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The SP PREP Data Preparation Package for the Hinode Spectro-Polarimeter

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The SP PREP Data Preparation Package for the Hinode Spectro-Polarimeter Solar Phys (2013) 283:601–629 DOI 10.1007/s11207-012-0205-4 The SP_PREP Data Preparation Package for the Hinode Spectro-Polarimeter B.W. Lites · K. Ichimoto Received: 25 September 2012 / Accepted: 26 November 2012 / Published online: 4 January 2013 © Springer Science+Business Media Dordrecht 2012 Abstract The Hinode/Spectro-Polarimeter (SP) is the first space-borne precision spectro- polarimeter for the study of solar phenomena. It is primarily intended for measuring the solar photospheric vector magnetic field at high spatial and spectral resolution. This objec- tive requires that the data are calibrated and conditioned to a high degree of precision. We describe how the calibration package SP_PREP for the SP operates. Keywords Instrumentation and data management · Polarization, optical 1. Overview The Focal Plane Package (FPP) of the Hinode/Solar Optical Telescope (SOT) includes a precision spectro-polarimeter (SP) that operates at the neutral iron lines at 6302 Å. Because of the nature of this dual-beam polarimeter, the data calibration is involved and requires many steps. This article outlines the polarimeter calibration procedures that are executed by the SolarSoft IDL package SP_PREP. Descriptions of the SP, FPP, SOT, and the Hinode mission are given by Lites et al. (2012), Tsuneta et al. (2008), and Kosugi et al. (2007). We refer to Lites et al. (2012) for on-orbit performance characteristics of the SP. In the same way that the SP instrument has inherited features from the Advanced Stokes Polarimeter (ASP: Elmore et al., 1992), some of the calibration procedures developed for the SP have substantial heritage in the data-reduction procedures of the ASP. Prior to de- tailed design of the Hinode/SP, a proof-of-concept spectro-polarimeter was implemented at the National Solar Observatory. That instrument led to the development of the Diffraction Limited Spectro-Polarimeter (DLSP: Sankarasubramanian et al., 2006). The data-reduction procedure developed for the DLSP was formulated as a prototype for the SP data-reduction B.W. Lites () High Altitude Observatory, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307, USA e-mail: [email protected] K. Ichimoto Hida Observatory, Kyoto University, Takayama, Gifu 506-1314, Japan 602 B.W. Lites, K. Ichimoto package described here. Even though the DLSP and SP data-reduction packages share some common elements and routines, in reality the instruments and the data they produce differ sufficiently such that most of the structure of the DLSP package had to be completely refor- mulated. A similar history is occured with the polarization calibration: the SP polarization- calibration procedure (Ichimoto et al., 2008) has some heritage in that of the ASP (Sku- manich et al., 1997), yet in the end the two procedures have little in common. Most SP observations are carried out in “operations” of typically 30 minutes or longer. The thermal characteristics of the instrument discovered shortly after launch necessitated an elaborate two-stage calibration procedure that requires two passes through each typical SP operation. The first pass finds empirical drifts of the image in the CCD focal plane during an operation and, after temporally smoothing these empirically determined drifts, they are used in the second pass to re-position the image in the focal plane and correct for gain variations uniquely associated with the drifting image. The calibration pipeline routinely processes all Hinode raw level0 SP data. The level0 data are organized into FITS files, each containing data resulting from the onboard accumulation/demodulation of a sequence of 0.1-second exposures of the CCD. Typical level0 FITS files result from accumulation of a few to more than ten seconds. In addition to fully calibrated Stokes profiles (the SP level1 data), SP_PREP provides quick-look output that is suitable for scientific analyses that require neither quantitative measures of the vector magnetic field, nor other analyses that require full spectral resolution of the Stokes profiles. This level1 processing comprises the following sequence of corrections to the data: i) digital wrap-around of the Stokes-I profiles and restoration of the spectra for onboard bit-shifting, ii) dark- and flat-field corrections, iii) removal of instrumentally induced polarization, iv) rectification of spectra so that the dispersion is along a pixel row and alignment of the spec- tra vertically between the two beams of the dual-beam polarimeter, v) merging of the two polarization beams from the dual-beam polarimeter, vi) correction of the spectral-line curva- ture, the thermal drift of the spectrum in the wavelength direction, and orbital Doppler shift, vii) rotation of the polarization frame of reference to the standard frame (+Q along solar East–West), viii) reversal of the spectrum direction so that increasing