The ability of NICMOS to perform high accuracy polarimetry is currently
hampered by an uncalibrated residual instrumental polarization at a level of
1.2-1.5%. To better quantify and characterize this residual we obtained
observations of three polarimetric standard stars at three separate space-craft
roll angles. Combined with archival data, these observations were used to
characterize the residual instrumental polarization to enable NICMOS to reach
its full polarimetric potential. Using these data, we calculate values of the
parallel transmission coefficients that reproduce the ground-based results for
the polarimetric standards. The uncertainties associated with the parallel
transmission coefficients, a result of the photometric repeatability of the
observations, dominate the accuracy of p and theta. However, the new
coefficients now enable imaging polarimetry of targets with p~1.0% at an
accuracy of +/-0.6% and +/-15 degrees.Comment: 5 pages, 2 figures. Contributed talk, "Astronomical Polarimetry 2008.
  Science from Small to Large Telescopes" La Malbaie, Quebec, Canada, 200

Axon, D. J.

Batcheldor, D.

Hines, D. C.

Robinson, A.

Schmidt, G. D.

Schneider, G.

Sparks, W.

Tadhunter, C.

English

arXiv

The ability of NICMOS to perform high accuracy polarimetry is currently hampered by an uncalibrated residual instrumental polarization at a level of 1.2−1.5%. To better quantify and characterize this residual we obtained observations of three polarimetric standard stars at three separate space-craft roll angles. Combined with archival data, these observations were used to characterize the residual instrumental polarization to enable NICMOS to reach its full polarimetric potential. Using these data, we calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, dominate the accuracy of p and θ. However, the new coefficients now enable imaging polarimetry of targets with p   1.0% at an accuracy of ±0.6% and ±15°

Batcheldor, Daniel

Hines, D.C.

Axon, David J.

