin, if your field of view is smaller - you should edit the transient detection script util/transients/search_for_transients_single_field.sh .
  • The magnitude calibration is based on V magnitudes of Tycho-2 stars. If this is not appropriate for your system, please contact me and I'll try to develop an alternative calibration strategy for your setup.
  • Reference images should be two different images obtained during one night. The reason behind using two reference images (instead of one) is the attempt to minimize the number of cases where SExtractor fails to deblend two stars on the reference image, but can successfully perform deblending on the second-epoch images. In this case, one of the two stars that "appeared" in the detection lists for the second epoch will be erroneously marked as a transient. The other measure that can minimize such errors is to try to use images with the best seeing as reference.
  • There are two files controlling the star-detection behavior of SExtractor: default.sex and default.conv . You may refer to SExtractor documentation for a detailed description of these files. You may find a few examples of default.sex and default.conv files in the VaST directory. The default files are meant to work for a wide variety of CCD images, however they may not provide an optimal star detection efficiency for a particular instrument. The two files fine-tuned for the "transient_detection_test_Ceres" dataset are default.sex.telephoto_lens and default.conv.telephoto_lens . To use them instead of default files, just copy these files:
    cp default.sex.telephoto_lens default.sex
    cp default.conv.telephoto_lens default.conv
    You may want to edit these file to match your own setup.


Some switches that you may use with VaST

"-P" — perform PSF-fitting photometry with SExtractor and PSFEx.
"-9" — use DS9 instead of VaST's own viewer pgfv to view FITS files.
"-u" — always assume UTC time system, don't perform conversion to TT.
"-k" — force VaST to ignore the "JD" keyword in FITS headers.
"-x2" — accept stars with non-zero SExtractor flag: 2 - blended stars, 4 - saturated stars, 99 - accept everything.
"-e" — failsafe mode: only stars detected on the reference frame will be processed.
"-p" — tell VaST that it deals with a linear detector. This flag is useful for well calibrated CCD images, but for some CCD images it is better not to use it. Test what works best for your particular dataset!
"-o" — tell VaST that it deals with a digitized photographic plate. The magnitude calibration between frames will be done using the relation proposed by Bacher et al. (2005, MNRAS, 362, 542).
"-l" — perform sigma-filtering of lightcurves.
"-r" — assume that images are not rotated with respect to each other by more than 3 degrees.
To get the full list of the available command l