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Reading HPOL Data
The HPOL data files (see also Data
Products) are written in FITS format.
The fluxes for the
ultraviolet spectrum are stored in the primary array. The
wavelengths, Stokes parameters Q and U (in percent), and a combined error
estimate for Q and U (also in percent), are stored in a binary
table extension. If no polarization data are available, there is no
binary table extension.
Various FITS readers may be used to read the data files.
The IUEDAC software provides general FITS file reading tools written
in IDL , such as
ifitsrd, which can be used to read the HPOL data files.
The header, extension header, flux, wavelengths, Stokes parameters
U and Q, and an error estimate
are returned. The Stokes parameters must be converted to provide
polarization and position angle as a function of wavelength. For
instance, to read the file hpolret_ab-aur_19940401_hw.fits:
The linear polarization and position angle may then be computed from
Q and U. The following is an Fortran subroutine
C Convert Polarization Q,U to Pol,PA (degrees)
C Temporarily renormalize values to avoid floating over/underflow
P = 0.
T = 0.
FF = Abs(Q) + Abs(U)
IF (FF.ne.0.) THEN
P = FF*Sqrt((Q/FF)**2 + (U/FF)**2)
T = Amod( 0.5*ATAN2(U,Q)/Raddeg + 180., 180. )
For data with flux data but no polarization data, one can create the wavelength
array from the FITS keywords CRVAL1 (starting wavelength in Ångstroms)
and CRDELT1 (wavelength array increment).
(Note for CCD data the "for loop" should be from 0 to 1199.)