Robinson, Andew

RIT Digital Institutional Repository

Rochester Institute of Technology RIT Digital Institutional Repository Articles Faculty & Staff Scholarship 11-6-2008 High Accuracy Imaging Polarimetry with NICMOS Daniel Batcheldor Rochester Institute of Technology G. Schneider University of Arizona, Tuscon D.C. Hines Space Science Institute G. D. Schmidt University of Arizona, Tuscon David J. Axon Rochester Institute of Technology See next page for additional authors Follow this and additional works at: https://repository.rit.edu/article Recommended Citation Batcheldor, D. et al. 2008, PASP, 449, 44 This Article is brought to you for free and open access by the RIT Libraries. For more information, please contact repository@rit.edu. Authors Daniel Batcheldor, G. Schneider, D.C. Hines, G. D. Schmidt, David J. Axon, Andew Robinson, W. Sparks, and C. Tadhunter This article is available at RIT Digital Institutional Repository: https://repository.rit.edu/article/1477 arXiv:0811.0843v1  [astro-ph]  6 Nov 2008Astronomical Polarimetry 2008: Science from Small to Large TelescopesASP Conference Series, Vol. 4**, 2009Bastien and MansetHigh Accuracy Imaging Polarimetry with NICMOS1D. Batcheldor,2 G. Schneider,3 D. C. Hines,4 G. D. Schmidt,3 D. J.Axon,5 A. Robinson,5 W. Sparks6 & C. Tadhunter7Abstract. The ability of NICMOS to perform high accuracy polarimetry iscurrently hampered by an uncalibrated residual instrumental polarization at alevel of 1.2!1.5%. To better quantify and characterize this residual we obtainedobservations of three polarimetric standard stars at three separate space-craftroll angles. Combined with archival data, these observations were used to char-acterize the residual instrumental polarization to enable NICMOS to reach itsfull polarimetric potential. Using these data, we calculate values of the paralleltransmission coe!cients that reproduce the ground-based results for the polari-metric standards. The uncertainties associated with the parallel transmissioncoe!cients, a result of the photometric repeatability of the observations, domi-nate the accuracy of p and !. However, the new coe!cients now enable imagingpolarimetry of targets with p " 1.0% at an accuracy of ±0.6% and ±15!.1. IntroductionThe Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is theonly near infrared (NIR) instrument capable of the high resolution, high fidelitypolarimetry needed to examine the scattering geometry and materials in manytypes of astronomical objects. However, there have been recent reports of a sys-tematic residual instrumental polarization of # 1.5% (Ueta, Murakawa & Meixner2005) that would seriously compromise studies of objects such as active galacticnuclei and regions within circumstellar protoplanetary disks; they typically ex-hibit polarizations of < 5%. A comprehensive study of this residual polarizationwas carried out by Batcheldor et al. (2006, B06), who found a residual excesspolarization of " 1.2% in the NIC2 camera.1Based on observations made with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Association of Universities forResearch in Astronomy, Incorporated, under NASA contract NAS 5-26555.2Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive,Rochester, NY, 14623, USA3Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ, 85721,USA4Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA5Department of Physics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester,NY, 14623, USA6Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA7Department of Physics & Astronomy, University of She!eld, She!eld, S3 7RH, UK12 Batcheldor et al.Table 1. Details of Polarimetric StandardsTarget Type mv mj mk 0.55µm 2.00µm 2.04µm(p%, !)VR 84c (WKK F7) B5V 10.3 9.34 9.13 5.98,118! 1.25,126! 1.19,126!VSS VIII-13 K1III 12.7 9.65 8.68 2.25,102! 0.91,102! 0.86,102!HD 331891 A4III 9.3 8.80 8.72 0.04,n/a ... ...Prior to this work, observations of only one polarized and one unpolarizedstandard have been made, each at only two celestial orientation angles. There-fore, until now, it has been impossible to entirely characterize the NICMOSresidual polarization. Polarization measurements at three well separated rollangles are necessary to remove the dependence of the measured Stokes param-eters on the relative transmission of each of the filters. We report here thefindings of a calibration program that collected such data.2. Observations and Data ReductionThe observations addressed the NICMOS residual polarization and several otherkey issues, including possible dependence with detector quadrants, source