||Department of Physics
Overview of Procedures for Photometry
- if necessary, i.e. for ".ST9" format images (not DRACO or EAST-14),
convert to ".fits" format using the routine "sbig2ndf"
- for DRACO and EAST-14 images, subtract a master dark frame
- if necessary remove 'hot' pixels by interpolation using
the Starlink Kappa "glitch" routine
- if required flatfield the images using Starlink's ccdpack
(make a flatfield image from averaging all images)
Measure the aperture photometry of objects on an image by one of
To understand the photometry process, it is a useful exercise to use GAIA for a few images, even if you mainly use "photom.py". The interactive approach of GAIA will also give enable you to choose appropriate aperture sizes and give an double check for the automated processes.
- interactively with GAIA and enter derived values
as a line in the "summary.obs" file
- fully automatically with the "photom.py" script
After Aperture Photometry
- use the "raw2dif" routine, which adds an appropriate
constant to the target magnitudes in order to place the photometry on a standard system. This is usually not possible for asteroids which have long timescale magnitude variations, due to their distance from Earth, as well as the variation due to rotation.
- plot the initial results with the "qplot" routine
- remove discordant points with a text editor
- if required average observation lines with the "average_lines.py"
- if required convert the observation times to Heliocentric JD with
the "cor2hjd" routine
The automated "fast_solve" routine
effectively uses phase dispersion minimisation to both find
the 'best' period and determine the run-to-run zero-point
offsets that best matched the data. The routine outputs (among others):
- if a suitable model light curve is available use the "bforce"
routine to look for the best fitting period
- overplot the model with derived period with the "bfplot" routine
Note: the model file output by "fast_solve" only contains lines for phase bins with observed data - however, "bfplot" requires a line for every phase bin. Edit the file and add arbitrary values for phase bins with no data.
- zero-point corrected lightcurves for each run
- a file ("fixed") with zero-point corrected magnitudes, which can be plotted by "bfplot" to obtain the combined lightcurve
- a model file (which is also required by "bfplot" for the above)