'Limovie' software for automated photometry of video occultations
At the high time resolution
made possible by video, most video-recorded occultations appear not to be
instantaneous. This is due to
significant angular size or duplicity of the star, diffraction effects, poor
seeing, and camera (and other types of) noise.
Thus, it may not be possible correctly to identify the instant of
occultation purely by subjective viewing of the tape at full speed, or by
single frame play back, on a VCR. In
order to extract the maximum possible timing accuracy from time-inserted video
recordings of occultations, it is necessary to examine the recordings frame by
frame (25 frames/sec in
The acronym stands for "LIght Measurement tool for Occultation observation using VIdeo rEcorder"! Limovie fulfils a longstanding need of video observers to *automatically* derive light curves from captured video recordings of lunar and asteroidal occultations. Limovie is also ideal for producing photometry of planetary satellite mutual events (occultations and eclipses), and can be used for meteor photometry. (A four-year series of mutual events of the Uranian satellites commenced in July 2006.)
Limovie (for Windows), which is currently up to version 0.9.24, can be downloaded from here.
The web page also contains a general description of Limovie and a downloadable English language 'Word' manual written by David Herald, author of the well-known 'Winoccult' occultation package.
Basically, occultations recorded on video or camcorder tape are captured to a PC as an AVI file using any standard capture method. The ability to capture DV video via a 'firewire' socket has just been added. The video needs to be captured without 'lossy' compression but, as explained on the site, suitable codecs are readily available for this.
Limovie only needs to be unzipped into a suitable directory - there is no further 'installation'. It can then be run using Start|Run... or from a shortcut. On starting Limovie, one is confronted by a rather cluttered single window [see screenshot below]. This may appear daunting at first but when read in conjunction with the documentation, the function of each option becomes reasonably clear. The AVI file is opened and the first frame/field of the video clip is displayed in a window. There is the usual range of video control buttons and the video can be run directly for an initial inspection if required. Clicking on the target star centres two circular photometric apertures on it - a circle for the star, and an annulus for the sky background. The pixel radii of these can be adjusted using spin controls, and a 3-D view is available to ensure best fitting of the apertures. Once set, clicking 'Start' from the Measurement buttons launches an automated run through the entire video clip. The analysis runs almost as fast as the original video clip. The pixel values in the object and sky apertures in each field/frame are counted and displayed in tabular form in the right hand window. Limovie tracks the star to counteract seeing and telescope tracking defects and ensure it remains centred. Once completed, the resulting table can be saved as a .CSV file. This is then imported into any spreadsheet package, where standard procedures can be used to produce a graph. The screenshot shows apertures centred on the target star involved in the Hermione occultation recorded by the author in December 2005. The graph shows the resulting light curve. Attaching accurate timing is only a matter of finding the precise time shown on the first frame or field in the clip window and making a simple equation for finding the time of all the other frames/fields, as shown in the example. There is actually a facility for automatic recognition of the second's display of certain time inserters, but the current version, does not support time inserters used by European observers.
Limovie only carries out simple photometry on the target star: it cannot carry out simultaneous photometry of comparison objects as this was not initially seen as important for occultation work. However, if required, the same video clip can be re-run through Limovie using different comparison stars and the results imported into the spreadsheet for differential photometry.
Limovie has already been used successfully by European Asteroid Occultation Network (EAON) observers to extract precise timings for a number of asteroid occultations. I am sure the author would not claim it matched more professional photometry software, but it does its job 'out of the box' and it's free to download.
ARPS Assistant Director (Occultations)