colordependence, the inter-pixel response functions (IPRFs) and latent image per-sistence. All orbits were scheduled so not to be impacted by the e"ects of theSouth Atlantic Anomaly. Persistence can be mitigated with an e"ective imagedithering strategy. However, a precise dithering pattern has to be chosen toavoid image raster overlaps of the PSF di"raction spikes. The observations weredithered in each detector quadrant to circumvent the e"ects of the IPRFs andsample around defective pixels. The pointing o"sets covered a phase spacing of-1/3, 0 and +1/3 pixels to best tile the IPRFs for the dither pattern employed.The dither steps were su!ciently large so that the 2nd Airy PSF maxima fromeach dither (0.!!58, 7.6 pixels), did not overlap from position to position.The three polarimetric standard stars presented in Table 1 were observedat three separate space craft roll angles. This provides the data for a uniquedetermination of any residual instrumental polarization relative to the equatorialreference frame. The technique allows the polarimetric analysis, derived from thethree polarizers at each of the field orientations, to be decoupled and checked forconsistency. For comparison with the NIC2 data, the ground-based polarizationmeasurements are corrected to 2.00µm using the known wavelength dependenceof interstellar polarization (Serkowski, Mathewson & Ford 1975).It is essential to inspect the fidelity of each pointed observation. Instru-mental sources of variation in the photometry, not directly resulting from theintrinsic polarization of the target, must be mitigated. Therefore, radially in-creasing aperture photometry was performed at each pointing and the encircledcount rates extracted. Radial profiles that showed more than a 2" deviationfrom the average profile were removed from further study.The determination of p and the orientation of polarization (!) is addressedfor NICMOS by Hines, Schmidt & Schneider (2000, HSS) and B06. In orderNICMOS Polarimetry 3VR84c012345p (%)-178.6o  -61.4o   58.4oVSS VIII-13179.6o113.6o  53.6oHD 331891-86.9o-22.4o  37.5o0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40306090120150180θ (o )0.2 0.4 0.6 0.8 1.0 1.2 1.4Aperture radius (")0.2 0.4 0.6 0.8 1.0 1.2 1.4Figure 1. The results using the previous calibration. The thick solid linesshow the ground based results and the grey shaded areas show the uncertain-ties associated with those ground-based results. Error bars are only shown forthe cases where they do not overlap the ground based results. The verticalsolid lines mark the radii of the 1st, 2nd and 3rd dark minima of the PSF. Eachline marks the three celestial field orientations for each target indicated.to assess the previous polarimetric accuracy, we have applied the coe!cientsderived by HSS to the new NIC2 calibration data. The results are presented inFigure 1, which demonstrates that at only a few roll angles does the previouscalibration reproduce the ground-based results.3. The CalibrationSince we have observed each standard at multiple space craft roll angles, theuncertainties due to the relative filter transmissions can be removed; each ofthe three polarizers, separately, was used to collect all the necessary polarimet-ric data. With these unknowns nullified by the multiple orientations, the onlyremaining explanation for these residuals (beyond intrinsically smaller uncer-tainties in the ground-based calibration) is an instrumental polarization (pi).As pi is fixed to the space-craft (Q,U)i are in the instrumental frame; pi mustbe determined in Stokes space. Therefore, we determine the actual values anduncertainties of Q and U (Q,U)ac, given the ground-based results, and deter-mine (Q,U)i assuming (Q,U)i = (Q,U)ob!(Q,U)ac. Using this method we findthe average instrumental polarization is 0.6 ± 0.1% at 116 ± 15".With pi determined (and subtracted), the parallel transmission coe!cients(tk) that reproduce the ground-based results can then be determined numeri-cally. The parallel transmission for POL240L (t240) is held at 0.9667 as per HSS.B06 also performed this process on archival data, so we include the values oftk derived by those authors. Table 2 presents the newly derived polarimetriccoe!cients from this study. There are 13 independent determinations of the4 Batcheldor et al.VR84c012345p (%)-178.6o  -61.4o   58.4oVSS VIII-13179.6o113.6o 53.6oHD 331891-86.9o-22.4o 37.5o0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40306090120150180θ (o )0.2 0.4 0.6 0.8 1.0 1.2 1.4Aperture radius (")0.2 0.4 0.6 0.8 1.0 1.2 1.4Figure 2. The results from the new