Документ взят из кэша поисковой машины. Адрес
оригинального документа
: http://www.stsci.edu/documents/dhb/web/c20_nicdataanal.fm4.html
Дата изменения: Tue Nov 18 00:20:25 1997 Дата индексирования: Sat Dec 22 16:37:09 2007 Кодировка: Поисковые слова: р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п р п |
The images in the three filters need to be compared, in order to produce the Stokes' parameters. To do so, the three images need to have the same dimensions and to be registered (have the source centered at exactly the same location in each image). In principle they should already be registered, provided the images in each filter were taken in the same visit without any changes of guide stars. If there is any displacement of the source between images, then each image can be registered using the IRAF task imlintran. When imlintran is used, the origin for the three input images should be set to the location of the source in each, so that in the output images the origin and the parameters nlines and ncolumns are identical.
To generate Stokes' parameters, the relative differences in flux between images in the different polarizing filters are used. Where the signal level is very faint, and the signal-to-noise ratio is very low, the differences will be very large but dominated by noise. If you attempt to calculate the Stokes' parameters using such data, you will obtain large and entirely spurious polarizations. To avoid this problem, it is advisable to estimate the noise in an area of the image free of sources, and then set a threshold at a value of order five to ten times this noise level. Using the IRAF task imreplace, all pixels with signals below this threshold should be set to some arbitrary value, probably close to the measured noise level. This action will cause all areas of the image where the signal level is very faint to show zero polarization.
and the Stokes parameters Q and U:
The statistical uncertainties are obtained by straightforward propagation of errors:
The Stokes parameters can then be combined to yield the polarized intensity, Ip:
and the degree, P, and position angle of polarization, q, using:
An interactive procedure to derive relevant parameters from NICMOS polarization images has been developed by Hines et al. (1997).1 In addition to taking into account instrumental polarization, this routine corrects for flatfield uncertainties and for small shifts between the images.
However, first approximation results can be obtained with a straightforward IRAF script. The approach we follow here is the simplest possible path to determining the polarization properties from the data. It does not take into account the instrumental polarization and does not allow for systematic errors in the data. The script will yield the correct morphology but not the exact intensity of the polarization. There are more sophisticated tools available in the community, including the one referenced above, which have been developed specifically to analyze polarization images, and which will yield better results than this very simple approach. The IRAF script is included below, and is commented.
Figure 18.4: Polarization Script
stevens@stsci.edu Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 11/13/97 17:28:21