fast_solve

On a CCD image the brightness of an asteroid is typically measured relative to two comparisons stars. From night-to-night as the asteroid moves different comparison stars must be used. The "fast_solve" programme solves for the night-to-night magnitude zero-point offsets using a very simple version of the phase dispersion minimisation technique. This assumes that the shape of the light curve does not change from night-to-night (run-to-run).

Program overview:

• Input a list of the summary.obs files (in file 'obs_list') from up to 20 runs.
  Columns: [filename, light travel time (mins)]
  The light travel time can be found from the JPL Horizons On-line Ephemeris.
• Input the literature period.
• Output postscript file 'comp_dif.ps' displays the difference of the two comparison stars. This needs to carefully examined to check that the selected comparison stars are not varing in brightness during the run. Inspect this plot with the ghostscript postscript viewer, i.e. "gv" at the command line prompt.
• Using the input period, data points are split into 100 equal phase bins.
• All runs are simultaneously matched to the first run by comparing data points in equivalent phase bins.
• The offset for each run is calculated by minimising the chi-squared of data point residuals (assuming an error of 0.02 mag per data point).
• File 'fixed' outputs the corrected magntiudes for all data points.
  Columns: [time, corrected mag, mag error, run number]
• Using corrected magnitudes, search around input period for a better period. The new period, along with new minimum chi-squared is displayed.
• A crude model is created from the median point in each phase bin. The model is saved in file 'new_model.dat'
  Columns: [phase bin, model mag, #points]
• The programme reads a file called 'fast_solve_driver' for the plot parameters.
• Output postscript file 'lightcurves.ps' has the corrected lightcurve for each run. If requested the "new_model.dat" curve is plotted over the data points; clearly this is only useful when the all the phases of the light curve have been well sampled. Example output plot for the asteroid Gaspra.

Note: "fast_solve" does not currently correct for the phase shift due to the relative viewing angle of the asteroid as observed from the moving Earth. Typically this is amounts to +/- 7 degree for a main-belt asteroid around the time of opposition as this planview of the asteroid's Gaspra 2015 opposition shows.

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jrl

2020-Jun-17 13:39:36 UTC