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Дата изменения: Mon Jun 12 19:04:27 2006 Дата индексирования: Mon Oct 1 20:56:07 2012 Кодировка: |
D. C. Hines1 Steward Observatory, The University of Arizona, Tucson, AZ 85721
(1)NICMOS Project, The University of Arizona
NICMOS, Polarimetry, Data Reduction, IRC +10216, CRL 2688
The Near Infrared Camera and Multi-Object Spectrometer
(NICMOS) contains optical elements which enable high spatial
resolution, high sensitivity observations of linearly
polarized light from
m. The filter wheels
for Camera 1 (NIC1) and Camera 2 (NIC2) each contain three
polarizing elements sandwiched with band-pass filters. The
design specifies that the position angle of the primary axis
of each polarizer (as projected onto the detector) be offset
by 120 from its neighbor, and that the polarizers have
identical efficiencies. While this clean concept was not
strictly achieved, the reduction techniques described below
permit accurate polarimetry using both the short- and
long-wavelength cameras over their full fields of view.
Polarizing efficiencies12.1 and absolute polarizer position angles (relative to the NICMOS entrance aperture) were derived for each polarizer in NIC1 and NIC2 from images obtained at 20 increments of the calibration polarizer position angle. The same method, but without the NICMOS polarizers in place, was used to to evaluate the polarization signature imparted by the mirrors which comprise the NICMOS imaging system, and to characterize the sensitivity of the NIC3 Grisms to polarized light.
The Thermal Vacuum tests showed that:
The ``standard theory'' polarimetry reduction algorithm outlined in the original NICMOS Manual (Axon et al. 1996) assumes that the polarizers have uniform and perfect (100%) polarizing efficiencies, and that the position angles of the primary axis of the polarizers are offset by exactly 120. The thermal vacuum tests showed that the NICMOS polarizers are not ideal, so a more complex technique is required. The new algorithm developed by Hines, Schmidt & Lytle (1997; hereafter HSL) is presented below.
The observed signal from a polarized source of total
intensity I and linear Stokes parameters Q and U
measured through the kth polarizer oriented with a
position angle12.2
is
![]() |
(12.1) |
Table (1) presents the properties of the individual polarizers as determined in preflight thermal vacuum tests and by the on-orbit standard star observations. Table (2) lists the coefficients derived from these parameters for use solving Eq. (1).
|
After solving the system of equations (Eq. 1) to derive the
Stokes parameters at each pixel (I, Q, U), the percentage
polarization (p) and position angle ()
at that
pixel are calculated in the standard way:
[Note that the arc-tangent function is implemented differently on different systems and programming environments, so care must be taken to ensure that the derived angles place the electric vector in the correct quadrant.]
Observations of a polarized star (CHA-DC-F7: Whittet et al.
1992) and an unpolarized (null) standard (BD 32 3739:
Schmidt et al. 1992) were obtained with NIC1 and NIC2
(Cycle 7 CAL 7692, 7958: Axon). The observations used a
four position, ``spiral-dither'' pattern with 20.5 pixel
offsets to improve sampling and alleviate the effects of bad
pixels, cosmic rays, some persistence, and other image
artifacts. Two epochs were chosen such that the
differential telescope roll between observations was
.
Since the thermal vacuum tests showed that the imaging
system had little effect on the observed polarization, any
measured polarization in the null standard was attributed
the tk term in the HSL algorithm. Applying our refined
coefficients to the polarized star data yielded a measured
percentage polarization within 0.3
of the published
value. Table 3 presents the results.
bWhittet et al. (1992) |
![]() |
Figure 1 presents the NICMOS polarimetry results for
IRC +10216 (Skinner et al. 1997) compared with the
ground-based data from Kastner & Weintraub (1994). The
polarization map derived by processing the NICMOS data with
the new HSL algorithm (center panel) agrees well with the
ground based data. In contrast, polarization images derived
by using the ``standard theory'' underestimate the
polarization and lead to incorrectly oriented electric
vector position angles. Variations of the percentage
polarization in relatively uniform regions of the
HSL-reduced IRC +10216 data suggest uncertainties
(in percentage
polarization per pixel), and comparison with the
ground-based data suggests an uncertainty in the position
angles
in a
pixel bins (Fig. 1).
![]() |
Figure 2 presents the NICMOS polarimetry results for CRL 2688 compared with observations obtained from the ground (Sahai et al. 1998). In this case the ground-based data are of exceptional quality and allow a more detailed comparison than for IRC +10216. Overall, the NICMOS and ground-based data agree quite well and show centrosymmetric patterns of position angle within the polar lobes.
Other, more subtle, features of the polarization morphology
that are seen in the ground-based polarization map are
reproduced precisely in the NICMOS map, confirming that the
NICMOS polarimetry is well calibrated. However, the
superior resolution of the NICMOS data reveals polarization
features that are not apparent in the ground-based
polarization map. In particular we note the very high
polarizations (
)
in the arcs and filamentary
structures -- features that are washed out (beam averaged)
in the ground-based images resulting in lower observed
polarization. As for IRC +10216, uncertainties in the
spacecraft data are estimated to be
in
percentage polarization, and
in the position
angles.
Limiting Polarization: Because the errors for
percentage polarization follow a Rice distribution, precise
polarimetry requires measurements such that
(Simmons & Stewart 1985).
Therefore, uncertainties
0.5-3% (per pixel) imply that objects should have minimum
polarizations of at least 2-12% per pixel. Binning the
Stokes parameters before forming the percentage polarization
(p) and the position angles reduces the uncertainties by
,
where N is the number of pixels in the
bin. Uncertainties as low as
should be achievable
with bright objects.
Limiting Brightness of the Target: In a perfect
photon-counting system,
,
where E is the total number of photons
counted. For CRL 2688, the signal strength even in regions
of low intensity (e.g. the H2-emitting torus) should
have produced
1%. We
measure
,
which
suggests the presence of other noise sources (e.g.
flat-field errors).
![]() |
Position Angle of Incoming Polarization Relative to
NICMOS Orientation: The non-optimum polarizer orientations
and efficiencies cause the uncertainty in polarization to be
a function of the position angle of the electric vector of
the incoming light. For observations with low
signal-to-noise ratios (per polarizer image), and targets
with lower polarizations, the difference between the signals
in the images from the three polarizers becomes dominated by
(photon) noise rather than analyzed polarization signal.
Therefore, observations that place important incoming
electric vectors at
45 and
135
in the NICMOS aperture reference frame should be avoided in
NIC1. No such restriction is necessary for NIC2.
We have demonstrated that NICMOS can produce highly accurate images in polarized light despite its non-ideal polarimetry optics. The HSL algorithm may be useful in processing data from other instruments that use polarimetry designs like NICMOS, such as the Faint Object Camera and the Advanced Camera for Surveys.
It is a pleasure to thank B. Stobie, L. Bergeron and A. Evans for assistance with the (non-polarimetric) data calibration. Special thanks to Joel Kastner for the use of his COB observations of CRL 2688 in advance of publication, and to the late Chris Skinner for his initial processing of the IRC +10216 data. DCH acknowledges support from the NICMOS project under NASA grant NAG 5-3042.