The Hinode Spectro-Polarimeter
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The joint Japan/US/UK Hinode mission includes the first large-aperture visible-light solar telescope flown in space. One component of the Focal Plane Package of that telescope is a precision spectro-polarimeter designed to measure full Stokes spectra with the intent of using those spectra to infer the magnetic-field vector at high precision in the solar photosphere. This article describes the characteristics of the flight hardware of the Hinode Spectro-Polarimeter, and summarizes its in-flight performance.
KeywordsInstrumentation and data management Polarization, optical
1 Overview of the Spectro-Polarimeter
The Focal Plane Package (FPP) of the Solar Optical Telescope (SOT) instrument (Tsuneta et al. 2008) on the Hinode mission (Kosugi et al. 2007) includes the first precision Spectro-Polarimeter (SP) for high-resolution solar observations from space. Data from the many ground-based spectro-polarimetric instruments developed within the last 20 years have demonstrated the power of this observational technique to permit not only the measurement of the magnetic-field vector to much higher precision than traditional vector magnetographs, but also the variations of the magnetic field and other properties of the plasma along the line-of-sight (LOS) and even inferences of unresolved atmospheric structure (see, for example, Bellot Rubio, 2006). Such inferences follow from the high information content of the spectrally resolved line profiles recorded in their full state of polarization (i.e. recorded in all four Stokes parameters). The limitation of the SP to record only a one-dimensional slice of the solar surface at a time is offset to some extent by the simultaneous polarimetric measurements of the accompanying Narrowband Filter Instrument (NFI). The NFI is also capable of full Stokes polarimetry: polarimetric calibration for the NFI is given by Ichimoto et al. (2008), and polarimetric calibration of the FPP Broad-band Filter Instrument (BFI) has also been accomplished (Snik et al. 2010). These instruments acting in concert are able to provide a more complete picture of the magnetic and dynamic state within the solar atmosphere than has heretofore been possible from the ground.
This closely spaced pair of lines arising from the same multiplet (816) have similar formation properties, yet they differ in sensitivity to the Zeeman effect. When analyzed together, they permit a much more accurate determination of field strengths, magnetic filling factors, and field orientations to the LOS than is possible from a single line.
Being higher excitation lines, they have smaller sensitivity to temperature than other lines such as those near 5250 Å, hence they are more amenable to simple analyses such as are permitted by a Milne–Eddington model atmosphere.
The lines differ in oscillator strength by a factor of 2.6, and hence the two lines subjected to the same analysis permit some discrimination of the variation of atmospheric properties along the LOS.
The lines are only separated in wavelength by 1 Å, thus considerably reducing the spectral coverage of the detector relative to other line pairs.
The lines fall in the red where CCD detectors typically have their highest quantum efficiency.
These lines have seen extensive use as magnetic-field diagnostics from ground-based instruments, so analysis techniques were in place and well-understood before the launch of Hinode.
The SP instrument was realized through collaboration of the principal contractor to NASA for the FPP (Lockheed Martin Solar and Astrophysics Laboratory) and the High Altitude Observatory of the National Center for Atmospheric Research, with significant interaction at all phases of design and construction by scientists at the National Astronomical Observatory of Japan.
2 Overall Design Concept
Spectro-polarimeter requirements and as-built performance.
≤ 35 mÅ
Line widths in umbrae
≤ 25 mÅ
Resolve Stokes Profiles
≥ 2 Å @ 6302 Å
2.39 Å (normal)
4.77 Å (max)
Cover both 6301.5, 6302.5 Å plus high velocity events
Cover a typical active region
10−4 (average of 45 s, 0.5″ sample)
Weak internetwork fields
Polarization S/N, contin.
