1. Acknowledgments 7. Scenarios for the Source of Residual Polarization 2. The Hinode SOT/SP Optical System 5. Raw (Uncalibrated) Polarization Images of Focal Plane of SP Throughout Mission Long-Term Variability of the Polarization Response Matrix of the Hinode Spectro-Polarimeter Bruce W. Lites: HAO/National Center for Atmospheric Research†, P.O. Box 3000, Boulder, CO 80307-3000, USA † The National Center for Atmospheric Research is sponsored by the National Science foundation 1.Abstract The Spectro-Polarimeter 1 (SP) of the Solar Optical Telescope 2 (SOT) on the Hinode mission 3 has been providing precision polarization measurements at high angular resolution consistently since October 2006. Starting in 2009, the Hinode team has performed a sequence of annual calibration measurements aimed at determining the flat-field response of this instrument. From these measurements we not only determine the flat-field response for the entire system (telescope, optics, and detector), but using the average disk-center quiet Sun as a standard candle, we determine the history of overall changes in throughput of the instrument over its active lifetime. The throughput has decreased nearly linearly by about 26% between 2006 and the present. This decrease in throughput is also reflected in a corresponding increase in the noise present in the polarization measurements. By examining the polarization signature of the spectral continuum in the annual flat-field measurements, we are also able to measure precisely any changes in the intensity-to-polarization crosstalk of the SP over its full scan range. We find a steady increase in residual instrumental polarization, particularly in Stokes Q, starting from undetectable levels at the start of the mission to a few ✕ 10 -3 (relative to Stokes I) in recent years. This polarization might be explained by degradation of the anti-reflective coatings of the collimator lens. 3. Polarization calibration of the SOT/SP 6. Characteristics of the Varying Instrumental Polarization 8. Conclusions and Outlook SOT/SP is experiencing I  Q, U, V crosstalk increasing from nearly undetectable levels early in the mission to present values of a few × 10 -3 in Q and a few × 10 -4 in U,V The residual polarization is distributed in the focal plane of SOT/SP in the form of an offset, out-of- focus image of the SOT entrance pupil The residual polarization must arise from optics in advance of the polarization modulator; i.e. from either the telescope primary and/or secondary mirrors or the collimator lens Several aspects of the observed polarization suggest that it arises as a result of ghost images within the Collimator Lens Unit as a result of degradation of the anti-reflective coatings The SOT/SP throughput has decreased by about 25% since launch For now, this residual polarization is (and has been) compensated effectively by the routine SOT/SP data processing procedures (SP_PREP 5 ) We will continue to monitor the progression of this anomalous polarization Figure 5. HISTORY OF RAW STOKES Q/I,U/I,VI IMAGES: Each of the 48 images shown is a map of the full field-of-view spectral continuum polarization signal measured with SOT/SP. The gray scale saturates at ±0.2%. Images of each of the two polarization beams (Left, Right) are shown. Data from 2009-2014 were obtained using a standard observing procedure as part of the annual flat-field sequence for SOT/SP: maps covering the full scan range of the slit with 256 equally- spaced scan steps. Data from 2006-2007 differ in appearance from the later data because they were obtained using different observing modes. In particular, the data from 2007 have considerably less noise because the scan comprised 2048 slit positions. No comparable data were obtained during 2008. References 1.Lites, B. W., et al. 2013, Solar Phys, 283, 579 2.Tsuneta, S., et al. 2008, Solar Phys, 249, 167 3.Kosugi, T., et al. 2007, Solar Phys, 243, 3 4.Ichimoto, K., et al. 2008, Solar Phys, 249, 233 5.Lites, B. W. and Ichimoto, K. 2013, Solar Phys, 283, 601 Thanks are extended to A. De Wijn and T. Tarbell for helpful suggestions. Support for this work was provided under NASA contract NNM07AA01C for the Hinode program at LMSAL and HAO. Figure 1. THE OPTICAL SYSTEM OF THE HINODE SOLAR OPTICAL TELESCOPE: Measurements in this work are acquired by the Spectro-polarimeter (SP), shown in magenta in the upper-left portion of the diagram. In addition to the SP, the relevant optical systems are the Optical Telescope Assembly (OTA, in red below) and the common optics to all systems (the Reimaging Lens and the Beam Distributor prisms shown at the left in yellow. From the standpoint of instrumental polarization, the SOT/SP has several important characteristics: 1.Symmetric optical system in advance of the polarization modulator: In a symmetric system it is difficult for optical devices to produce significant polarization. Linear polarization of any ray making an oblique angle with the mirror reflections will be cancelled by equivalent rays passing through the system at rotation angles about the optical axis of the telescope of ±90°. Similarly, any stress-induced birefringence that is symmetric about the optical axis will lead only to a net depolarization. 2.The polarization modulator, a rotating retarder, encodes the incident polarization state (Q, U, V ) into a temporal modulation of the linear polarization of the beam that follows. The encoded signal is temporal variations of differing frequencies and phases of the incident Q, U, V. 3.The two orthogonal linear polarization states exiting the polarization modulator are subjected to simultaneous, separate polarization analysis by the polarizing beam splitter just prior to the SP camera, and separate detection/demodulation by the SP CCD camera. This is a “dual-beam”polarimeter that doubles the overall efficiency of the system and reduces its sensitivity to polarization crosstalk arising from residual image motion. The basic polarization calibration of the SOT/SP occurred in June 2005 before integration of the SOT to the spacecraft. This calibration was accomplished by placing three large polymer sheet polarizers in front of the telescope as illuminated by natural sunlight: Figure 3. SCHEMATIC OF POLARIZATION CALIBRATION SETUP FOR THE SOT (illustration from Ichimoto et al. 2008 4. Three sheet polarizers (linear polarizer, left- and right-circular polarizers) were alternately placed in front of the telescope aperture, then rotated to eight stations at 45° intervals. Data so collected was used to determine the instrument response matrix X for each of the dual-beam polarimeter channels (left and right), as described by Ichimoto et al. 2008 4 : 2006 2007 2009 2010 2011 2012 2013 2014 Slit Scan Direction Distance Along Slit Left Right Q/ I U/I V/I L eft ChannelRight Channel Figure 2. TECHNICIANS PLACING A SHEET POLARIZER IN FRONT OF THE SOT ENTRANCE APERTURE The inverse of X (X -1 ) is applied to the polarimeter output Stokes vector to apply the polarization calibration. The science goals of Hinode/SOT dictate the tolerence limits |ΔX| are dictated by the science goals (Ichimoto et al. 2008). Note that the response matrices are nearly diagonal and most off-diagonal elements are smaller than their corresponding tolerance limits. Were they all smaller than |ΔX|, there would have been no need to perform or apply polarization calibration. The first column of X indicates the crosstalk from I  Q,U,V. These values are not well determined by the calibration because nearly unpolarized light could not be introduced. On-orbit, however, the spectral continuum near the center of the solar disk is an excellent source of unpolarized light. The analysis presented here uses the spectral continuum to determine the first column of the SOT/SP response matrix, and its behavior during nearly eight years of operation in space. Tolerance Limit The raw polarization images of the spectral continuum for the two channels of the dual-beam SOT/SP system are shown in Fig. 5. Early in the mission the instrumental polarization was very low, indicating a nearly ideal polarimetric response. However, in recent years there has been a clear increase in polarization for Stokes Q/I having the form of a somewhat out-of-focus image of the pupil, displaced downward from the center of the field-of-view. 4. Throughput History for SOT/SP Figure 4 at left displays a nearly linear decrease of continuum intensity recorded by the SOT/SP since its launch in late 2006. In all, the throughput at 6304 Å has decreased by about 25%. This decrease has been accompanied by a corresponding increase of the rms noise measured in the spectral continuum of the polarization images Q, U, V (colored data points). Prior to mid- 2008, data were obtained with settings of the camera electronics that differed from the later data, resulting in a higher level of measurement noise. From mid-2008 onward, data at the center of the solar disk were taken each month using the Hinode/SOT synoptic “Irradiance Program” (HOP 79). Figure 4. THROUGHPUT, NOISE HISTORY OF SOT/SP: The average level of the quiet-Sun continuum at disk center as seen by the SP is shown by the black diamonds. The black, straight line represents a linear fit through these points. The vertical axis at left shows the signal levels in DN for the SP per pixel per sec. Colored squares represent the rms fluctuation in the polarization continua relative to the Stokes I continuum: Q (green), U (blue), and V (red) for the typical SP observation of 4.8 second exposure. Open squares early in the mission flag measurements with analog-digital converter offset parameter 7, whereas later in the mission the small filled squares represent observations at a different offset (offset parameter 5). The vertical axis labels at right indicate the level of fluctuation in the polarization continua. (Figure is updated version of Fig. 11 of Lites et al. 2013 1 ) Q/IU/I V/I 201 4 201 3 201 2 201 1 201 0 200 9 200 7 200 6 Figure 7. HISTOGRAMS OF RESIDUAL Q/I POLARIZATION FROM Q/I IMAGES OF FIG.6 Figure 6. HISTORY OF CALIBRATED, FLAT-FIELDED, MERGED CONTINUUM POLARIZATION. The grey scale for Q/I polarization images saturates at ±0.4%, whereas it saturates at ±0.05% for U/I, V/I. Fig. 6 shows the result of application of the calibration procedure 3 for SOT/SP to the flat-field data shown in Fig. 5, except that no residual I  Q, U, V crosstalk correction is applied. The amplitude of the residual polarization signal is about a factor of two larger than in Fig. 6 because of correction for the modulation efficiency (diagonal elements of the response matrix X). Fig. 7 shows the histograms of the residual Q/I polarization for each year’s data. The increase of the positive residual shows no evidence for slowing, and importantly, there is a component that is increasing to negative values with time. Characteristics of the Residual Polarization: Q-polarization dominant U, V residual polarization smaller than that of Q by more than a factor of 10 Residual has form of downward-displaced, out-of-focus pupil image Negative as well as positive residuals occur, but positive dominant for Q, U and negative is dominant for V The residual polarization signature is a modulated polarization signal that has occurred where initially there was none. The polarization must arise from changes in optical elements prior to the polarization modulator: either the telescope mirrors or the collimator lens. Scenario 1: SOT Primary and Secondary Mirror Degradation Mirror degradation leads to increase in polarization upon reflection Secondary mirror supported by three vanes – asymmetric illumination in the N-S direction (like off-axis telescope) Lower reflectivity would lead to +Q bias Degradation of primary does not explain appearance of pupil image in polarization Would not explain appearance of spatially-offset pupil image Would not cause enhancement of both +Q and -Q in separate regions of the image Scenario 2: Degradation and Ghost Images in the Collimator Lenses Unit (CLU) Degradation of anti-reflective coatings might increase both reflectivity of surfaces (ghosts) and polarization The CLU is known to have “a measured tilt of image plane due to a fabrication error”. This might explain the offset of the pupil image seen in polarization There is no model that provides a quantitative prediction of polarization of ghost reflections in the CLU It is not clear how both +Q (–Q) would be produced in bright (dark) regions of a ghost image as a result of degradation of multilayer antireflective coatings