De La Peña, M., Bushouse, H., Storrs, A., Koratkar, A., Keyes, C., & Allen, R. 1999, in ASP Conf. Ser., Vol. 172, Astronomical Data Analysis Software and Systems VIII, eds. D. M. Mehringer, R. L. Plante, & D. A. Roberts (San Francisco: ASP), 474
The Faint Object Spectrograph Post-COSTAR Spectropolarimetry Correction
M. De La Peña, H. Bushouse, A. Storrs, A. Koratkar, C. Keyes
Space Telescope Science Institute, Baltimore, MD 21218
R. Allen
Steward Observatory, University of Arizona, Tucson, AZ 85721
Abstract:
While the COSTAR mirror assembly installed on the Hubble Space Telescope (HST)
during the second servicing mission effectively restored HST optical
capabilities, off-axis reflections from the COSTAR mirrors also introduced
a measurable amount of instrument polarization. We developed a methodology
to quantify and remove the induced polarization as detected by
the FOS spectropolarimeter (Allen 1995), and modified the FOS
calibration pipeline to accommodate post-COSTAR FOS Blue spectropolarimetry
observations.
Since the COSTAR mirrors have different reflectivities for light vibrating
parallel and perpendicular to the line of centers between the mirrors, the
polarization state of the incoming light to the FOS spectropolarimeter was
altered. In addition, the parallel vibrations were shifted in phase relative
to the perpendicular vibrations, so some portion of incoming linear
polarization was converted to circular polarization, and vice versa.
The effects of the COSTAR mirrors were determined from observations of
known unpolarized and polarized standard stars. While FOS Blue observations
of an unpolarized standard, BD+28D4211, provided the relative measures of the
parallel and perpendicular reflectivities, corresponding observations of a
polarized standard, BD+64D106, were used to determine the phase shift. Once
the induced polarization effects were determined, the proper polarization
for an unknown source could then be recovered by numerically inverting the
reflection process during pipeline processing.
Quantifying the induced polarization was accomplished by
processing observations for both the unpolarized and polarized standards
through a special version of the FOS calibration pipeline
(Storrs et al. 1998). In this special processing, the data were calibrated
with the typical pre-COSTAR calibrations, but the final output data
were forced to remain in the instrument frame, and were not rotated onto the
sky frame which would be done in typical processing.
While the standard version of the FOS calibration pipeline, calfos, is
implemented as an IRAF/STSDAS task, the special version was used
strictly to derive the post-COSTAR corrections. Consequently, the
post-COSTAR corrections are boot-strapped from the pre-COSTAR calibrations.
Once post-COSTAR corrections were generated by the special
pipeline, it was necessary to smooth and interpolate the data to derive
the final corrections.
The post-COSTAR corrections comprise a new FOS calibration file, PCPHFILE.
Table 1 lists the spectropolarimetric calibrated modes. There
are no flux and polarimetry calibrations defined for the FOS Red detector.
The post-COSTAR polarization correction has been implemented for FOS Blue
spectropolarimetry data in Version 3.0 and higher of both the calibration
pipeline, calfos, as well as in the stand-alone polarimetry processing
task, calpolar. The advantage of calpolar is that
it provides the user with the additional flexibility to manipulate the
flux calibrated data prior to polarization processing.
In order to obtain any calibrated spectropolarimetric data with
calfos, the keyword MOD_CORR must be set to PERFORM in the raw data
header file (D0H);
this is true for both pre- and post-COSTAR epoch processing. If the
spectropolarimetry data were acquired during the post-COSTAR epoch as
indicated by the header keyword ``KYDEPLOY = T'', which stands for COSTAR
deployed for
FOS (T or F), and a post-COSTAR calibration is available as noted in
Table 1, the post-COSTAR polarimetry correction obtained
from the PCPHFILE reference file will be applied. If the D0H file does not
contain the new PCPHFILE keyword, this keyword must be added for proper
post-COSTAR spectropolarimetric processing. The D0H file can be updated
to add this new keyword using the chcalpar task.
If no post-COSTAR polarimetry calibration file were provided for the PCPHFILE
calibration file keyword (or an incorrect file were supplied), calfos
will process all spectropolarimetric data with only the pre-COSTAR
corrections. In this instance if the data were from the post-COSTAR epoch,
warning messages will be issued regarding the application of only
pre-COSTAR corrections.
A proper post-COSTAR calibration depends upon the data being processed
using the standard pre-COSTAR corrections, with the exception that
the data are left in the instrument frame rather than being rotated
onto the sky frame. The Stokes parameters are then corrected to remove
the additional polarization imposed by the COSTAR mirrors; the linear and
circular polarization, as well as the phase angle are re-derived, and the
post-COSTAR corrected final results are rotated onto the sky frame.
The post-COSTAR corrections essentially remove the wavelength-dependent
COSTAR-induced instrumental polarization in the FOS Blue observations.
Figure 1 shows the polarization and Stokes vectors for
the situation when no post-COSTAR correction has been applied to observations
of v356 Sgr. The wavelength regime from 1900 - 2300 Å exhibits
weak linear polarization on the order of 2%; this polarization is
COSTAR-induced.
Figure 2 shows the polarization and Stokes vectors for the
situation when the post-COSTAR correction has been applied.
Figure 1:
v356 Sgr with only the Pre-COSTAR Correction.
 |
Figure 2:
v356 Sgr with the Post-COSTAR Correction applied.
 |
In self-consistency tests it was found that even after the observations
which were used to determine the post-COSTAR corrections were themselves
corrected for post-COSTAR induced polarization, there remained a residual
of 0.08% in Stokes Q.
In the post-COSTAR epoch, the combined effects of residual
geomagnetically-induced motion (GIM) and spacecraft jitter do not produce
polarization greater than 1%. Additional sources of error in polarization
are due to the photon statistics of the observation and the error in the
retardation calibration (Allen & Smith 1992). Since post-COSTAR observations
made with only four polarizer positions cannot be corrected
for COSTAR-induced circular polarization, these observations contain an
additional 0.4% uncertainty in Q. Only observations which have been made
with eight or sixteen polarizer positions can be adequately corrected for
the effects of the COSTAR mirrors.
Individual pre-COSTAR spectropolarimetric data should always be checked for
systematic errors due to motion perpendicular to the dispersion and the
large point spread function. Light loss from one polarizer position to
the next can introduce systematic effects that mimic the effect of a polarized
signal. In general, since the linear
polarization is biased towards positive values, observers of weakly polarized
objects should correct for this bias; this can be accomplished using the
polbias task. The calfos processing messages should always be
monitored. If the polarimetry processing is requested, but an incorrect
post-COSTAR calibration file is specified or none exists, the processing will
issue warning messages and continue. In this case, only the pre-COSTAR
calibrations will be applied.
References
Allen, R. G. 1995, in Calibrating Hubble Space Telescope
Post-Servicing Mission, ed. A. Koratkar and C. Leitherer,
(Baltimore: STScI),
60
Allen, R. G. & Smith, P. S. 1992, FOS Polarimetry
Calibrations (STScI FOS IDT Report CAL/FOS-078), (Baltimore: STScI)
Storrs, A., Koratkar, A., Keyes, C., Bushouse, H., De La Peña, M.,
& Allen, R. 1998, FOS Spectropolarimetry (STScI ISR
CAL/FOS-150), (Baltimore: STScI)
© Copyright 1999 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA