In addition to the quick--look on--line data reduction, there are
several programmes available to reduce TurPol data.
The main reduction programme is polred which computes the
polarisation parameters and raw magnitudes. A file with the
instrumental position angle and it variation from filter to filter is
needed. Default values, valid for observations made with field 0 set
at the TCS, are in the file cor.dat in the form:
17.4 .74 3.2 .0 -3.7 -3.6
where the first line is the zero point correction, and the second line
contains the wavelength dependent components, both given in
degrees. The corrections are for UBVRI from left to right. The file
is read in free format and a space has to be used as a separator
between values.
To reduce the data, type polred and you will be prompted for an
output filename, an input filename, and the angle off--sets
filename. When one file (usually one complete night of data) is
reduced polred asks you for another input filename, allowing several
nights of reduced data to be stored in one output file. To exit
polred, type end.
The sky background used by polred is the interpolated value between each sky measurement, if the star number and diaphragm have not been changed. In the case where either the diaphragm or ND filter have been changed, the first sky background value taken with the same setup is used.
The Stokes parameters p
and p
are calculated from the eight
sub--integrations, and the associated errors are computed from photon
statistics (E
), and from a least squares fit to the eight
sub--integrations (E
). If E
2E
the sub--integration
with the largest residual is discarded, and the remaining 7
sub--integration are used instead. This is done to eliminate bad data
points which can be due to transient electrical interference, or other
disturbances such as the star being close to the edge of the
diaphragm. If frequent rejections take place, the star was too close
to the edge.
The second line of the data file has to start with the code number 9,
and then have the date, while the third line starts with code 8 and
has the command for setting the polarimetric mode. The very last line
of a night's file should start with a 9 again to indicate the end of
the data.