spectral pixel corre- sponds to the direction toward longer wavelengths, ix) compensation for residual I → Q, U, and V crosstalk, x) shifting the spectrum along pixel columns to correct for thermal flexure of the instrument in that direction, xi) correction for the slowly varying intensity response of the instrument (vignetting of the image plane) in the slit-scan direction, and xii) conversion of the data back to the same format as the unprocessed data (integers, possibly bit-shifted). This article describes in some detail the procedures employed to acquire and process the data needed for calibration (Section 2). Section 3 describes how these calibrations are applied in practice to produce the calibrated level1 Stokes-profile images and the ancillary L level1 data products such as the longitudinal and transverse “apparent flux density” [Bapp T and Bapp] as derived from the level1 Stokes profiles. 2. Determining the Data Needed for Hinode/SP Calibration This section describes the methods used to acquire and construct data needed to carry out the routine calibration of maps from the Hinode/SP. 2.1. Adjustment for Unsigned Stokes I , Bit-Shifting Data from the SOT reformatting program are presented as level0 FITS files containing signed integers. The SP_PREP program converts these integers to 32-bit floating-point numbers for further processing. SP_PREP for the Hinode Spectro-Polarimeter 603 The reformatting program presents all four Stokes spectral images as signed 16-bit inte- gers in a single FITS file, but unlike Stokes Q, U,andV , the Stokes-I signal is processed and compressed on-orbit as unsigned integers. Owing to the high bias level of the Stokes-I signal (Section 2.3), the Stokes-I images frequently wrap beyond the signed integer 15-bit boundary and thus may appear to have negative values when standard FITS reading pro- grams are used. The SP_PREP procedure detects this apparent wraparound and adjusts the signal appropriately by adding 215 to Stokes I , where needed, prior to any further process- ing. If digital overflow occurs onboard in the unsigned Stokes-I 16-bit integers during longer exposures, it will cause double wraparound in the level0 signed integer FITS data. This dou- ble wraparound is difficult to correct for in an automatic manner. To avoid digital overflow of the summed images for longer exposures, the onboard FPP processor allows downward shifting by one bit of any of the four demodulated Stokes images (sums and differences of CCD images over one modulation cycle: a half-rotation of the rotating retarder, or eight CCD exposures) prior to summing in the onboard FPP “smart memory”. This bit-shifting is carried out most frequently for Stokes I , but it may also be done for both I and V ,or for all Stokes parameters I,Q,U,andV as indicated by the FPP keyword SPBSHFT.The SP_PREP routine restores the data to their unshifted values prior to any processing of the data. The process of bit-shifting truncates the least significant bit, leading to an average negative, non-recoverable bias of one-half of the least significant bit for each measurement summed onboard. It is possible to acquire data near the quiet-Sun center with digital overflow in Stokes I for integrations as short as 12.4 seconds (144 CCD exposures), and with the standard dark bias offset (Section 2.3). This wraparound of Stokes I is difficult to correct for in post- processing, and will contain serious artifacts when the data are taken with the usual case of significant on-orbit JPEG compression. The level1 SP data are written to FITS files in a manner identical to that of level0 data; that is, after all processing, the data are bit-shifted in the same way as the original data, then converted to 16-bit signed integers. Therefore, it is necessary to restore level1 SP data to their original form by following the same procedures for reading the data as carried out in SP_PREP, for example by using the SolarSoft routine READL1_SBSP. 2.2. Acquisition of Dark Images The dark level of the SP cameras has a first-order influence only on the Stokes-I profiles because the Hinode/SOT onboard polarization modulation/demodulation scheme performs differences of measured intensities to arrive at Stokes Q, U,andV . In order not to bias Stokes I , the dark offset needs to be known to an accuracy specified by the science goals of each observation. Most science requirements are met if the Stokes-I levels are known to about 1 %. However, in very dark umbrae where the mean continuum intensity is below 10 % of the quiet-Sun continuum, errors of about 1 % in the dark level might result in an erroroftheStokes-I profile that could adversely affect some analyses. The SP contains no shutter, consequently there is no way to precisely measure the dark images on a frequent basis from launch to the end of the mission. Images taken well beyond the east and west limbs do not provide a pure measure of the dark bias because they are slightly contaminated by scattered light from the solar disk (see, for example, Lites et al., 2010).
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