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qicpol, ALFOSC/FAPOL circular 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 V/I and the statistical error in percentage.

Using the qicpol-tool

  • on a sequencer-window type: qicpol image_name
    e.g. qicpol ALqb280078.fits
    or if you want to check data from previous night
    e.g. qicpol ALqb270076.fits
    NOTE the image must be the "90.0-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 and 90.0 degrees. If any of these is not fulfilled the task will exit.
  • Launch ds9 image display with the first image
  • the 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_90 are calculated by f_0=(f_0_o - f_0_e)/(f_0_o + f_0_e) etc. for all apertures.
  • the V/I is obtained from V/I=0.5*(f_0 -f_90) * 100 %
  • 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*flux[in ADU]) %
References for data reductions e.g.:
Bagnulo S., et al A&A 450, 777 (2006) "Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1"
E. Landi Degl'Innocenti et al ASPC, vol 364, p495 (2007) "Polarimetric Standardization" in "The future of photometric, spectrophotometric and polarimetric standardization" ed. C.Sterken ASPC, vol 364, p495 (2007)

Known short comings

  • The polarization is measured from the raw data, i.e. no overscan/bias subtraction or flat fielding before photometry
  • Only one set of data, the programme can't handle multiple exposures of the target
  • The calculations are carried out for lambda/4 angles: 0 and 90.0 degrees. If one of the angles is missing the task aborts.

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