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:

ifitsrd,'hpolret_ab-aur_19940401_hw.fits',-1,header,exthead,flux
ifitsrd,'hpolret_ab-aur_19940401_hw.fits',1,header,exthead,wave,q,u,err

The linear polarization and position angle may then be computed from Q and U. The following is an Fortran subroutine

SUBROUTINE QUPT(Q,U,P,T)

C Convert Polarization Q,U to Pol,PA (degrees)
C Temporarily renormalize values to avoid floating over/underflow

Data Raddeg/.01745329/

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. )
ENDIF
RETURN
END

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.)

ifitsrd,.fits',-1,header,exthead,flux
stpar,header,'crval1',crval1
stpar,header,'cdelt1',cdelt1
print,crval1,cdelt1           (yields 1366.00, 2.0000)
wave = fltarr(1024) + crval1
for i=1,1023 do wave(i) = wave(i-1) + cdelt1
plot,wave,flux