910 in 4.8 s
Active region fields
Undispersed light scatter
0.1585″ along slit
0.1476″ slit step
Critical sampling of SOT resolution @ 6302 Å
≥ 10 Hz
Crosstalk arising from solar proper motions
3 Elements of the SP
3.1 The Polarization Modulator
The heart of the polarimetric systems for the FPP is the rotating retarder Polarization Modulator. It is located on the optical axis of the telescope between the Collimating Lens Unit of the Optical Telescope Assembly (OTA: Suematsu et al., 2008, shown in red in Figure 1) and the Tip–Tilt Mirror. This placement of the modulator minimizes instrumental crosstalk among the Stokes parameters owing to the symmetry of the optical system and the low angles of reflection within the SOT. The modulator is a continuously rotating, multi-order bi-crystalline retarder with design retardations of (5+0.35) waves at 6300 Å and (7−0.35) waves at 5170 Å. This particular design (Guimond and Elmore 2004) has several advantages. The two crystalline sapphire and quartz elements are radiation-tolerant and therefore are not expected to darken over an extended lifetime of the Hinode mission. This design has very low variation of retardance with both temperature and field-of-view. The quartz and sapphire optical elements are thin enough (1.597 and 1.371 mm, respectively) that any fringes present in monochromatic light will have a wavelength spacing much larger than the characteristic width of the solar spectrum lines, yet they are thick enough that they withstood dynamic testing and launch. Fringing from each of these vacuum-spaced elements is minimized by broadband anti-reflective coatings on the four surfaces. The selected retardance provides nearly equal modulation of Stokes polarization states Q,U, and V both at the SP wavelength (6302 Å) and at the additional wavelength 5173 Å where the narrow-band filter (NFI) instrument also makes polarization measurements.
3.2 The Slit Scanning Mechanism
After the polarization modulator, the 45-degree reflections that divert the beam from along one of the spacecraft (x,y,z) axes to along another will not introduce crosstalk among the Stokes polarization parameters (Q, U, and V), provided that the final polarization analysis is also performed along these axes. In the case of the SP, this analysis is performed by the calcite polarizing beam splitter near the detector. The reflections of the Tip–Tilt mirror, the beam-distribution prism, and the slit-scan mirror are so oriented as to introduce minimal crosstalk. Furthermore, the slit-scan mirror is coated with a high-reflectivity protected silver coating, thereby also minimizing the polarization from this oblique reflection.
The FPP reimaging lens forms an image of the Sun at the spectrograph slit. The slit-scan mechanism can displace the solar image on the slit by up to ± 239″ (on the Sun) in increments that average 0.148″. To accomplish this small angular motion, a stepper motor drives a ball screw, which in turn drives motion-reducing levers mounted on flex pivots. The flex pivots minimize backlash. The image scanning does not occur at a pupil, so that the scan motion displaces the pupil image on the spectrograph grating. However, the range of the scan motion is small enough that the pupil displacement may be ignored, especially since the grating is significantly over-filled (see Section 3.4).
3.3 The Pre-Slit and Bandpass Filter
A reflective pre-slit assembly precedes the entrance slit of the spectrograph to reflect most of the broadband light and heat to a heat dump. A bandpass filter (FWHM=10.0 Å, transmission=0.934 at 6302 Å) is located at the pre-slit. This filter isolates the desired spectral order around 6302 Å to minimize stray undispersed light within the spectrograph.
3.4 An Off-Axis Reflecting Littrow Spectrograph
Weight and size constraints for space instrumentation dictate that the spectrograph be as compact as practical while still serving the scientific needs. A standard refracting Littrow spectrograph is appealing in that a single lens is employed for both collimation and reimaging of the beam. The Hinode/SP, as shown in Figure 3, uses a variant of the Littrow configuration where the lens is replaced by an off-axis paraboloid coated with protected silver. This makes the design more compact in the long dimension (925 mm focal length). It has the added advantages that it avoids stray-light reflections from the lens surfaces, and that focal properties are the same in either air or vacuum. The spectrograph slit is a 12 μm gap in an evaporated chromium layer on the flat surface of the plano-convex field lens (fused silica). The field lens immediately behind the slit images the pupil on the grating. Diffraction from the narrow slit fills the grating; without diffraction the f/31 beam incident on the slit would demand a much longer spectrograph in order to illuminate the grating and thus achieve the required spectral resolution. In this design only about 15 % of the light diffracted from the slit spills off the sides of the collimator and grating. The 79 line mm−1 echelle grating (aluminum coating) causes very little polarization at 6302 Å because it transmits s- and p-polarizations with nearly equal efficiency. A pick-off mirror directs the dispersed light returning from the grating toward the detector. This mirror surface is a dielectric coating optimized for high reflectivity at 6302 Å. It also assists in rejection of stray light at other wavelengths.