calibration. The same convention isused in Figure 1. All results are now consistent with the ground-based data.Table 2. The New Polarimetric coe!cientsPolarizer #k (!) $k lk tkPOL0L 8.84 0.7313 0.1552 0.882 ± 0.008POL120L 131.42 0.6288 0.2279 0.837 ± 0.004POL240L 248.18 0.8738 0.0673 0.9667coe!cients from each orbit (nine new, four archival), the standard deviation ofwhich is used to define the uncertainty in the coe!cients about the mean. Theseuncertainties are propagated through the calculation of p and !. As a check, thenew coe!cients have been applied to the calibration data and presented in Fig-ure 2. This calibration gives results consistent with the ground-based data.4. DiscussionsThere are many potential (known and conjectured) sources for the estimatederrors on p and !. However, the uncertainties in the derived transmission co-e!cients dominate the statistical uncertainties from the photometry. Couldthese uncertainties be responsible for the observed instrumental polarization?A more accurate way to determine pi requires observations of an unpolarizedflux standard across the whole detector focal plane; this would map the two-dimensional relative transmission di"erences of the single polarizers. If there isa relative transmission across the focal plane, then this could mimic an instru-mental polarization. Therefore, it is appropriate to present the 0.6% value asan instrumental upper limit; pi could be zero.NICMOS Polarimetry 5When compared with the previous calibration, it can be seen that t120 isconsistent; t0 has been changed by 0.5%. Therefore, applying the new valuesback to previous studies of targets with p > 5% will not significantly alter theresults. However, when applied to targets with p < 5%, the new value of t120results in a very significant improvement in polarimetric accuracy.We recommend specific observing and data analysis strategies for spatiallyunresolved, and resolved, targets. For a spatially resolved target (without abright point source) observers should bin their data to 3$3 pixels (the di"ractionlimit of the instrument). Dithering can be used to increase spatial resolution andsample around bad pixels. However, high signal to noise ratio data is still criticalin determining p and ! to high accuracy. The case of a spatially unresolvedsource is more complicated. Due to uncorrectable bad pixels, the dithered datamust be inspected to identify deviant profiles. This must be done by executinga large number of pointed dithers (with a step size greater than the PSF toavoid persistence). Encircled energy profiles should be derived independentlyfrom each observation. A sigma clip should then be used to determine whetheran individual profile is deviant or not. The greater number of dithers used, themore accurate the flagging of deviant profiles will be. For small numbers ofdither points (not optimally recommended) medianing can be used to coarselyreject outliers. Outside an aperture of radius 0.!!57, the PSF a"ects disappear.Inside this radius, the uncertainties in p and ! rise rapidly. Observers mustweigh the required polarimetric accuracy to the required resolution.5. ConclusionsUsing the multi-roll technique, we have placed an upper limit to the NIC2 instru-mental polarization of 0.6%. The instrumental polarization has been subtractedfrom the target data in the Q,U plane. New parallel transmission coe!cientswere then derived by forcing p and ! to the same values of the calibration stan-dards. The new coe!cients are such a minor change from the previous that itdoes not warrant the re-analysis of previous NIC2 imaging polarimetry data ofhighly polarized (p > 5%) targets. Importantly, this improved calibration opensa new domain for observational investigations with HST by enabling very highprecision polarimetry of intrinsically low (p < 5%) polarization sources.Acknowledgments. Support for Proposal number HST-GO-10839.01-A wasprovided by NASA through a grant from the Space Telescope Science Institute,which is operated by the Association of Universities for Research in Astronomy,Incorporated, under NASA contract NAS5-26555.ReferencesBatcheldor, D. et al. 2006, PASP, 118, 642Hines, D. C., Schmidt, G. D. & Schneider, G. 2000, PASP, 112, 983Serkowski, K., Mathewson, D. L., & Ford, V. L. 1975, ApJ, 196, 261Turnshek, D. A., Bohlin, R. C., Williamson II, R. L., Lupie, O. L., Koornneef, J., &Morgan, D. H. 1990, AJ, 99, 1243Ueta, T., Murakawa, K., & Meixner, M. 2005, AJ, 129, 1625Whittet, D. C. B., Martin, P. G., Hough, J. H., Rouse, M. F., Bailey, J. A., & Axon,D. J. 1992, ApJ, 386, 562

