-John Biretta and Matt McMaster
(Last update: 23 December 1997)
97 Dec. 23: Much progress has been made during the last few months. We have improved the instrumental polarization model in both the simulator and calibration tools. Improvements include detailed modeling of the MgF2 coating on the pick-off mirror, and correction of an error found in one of our references on polarization theory. Comparison of data processed with the old and new versions typically shows differences in the fractional polarization of 1% or less, and 1 to 2 degrees in the polarization position angle. The new model predicts the relative counts observed on-orbit in different settings of the polarizer filter to 1.5 percent RMS accuracy.
Upgraded Simulator Tool.
Upgraded Calibrator Tool.
A new Instrument Science Report describing the WFPC2 Polarization Calibration is available in PostScript form (1.3 Mbytes, 67 pages).
An example IRAF script for calculation and display of polarization images.
97 Jan. 28: The bit about changing the sign on U when taking polarization data into AIPS turns out to be a red herring. What one really needs to do, is make sure the sign on the X pixel size (CDELT1) is negative after you get the data into AIPS. AIPS uses a convention where the RA axis has a negative pixel scale. If instead you had a positive RA pixel scale, the polarization vectors would point the wrong way. (See example below for generating and displaying polarization images.)
97 Jan. 27: Added example showing how to generate Stokes images and a vector plot for R Mon.
We did more work with the on-orbit calibrators and tried to develop some simple aperture-dependent corrections to improve the accuracy below 1%, but this was not very successful. The corrections will probably need to contain at least parallel and perpendicular terms for each aperture. We will probably need to wait for more on-orbit data to derive those corrections.
96 Nov. 20: It now appears that the accuracy in the rotated POLQ is the same as the other settings (about 1 to 2%). The strange results we got for the POLQN33 and POLQP15W settings on the polarization calibrator stars was due to a bug in CALWP2 (WFPC2 pipeline calibration program) which caused the PC1 flat to be applied to WF2 data when only one CCD is read-out. We've re-calibrated some of the POLQN33 data by hand, and it looks fine. It will probably take a few weeks to recalibrate everything. More info then.
96 Nov. 19: The large errors for the rotated positions of the
POLQ have been traced to a strange error in CALWP2 (WFPC2 pipeline
calibration program). Under certain circumstances the wrong flat
is applied to the data. More details soon.
96 Nov. 14: Test version of new (second) calibration tool made available. General re-work / update of this page.
96 Nov. 11: WWW access to polarizer flats discontinued. They are instead available through the HST archive.
96 Oct. 10: HST archive names for polarizer flats added. Pol. WWW tool upgraded to computed (predicted) - (observed) counts.
96 Sep. 25: We've added a (somewhat crude) WWW tool for photometric calibration, and some discussion of photometric results.
96 Sep. 18: We've updated the matrices for the polarizers and POM with more accurate expressions.
Polarizer Flat Fields:
The first step in polarizer calibration is to flat field the data.
The polarizer flats are available though the HST archive. Archive file names are given below. Note that the flats have suffix R4H (i.e. file GA41233FU.R4H) and their data quality files have suffix B4H (i.e. file GA41233FU.B4H). Find the appropriate flat in the list below for your spectral filter and polarizer setting. Get the flat file from the HST archive (via STARVIEW, etc.) and *multiply* it into your data.
Filter1 Filter2 File F300W POLQN18 GA21514JU F300W POLQN33 GA21514MU F300W POLQP15 GA21514PU F300W POLQ GA21514SU F336W POLQN18 GA215151U F336W POLQN33 GA215154U F336W POLQP15 GA215157U F336W POLQ GA21515BU F390N POLQN18 GA21515EU F390N POLQN33 GA21515HU F390N POLQP15 GA41233FU F390N POLQ GA41233IU F410M POLQN18 GA41233LU F410M POLQN33 GA41233OU F410M POLQP15 GA41233QU F410M POLQ GA41233TU F439W POLQN18 GA412342U F439W POLQN33 GA412345U F439W POLQP15 GA412348U F439W POLQ GA41234BU F547M POLQN18 GA711082U F547M POLQN33 GA711087U F547M POLQP15 GA71108BU F547M POLQ GA71108FU F555W POLQN18 GA71108KU F555W POLQN33 GA71108OU F555W POLQP15 GA71108TU F555W POLQ GA711093U F606W POLQN18 GA711097U F606W POLQN33 GA71109AU F606W POLQP15 GA713053U F606W POLQ GA713058U F656N POLQN18 GA71305DU F656N POLQN33 GA71305HU F656N POLQP15 GA71305MU F656N POLQ GA71305RU F675W POLQN18 GA713061U F675W POLQN33 GA713065U F675W POLQP15 GA713069U F675W POLQ GA71306DU
Theory of WFPC2 Polarizer Calibration:
A new Instrument Science Report describes our current understanding of the Photometric Calibration. It is available in PostScript form (1.3 MBytes, 67 pages). Comments are welcomed.
The older HTML version of the theory discussion from January 1997 is also available.