3.5 The Modified Savart Plate Polarization Analyzer
3.6 The CCD Camera
The camera readout electronics allow for summing one, two, or four pixels in either the serial or parallel direction during readout, but the flight software allows only for summing one or two pixels in the serial direction. The signals are digitized to a 12-bit precision. Digitization of selected sub-areas is permitted. From dark images taken on orbit, a read noise at the operational 10 Hz rate of 133 e− is achieved. The typical scale factor for SP data numbers (DN) using the default camera settings adopted since 2009 is about 76 e− per data number. The full-well capacity of the CCD is about 140 000 e−, and typical quiet-Sun disk-center continuum levels at the start of the mission (see Figure 11) collected about 46 000 e− each 0.1-second exposure. Thus, the SP exposures always sit safely under the full-well limit.
3.7 De-Modulation Electronics and On-Board Data Processing
The SP CCD camera records images and transmits them in digital form to the SP “smart memory” at a 10 Hz rate. The function of the smart memory is to “de-modulate” the polarization states that are temporally encoded, and to accumulate the polarization signals over several modulation cycles until the target S/N ratio is achieved. As illustrated by Lites (1987), the basic modulation cycle of the rotating retarder is half of one rotation, during which time eight images are transmitted from the CCD to the smart memory. The smart memory then adds or subtracts each image into four separate memory buffers, each buffer corresponding to one of the measured I, Q, U, and V Stokes parameters. The sign applied to the added value depends upon the buffer being addressed and upon the rotational phase of the modulator. The demodulation scheme is shown graphically and in table form for the eight-exposure scheme shown in Figure 22 of Ichimoto et al. (2008). This same demodulation scheme was used for the ASP (Elmore et al. 1992). The size of the smart memory for the SP is eight Mbytes to allow for double-buffering of the data.
At the end of the accumulation cycle, typically three full rotations of the modulator or six modulation cycles, the raw accumulated-demodulated Stokes spectral images attain a polarization precision of about 10−3 (see Table 1). Each word in the smart memory is a two-byte integer, unsigned for Stokes I and signed for Stokes Q, U, and V. With the 12-bit digitization of each exposure and the actual intensity levels, digital overflow in the Stokes-I continuum occurs when the SP is programmed to carry out significantly longer accumulations than the typical 4.8 second “Normal Map” (see Section 6). This overflow is even more serious for data that are binned spatially (i.e. in the “Fast Map” observing mode). The smart memory therefore has the capability to bit-shift any of the buffers in order to avoid this digital overflow of the two-byte integers. Furthermore, the smart memory is able to “window” the data in the spatial direction (along the slit). The observer has a choice between 16 separate window sizes. Windowing permits the data volume to be reduced at its source for science programs that do not require the full 160″ spatial coverage along the slit.
4 As-Built Performance of the SP
4.1 Spectral Resolution and Undispersed Stray Light
Prior to launch, the SP spectral-response profile was measured using a tunable laser, tuned to the vicinity of the 6302 Å Fe i lines. Due to wavelength drift of the tunable laser, short exposures were necessary to achieve reliable measures of the profile. Further, a rapidly rotating diffuser placed in front of the laser minimized the laser speckle pattern, and allowed more uniform illumination of the slit.
4.2 Slit Scan Linearity
In a detailed analysis of coalignment between the SP and the Filtergraph instruments, Centeno et al. (2009) derived the on-orbit behavior of the periodic error in the slit scan to high accuracy. Reported in that article are functions that not only characterize the periodic error, but other spatial effects as well.
4.3 Throughput and Polarization Signal-to-Noise (S/N)
On-orbit one is able to quantify the S/N by measuring the fluctuation of the signal in the continuum of the demodulated Q,U, and V polarizations. The expected polarization signals of the intrinsic solar spectral continuum at disk center are very much smaller than the ≈ 10−3 noise level of a typical SP measurement. Polarization in the continuum arising from instrumental effects is removed prior to these estimations of rms fluctuation by the standard SP calibration procedures (Lites and Ichimoto 2012).
The observations shown with black diamonds in Figure 11 represent the mean signal level of the Stokes-I continuum, per pixel per second, in SP data numbers (DN – see scale at left of graph). The SP throughput is decreasing in a linear fashion, as shown by the linear fit of these data, at a rate of −251 DN yr−1. It has decreased about 17 % in the five years after launch shown in this analysis.
Beginning in 2008, the recommended settings for the SP camera digital–analog converter offsets (DAC) were changed. The offset used as the default during the early part of the mission (DAC ID=7) added a large value to the observations that occasionally resulted in digital wrap-around of the 16-bit integer intensities for long SP observations. To minimize the occurrence of this wrap-around, the default offset was reset to the offset assigned to DAC ID=5, resulting in much lower offsets.