High Accuracy Imaging Polarimetry with NICMOS

Name not available

Rochester Institute of TechnologyRIT Scholar WorksArticles11-6-2008High accuracy imaging polarimetry with NICMOSDaniel BatcheldorG. SchneiderD.C. HinesFollow this and additional works at: http://scholarworks.rit.edu/articleThis Article is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Articles by an authorizedadministrator of RIT Scholar Works. For more information, please contact ritscholarworks@rit.edu.Recommended CitationAstronomical Polarimetry 2008: Science from Small to Large Telescopes ASP Conference Series, Vol. 4**, 2009arXiv:0811.0843v1  [astro-ph]  6 Nov 2008Astronomical Polarimetry 2008: Science from Small to Large TelescopesASP Conference Series, Vol. 4**, 2009Bastien and MansetHigh Accuracy Imaging Polarimetry with NICMOS1D. Batcheldor,2 G. Schneider,3 D. C. Hines,4 G. D. Schmidt,3 D. J.Axon,5 A. Robinson,5 W. Sparks6 & C. Tadhunter7Abstract. The ability of NICMOS to perform high accuracy polarimetry iscurrently hampered by an uncalibrated residual instrumental polarization at alevel of 1.2−1.5%. To better quantify and characterize this residual we obtainedobservations of three polarimetric standard stars at three separate space-craftroll angles. Combined with archival data, these observations were used to char-acterize the residual instrumental polarization to enable NICMOS to reach itsfull polarimetric potential. Using these data, we calculate values of the paralleltransmission coefficients that reproduce the ground-based results for the polari-metric standards. The uncertainties associated with the parallel transmissioncoefficients, a result of the photometric repeatability of the observations, domi-nate the accuracy of p and θ. However, the new coefficients now enable imagingpolarimetry of targets with p ≈ 1.0% at an accuracy of ±0.6% and ±15◦.1. IntroductionThe Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is theonly near infrared (NIR) instrument capable of the high resolution, high fidelitypolarimetry needed to examine the scattering geometry and materials in manytypes of astronomical objects. However, there have been recent reports of a sys-tematic residual instrumental polarization of∼ 1.5% (Ueta, Murakawa & Meixner2005) that would seriously compromise studies of objects such as active galacticnuclei and regions within circumstellar protoplanetary disks; they typically ex-hibit polarizations of < 5%. A comprehensive study of this residual polarizationwas carried out by Batcheldor et al. (2006, B06), who found a residual excesspolarization of ≈ 1.2% in the NIC2 camera.1Based on observations made with the NASA/ESA Hubble Space Telescope obtained at theSpace Telescope Science Institute, which is operated by the Association of Universities forResearch in Astronomy, Incorporated, under NASA contract NAS 5-26555.2Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive,Rochester, NY, 14623, USA3Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ, 85721,USA4Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA5Department of Physics, Rochester Institute of Technology, 54 LombMemorial Drive, Rochester,NY, 14623, USA6Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, 21218, USA7Department of Physics & Astronomy, University of Sheffield, Sheffield, S3 7RH, UK12 Batcheldor et al.Table 1. Details of Polarimetric StandardsTarget Type mv mj mk 0.55µm 2.00µm 2.04µm(p%, θ)VR 84c (WKK F7) B5V 10.3 9.34 9.13 5.98,118◦ 1.25,126◦ 1.19,126◦VSS VIII-13 K1III 12.7 9.65 8.68 2.25,102◦ 0.91,102◦ 0.86,102◦HD 331891 A4III 9.3 8.80 8.72 0.04,n/a ... ...Prior to this work, observations of only one polarized and one unpolarizedstandard have been made, each at only two celestial orientation angles. There-fore, until now, it has been impossible to entirely characterize the NICMOSresidual polarization. Polarization measurements at three well separated rollangles are necessary to remove the dependence of the measured Stokes param-eters on the relative transmission of each of the filters. We report here thefindings of a calibration program that collected such data.2. Observations and Data ReductionThe observations addressed the NICMOS residual polarization and several otherkey issues, including possible dependence with detector quadrants, source colordependence, the inter-pixel response functions (IPRFs) and latent image per-sistence. All orbits were scheduled so not to be impacted by the effects of theSouth Atlantic Anomaly. Persistence can be mitigated with an effective imagedithering strategy. However, a precise dithering pattern has to be chosen toavoid image raster overlaps of the PSF diffraction spikes. The observations weredithered in each detector quadrant to circumvent the effects of the IPRFs andsample around defective pixels. The pointing offsets covered a phase spacing of-1/3, 0 and +1/3 pixels to best tile the IPRFs for the dither pattern employed.The dither steps were sufficiently large so that the 2nd Airy PSF maxima fromeach dither (0.