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qilpol, ALFOSC/FAPOL linear polarimetry, quick-look tool

The quick-look tool displays an image, then one needs to mark the target e- and o-beams and the programme calculates the linear polarization, degree of polarization, position angle and the normalized Stokes parameters, q and u.

Using the qilpol-tool

  • on a sequencer-window type: qilpol image_name
    e.g. qilpol ALqb280078
    NOTE the image must be the "67.5-degree image"
  • mark the e- and o-beam on the ds9 display, pressing any key followed by CTRL-D
  • check the polarization and the images

Detailed description of the programme

  • reads the file name from the command line and searches for this and three previous images from the /data/alfosc/ directory.
  • Check that the four frames have the same TCSTGT-keyword value, that there are no bias frames, and the retarder plate has positions 0, 22.5 45 and 67.5 degrees. If any of these is not fulfilled the task will exit.
  • Launch ds9 image display with the first image
  • user is asked to mark the e- and o-image of the target interactively on the ds9.
  • the fluxes of the images are measured using IRAF/APPHOT with fixed aperture radiuses of 5,8,11,16,26 and 37 pixels . The background annulus is 10 pixels wide starting from 38 pixels from the centre of the object.
  • Using the APPHOT flux values the f_0, f_22.5 etc are calculated by f_0=(f_0_o - f_0_e)/(f_0_o + f_0_e) etc. for all apertures.
  • initial normalized Stokes parameters q' and u' are calculated by q' = 0.5*(f_0 - f_45) and u' = 0.5*(f_22.5 - f_67.5)
  • the degree of polarization (P) is calculated by P = 100.* sqrt(q'*q' + u' * u') %
  • the position angle (PA) is calculated by PA=0.5*(u'/abs(u'))*arccos(q'/P)
  • the normalized Stokes parameters are calculated by q=P*cos(2*PA) and u=P*sin(2*PA)
  • The errors are estimated by the photon shot noise assuming that all the images have the same errors, P_err=100*0.5/sqrt(0.327*e) % and PA_err=28.65*(P_err)/P
References for data reductions:
E. Landi Degl'Innocenti et al in The future of photometric, spectrophotometric and polarimetric standardization ed. C.Sterken, ASP Conference series (in press) (astro-ph 0610262)
Fossati, L., et al in The future of photometric, spectrophotometric and polarimetric standardization ed. C.Sterken, ASP Conference series (in press) (astro-ph 0610637)

Known short comings

  • The polarization is measured from the raw data, i.e. no overscan/bias subtraction or flat fielding before photometry
  • Fixed position angle correction, i.e. the calculated Stokes parameters and the PA are not correct unless the ALFOSC field rotation is set to +90 (North up East left). Also, no wavelength dependent PA-correction.
  • Only one set of data, the programme can't handle multiple exposures of the target
  • The calculations are carried out for lambda/2 angles: 0,22.5, 45 and 67.5 degrees. If one of the angles is missing the task aborts.

Back to top Last modified: 21-Apr-2015