The rms fluctuation of the polarization continua in level1 data is shown as the colored open squares for DAC ID=7, and as the filled smaller squares for DAC ID=5. Evidently the newer offset reduces the noise somewhat especially considering that the data prior to 2008 were obtained with both CCDSIDEs, whereas the more recent data recorded only one CCDSIDE, but are summed two pixels along the length of the slit. The single-sided data selects CCDSIDE0 – the beam that has the higher intensity throughput of the two beams. To arrive at these estimates, only the spectral regions away from the lines were considered, and pixels having significant Stokes-V polarization were also excluded because the latter produce polarization throughout the entire 2.39 Å spectral range of a typical SP observation. The slight increase in noise over the years reflects the decreasing throughput of the SP. Note that the noise levels in Q are nearly identical to those in U, but that the noise is somewhat smaller in V, probably owing to the approximately 8 % higher modulation efficiency in Stokes V relative to Q and U (Ichimoto et al. 2008). As of mid-August 2011, the S/N stood at 890 for Stokes Q and U and 950 for Stokes V.
Figure 11 presents the history of the rms fluctuation in polarization applicable for two-sided level1 data without summing along the slit, or single-sided data that are summed two pixels along the slit, for the standard SP exposure time of 4.8 seconds (three full rotations of the polarization modulator). These data may be used to estimate noise levels in polarization for observations having different exposure times, different summing along the slit, or different binning in the slit scan direction (e.g. Fast Maps), when they are scaled according to the square-root of the total number of photons summed in the observation.
The severe reduction of the Hinode telemetry rate following failure of the onboard X-band transmitter in December 2007 (Shimizu 2009) dictated that new strategies in Hinode observing be implemented. In order to conserve telemetry, most observations after early 2008 have been carried out in single-beam mode, where data from only one of the CCDSIDEs are transmitted to the ground. This restriction on the data results in an increase in the noise level, but also some increase in I→Q,U, and V crosstalk arising from residual image motion and plasma motions on the Sun is experienced when using a single beam only. In practice, however, the Hinode correlation tracker has excellent performance (Shimizu et al. 2008) so that this additional source of crosstalk may be ignored except for observations of quiet regions requiring the smallest noise levels.
To further quantify this degradation of the data quality in single-beam mode, Figure 13 shows the rms fluctuation of the difference between the single- and dual-beam analyses as a function of wavelength. First consider the black solid line in that figure (the normalized mean spectral profile), and the black dashed line (the square root of the normalized Stokes-I profile). The latter represents the rms noise relative to the noise in the continuum were the signal arising entirely from photoelectrons. In fact, there is a small amount of read noise amounting to about 0.6 % of the signal in the continuum and 2.1 % in the core of the deeper of the spectral lines (see Section 3.6). For qualitative illustration here, these levels of read noise may be ignored. The colored line plots display the statistical fluctuation along the slit direction of the Q,U, and V single- minus dual-beam difference images as averaged over 100 separate observations of the quiet-Sun data set. If there were no additional crosstalk due to Doppler shifts and image motion, the colored plots would be expected to approximately follow the black dashed curve.
It is evident from the above analysis that the single-beam observations suffer a degradation of the polarization signals within the line cores of a few parts in 103. This will compromise observations of the very quiet Sun, but for maps of active regions the polarization signals are usually large enough that these small effects may be ignored.
4.4 Instrumental Scattered Light
The spacecraft orbital motion ensures that the apparent motion of the lunar disk across the Sun is very fast. In order to attain good statistics of the scattered-light intensity, the observations were planned for the full length of the slit (only one CCDSIDE) without binning along the slit. At the time of maximum eclipse when the motion of the lunar disk along the slit has halted, one may attain good resolution of the lunar limb as shown in Figure 14.
In order to determine the level of scattering, the dark level of the SP detector must be well known. The 11 July 2010 solar eclipse occurred during the height of the Hinode spacecraft eclipse season when the Sun is obscured by the Earth for a few minutes every orbit. Thus, SP measurements of the SOT viewing the Earth provide an excellent dark level (Lites and Ichimoto 2012). Such measurements were obtained between 21:35 and 21:40 UT, just prior to the observations shown in Figure 14 at 21:48 UT.