′′58, 7.6 pixels), did not overlap from position to position.The three polarimetric standard stars presented in Table 1 were observedat three separate space craft roll angles. This provides the data for a uniquedetermination of any residual instrumental polarization relative to the equatorialreference frame. The technique allows the polarimetric analysis, derived from thethree polarizers at each of the field orientations, to be decoupled and checked forconsistency. For comparison with the NIC2 data, the ground-based polarizationmeasurements are corrected to 2.00µm using the known wavelength dependenceof interstellar polarization (Serkowski, Mathewson & Ford 1975).It is essential to inspect the fidelity of each pointed observation. Instru-mental sources of variation in the photometry, not directly resulting from theintrinsic polarization of the target, must be mitigated. Therefore, radially in-creasing aperture photometry was performed at each pointing and the encircledcount rates extracted. Radial profiles that showed more than a 2σ deviationfrom the average profile were removed from further study.The determination of p and the orientation of polarization (θ) is addressedfor NICMOS by Hines, Schmidt & Schneider (2000, HSS) and B06. In orderNICMOS Polarimetry 3VR84c012345p (%)-178.6o  -61.4o   58.4oVSS VIII-13179.6o113.6o  53.6oHD 331891-86.9o-22.4o  37.5o0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40306090120150180θ (o )0.2 0.4 0.6 0.8 1.0 1.2 1.4Aperture radius (")0.2 0.4 0.6 0.8 1.0 1.2 1.4Figure 1. The results using the previous calibration. The thick solid linesshow the ground based results and the grey shaded areas show the uncertain-ties associated with those ground-based results. Error bars are only shown forthe cases where they do not overlap the ground based results. The verticalsolid lines mark the radii of the 1st, 2nd and 3rd dark minima of the PSF. Eachline marks the three celestial field orientations for each target indicated.to assess the previous polarimetric accuracy, we have applied the coefficientsderived by HSS to the new NIC2 calibration data. The results are presented inFigure 1, which demonstrates that at only a few roll angles does the previouscalibration reproduce the ground-based results.3. The CalibrationSince we have observed each standard at multiple space craft roll angles, theuncertainties due to the relative filter transmissions can be removed; each ofthe three polarizers, separately, was used to collect all the necessary polarimet-ric data. With these unknowns nullified by the multiple orientations, the onlyremaining explanation for these residuals (beyond intrinsically smaller uncer-tainties in the ground-based calibration) is an instrumental polarization (pi).As pi is fixed to the space-craft (Q,U)i are in the instrumental frame; pi mustbe determined in Stokes space. Therefore, we determine the actual values anduncertainties of Q and U (Q,U)ac, given the ground-based results, and deter-mine (Q,U)i assuming (Q,U)i = (Q,U)ob−(Q,U)ac. Using this method we findthe average instrumental polarization is 0.6 ± 0.1% at 116± 15◦.With pi determined (and subtracted), the parallel transmission coefficients(tk) that reproduce the ground-based results can then be determined numeri-cally. The parallel transmission for POL240L (t240) is held at 0.9667 as per HSS.B06 also performed this process on archival data, so we include the values oftk derived by those authors. Table 2 presents the newly derived polarimetriccoefficients from this study. There are 13 independent determinations of the4 Batcheldor et al.VR84c012345p (%)-178.6o  -61.4o   58.4oVSS VIII-13179.6o113.6o 53.6oHD 331891-86.9o-22.4o 37.5o0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40306090120150180θ (o )0.2 0.4 0.6 0.8 1.0 1.2 1.4Aperture radius (")0.2 0.4 0.6 0.8 1.0 1.2 1.4Figure 2. The results from the new calibration. The same convention isused in Figure 1. All results are now consistent with the ground-based data.Table 2. The New Polarimetric coefficientsPolarizer φk (◦) ηk lk tkPOL0L 8.84 0.7313 0.1552 