This tool computes the expected count rate for a given target in each of several settings of the POLQ filter.
It can be used to derive the polarization properties of a target, by iteratively guessing the polarization parameters of a target, running the tool and obtaining the estimated count rates in the various settings of the POLQ filter that were used (i.e. apertures PC1, WF2, WF3, WF4, POLQN33, POLQN18, POLQP15W), comparing these against the observed count rates, adjusting the target properties. and so forth. (An easier-to-use automatic tool is in the works...).
The user firsts inputs the polarization properties of the target. If SYNPHOT data are input (see separate help button), the units on I, Q, U, and V should be consistent with those of the SYNPHOT data. That is, give I in ergs cm-2 s-1 Hz-1 if you plan to use these units for the SYNPHOT data. One may also just input I, Q, U, and V in units of counts, and then leave the SYNPHOT parameters set to zero.
One may input the polarization properties as either (I, f, PA) or as the usual Stokes vector (I, Q, U, V).
The program then needs to know which spectral filter was used, so it knows what wavelength to use for estimating mirror attenuation and retardance.
Finally one inputs the apertures used for each image of the observation, and the PA_V3 value from the image header (which gives the rotation of HST on the sky). If desired, the observed count rates and their uncertainties may also be input, in which case the program will report the differences between the predicted counts and the observed counts, as well as a chi-square.
Clicking on "calculate" will then start the computations.
The results are a table of predicted count rate for each of the apertures used.
Accuracy:
Accuracy is about 1% of I for F555W and F675W, and 2% of I for other filters in the apertures PC1, WF2, WF3, and WF4. For reasons which are currently unclear, the accurracy in apertures POLQN33 and POLQP15W is poorer (2% to 5% of I).
How the tool works:
This tool uses Mueller matrices to convert the incident Stokes vector to a detected count rate on the CCD. Separate matrices are used for the rotation of HST on the sky, the 47 degree pick-off mirror in WFPC2, the rotations of the polarizer quads and filter wheel, and for the polarizer itself.
The matrix for the pick-off mirror include the effects of diattenuation and linear retardance in the mirror. The mirror is modeled as bare aluminum; the 250 Angstrom MgF2 coating is thought to have little effect (<<1%). Further study of this issue is needed, and the mirror model will be improved if needed.
The polarizer matrix includes leakage terms for the perpendicular direction.
-J. Biretta, STScI