The image at right in Figure 14 shows the raw SP data corrected only for dark level and displayed on a logarithmic scale to reveal structure of the image above the limb. The observations are “normal mode” (see Section 6) having a 4.8-second exposure. The Fe i lines are seen in absorption all the way to the extreme bottom of the detector. There, the depths of the spectrum lines are comparable to those of the same exposure on the solar disk, indicating that the dark-level correction is adequate. A plot of the mean continuum level as a function of height above the solar limb (corrected for the tangent to the limb at mid-eclipse) is shown in the left panel of Figure 14. The curve falls very rapidly just above the limb, presumably owing to the diffraction pattern of the Hinode/SOT. The excellent optical quality of the SOT/SP combination was already evident in the solar-limb measurements reported by Lites et al. (2010), as was the scattered-light spectrum from the solar disk. The scattered light in this measurement falls below 10−2 at about 10″ above the lunar limb.
This measurement will permit extraction of the contribution to the point-spread function for the SP arising from scattering. In turn, an accurate assessment of that function will permit more detailed analyses of magnetic-field structure below the angular resolution of the SP because the influence of scattering from its surroundings will be known.
5 SP Polarization Calibration
Polarization calibration of the SP is described by Ichimoto et al. (2008). Calibration procedures for flat-field and dark correction, residual polarization crosstalk correction, merging of the dual-beam images, correction for spectral curvature and skew, and compensation for thermal flexure of the instrument as determined on-orbit are described in the companion article (Lites and Ichimoto 2012).
6 SP Observing Modes
The Hinode/SP is simple in design, and allows only a few observing modes. Nonetheless, the flexibility of slit scanning and binning allows the instrument to address diverse problems in solar physics. The four basic SP observing modes are summarized as follows.
Typical integration times to reach continuum S/N=1000 in Q,U, and V is 4.8 seconds, or three full rotations of the modulator. The slit scanner then steps after accumulating data for three rotations. This mode is most useful for maps of the vector magnetic field of active regions. A 1024×1024 [151″×162″] map requires about 90 minutes to execute.
Fast Map Mode:
Frequently there is need to accomplish maps of the vector magnetic field more rapidly than the Normal Mode. The Fast Map mode bins two pixels along the slit at the camera, then sums each two successive integrations in Smart Memory, resulting in spatial samples 0.30″×0.32″. In this mode the slit scanner is stepped one step after each rotation of the modulator (1.6 seconds), thus each measurement comprises the sum of two successive slit position measurements. A 151″×162″ Fast Map requires about 30 minutes.
For exploration of very rapid changes in the photosphere at small scales, allowance was made for the Dynamics Mode in which integration is carried out for only a single rotation of the modulator. These data are collected at reduced S/N. Data-rate constraints at the spacecraft demand that the spectral images be windowed to eliminate at least 2/3 of the full 1024-pixel slit length of the Normal Mode.
Deep Magnetogram Mode:
This mode facilitates studies of weak internetwork. It permits the polarization signal to be integrated over many rotations of the modulator. For example, S/N=104 in Q,U, and V may be attained in four minutes with two-pixel summing along the direction of the slit. In extremely long integrations of the Deep Magnetogram Mode, one anticipates that both Stokes-I and regions of large Stokes-V polarization signal may digitally “wrap around” beyond the 16-bit dynamic range of the smart memory. Allowance is made in the flight software to bit-shift any or all of the four Stokes parameters as they are accumulated in order to avoid digital wrap-around (see Section 3.7). In order to avoid wrap-around for very long exposures, it is recommended to keep the total exposure of a Normal Map to less than 12.8 seconds, and of a Fast Map to less than 6.4 seconds. To achieve longer integrations, successive measurements must be summed in data processing of the downlinked data.
The authors thank the many other individuals in the USA and Japan for their dedicated work throughout development of the Hinode mission, and also those involved with the mission after launch. We also thank an anonymous referee for helpful comments, and S.W. McIntosh for useful comments prior to submission. Hinode is a Japanese mission developed and launched by ISAS/JAXA, collaborating with NAOJ as a domestic partner, NASA and STFC (UK) as international partners. Scientific operation of the Hinode mission is conducted by the Hinode science team organized at ISAS/JAXA. This team mainly consists of scientists from institutes in the partner countries. Support for the post-launch operation is provided by JAXA and NAOJ (Japan), STFC (UK), NASA, ESA, and NSC (Norway). The FPP project at LMSAL and HAO is supported by NASA contract NNM07AA01C. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
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