0.882± 0.008POL120L 131.42 0.6288 0.2279 0.837± 0.004POL240L 248.18 0.8738 0.0673 0.9667coefficients from each orbit (nine new, four archival), the standard deviation ofwhich is used to define the uncertainty in the coefficients about the mean. Theseuncertainties are propagated through the calculation of p and θ. As a check, thenew coefficients have been applied to the calibration data and presented in Fig-ure 2. This calibration gives results consistent with the ground-based data.4. DiscussionsThere are many potential (known and conjectured) sources for the estimatederrors on p and θ. However, the uncertainties in the derived transmission co-efficients dominate the statistical uncertainties from the photometry. Couldthese uncertainties be responsible for the observed instrumental polarization?A more accurate way to determine pi requires observations of an unpolarizedflux standard across the whole detector focal plane; this would map the two-dimensional relative transmission differences of the single polarizers. If there isa relative transmission across the focal plane, then this could mimic an instru-mental polarization. Therefore, it is appropriate to present the 0.6% value asan instrumental upper limit; pi could be zero.NICMOS Polarimetry 5When compared with the previous calibration, it can be seen that t120 isconsistent; t0 has been changed by 0.5%. Therefore, applying the new valuesback to previous studies of targets with p > 5% will not significantly alter theresults. However, when applied to targets with p < 5%, the new value of t120results in a very significant improvement in polarimetric accuracy.We recommend specific observing and data analysis strategies for spatiallyunresolved, and resolved, targets. For a spatially resolved target (without abright point source) observers should bin their data to 3×3 pixels (the diffractionlimit of the instrument). Dithering can be used to increase spatial resolution andsample around bad pixels. However, high signal to noise ratio data is still criticalin determining p and θ to high accuracy. The case of a spatially unresolvedsource is more complicated. Due to uncorrectable bad pixels, the dithered datamust be inspected to identify deviant profiles. This must be done by executinga large number of pointed dithers (with a step size greater than the PSF toavoid persistence). Encircled energy profiles should be derived independentlyfrom each observation. A sigma clip should then be used to determine whetheran individual profile is deviant or not. The greater number of dithers used, themore accurate the flagging of deviant profiles will be. For small numbers ofdither points (not optimally recommended) medianing can be used to coarselyreject outliers. Outside an aperture of radius 0.′′57, the PSF affects disappear.Inside this radius, the uncertainties in p and θ rise rapidly. Observers mustweigh the required polarimetric accuracy to the required resolution.5. ConclusionsUsing the multi-roll technique, we have placed an upper limit to the NIC2 instru-mental polarization of 0.6%. The instrumental polarization has been subtractedfrom the target data in the Q,U plane. New parallel transmission coefficientswere then derived by forcing p and θ to the same values of the calibration stan-dards. The new coefficients are such a minor change from the previous that itdoes not warrant the re-analysis of previous NIC2 imaging polarimetry data ofhighly polarized (p > 5%) targets. Importantly, this improved calibration opensa new domain for observational investigations with HST by enabling very highprecision polarimetry of intrinsically low (p < 5%) polarization sources.Acknowledgments. Support for Proposal number HST-GO-10839.01-A wasprovided by NASA through a grant from the Space Telescope Science Institute,which is operated by the Association of Universities for Research in Astronomy,Incorporated, under NASA contract NAS5-26555.ReferencesBatcheldor, D. et al. 2006, PASP, 118, 642Hines, D. C., Schmidt, G. D. & Schneider, G. 2000, PASP, 112, 983Serkowski, K., Mathewson, D. L., & Ford, V. L. 1975, ApJ, 196, 261Turnshek, D. A., Bohlin, R. C., Williamson II, R. L., Lupie, O. L., Koornneef, J., &Morgan, D. H. 1990, AJ, 99, 1243Ueta, T., Murakawa, K., & Meixner, M. 2005, AJ, 129, 1625Whittet, D. C. B., Martin, P. G., Hough, J. H., Rouse, M. F., Bailey, J. A., & Axon,D. J. 1992, ApJ, 386, 562

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High Accuracy Imaging Polarimetry with NICMOS

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