The Hinode/SOT Instrument

Hinode Overview

Hinode is a Japanese mission with significant contributions by team members in the United States, Europe, and United Kingdom. The spacecraft carries three instruments: the Solar Optical Telescope (SOT), the Extreme Ultraviolet Imaging Spectrometer (EIS) and the X-Ray Telescope (XRT). Together, they are designed to provide multi-wavelength data from the photosphere to the upper corona. The 875 kg spacecraft (originally designated Solar-B) was launched in 2006 into a polar, sun-synchronous orbit at ~600 km altitude with an inclination of −98° allowing nine months of continuous observations and a three-month eclipse season per year. Hinode is operated from the Institute of Space and Astronautical Science (ISAS) in Sagamihara, Japan, which is a division of the Japan Aerospace Exploration Agency (JAXA). More details about the Hinode mission are found in Kosugi et al. (2007).

The following table contains links to the top-level Hinode pages at the participating institutions:

National Astronomical Observatory of Japan
NASA's Marshall Space Flight Center (USA)
Mullard Space Science Laboratory (UK)
European Space Agency

The SOT Instrument

SOT optical telescope assembly

SOT is a diffraction-limited Gregorian telescope with a 0.5 m aperture. The SOT instrument package comprises a spectro-polarimeter (SP) and a filtergraph (FG), which each benefit from the image stabilization capabilities of the onboard correlation tracker (CT). Together, this instrumentation enables the evolution of photospheric dynamics to be studied in detail from a space-based environment that is free from blurring caused by turbulence in the Earth's atmosphere. Additional information about SOT is found in Tsuneta et al. (2008).

SOT-SP is a scanning-slit spectrograph that provides polarization spectra useful for inferring the vector (three-component) magnetic field at the solar photosphere. SOT-SP achieves this goal by obtaining line profiles of two magnetically sensitive lines, namely the Fe I 6302 Å doublet, and nearby continuum using a 0.16″×164″ slit as it scans a region of interest. Spectra are exposed and read out continuously 16 times per rotation of the polarization modulator, and the raw spectra are added and subtracted on board in real time to demodulate, generating Stokes IQUV spectral images. Two spectra may be simultaneously taken in orthogonal linear polarizations, and when these are combined in data analysis after downlink, spurious polarization due to any residual image jitter or solar evolution is greatly reduced. The slit is aligned in the north-south direction, and it can map a region of interest by scanning in the east-west direction, up to the full 320″-wide field of view. More details about SOT-SP observing modes are provided in this section.

SOT-FG was an imaging instrument that created images of the solar photosphere and chromosphere in various wavelengths of light, however, due to a camera anomaly in February 2016, SOT-FG no longer operates. The two primary imaging modes of SOT-FG, a broadband filter imager (BFI) and a narrowband filter imager (NFI), sampled different fields of view, passbands, and spectral regimes. The archive of SOT-FG data available for analysis by the scientific community spans almost a decade. More details about the observing modes of SOT-FG are provided in this section.

The BFI produced photometric images with broad spectral resolution in six wavelength bands (CN band, Ca II H line, G band, and three continuum bands) at high spatial resolution (0.0541″ per pixel) and at rapid cadence (less than 5 s was possible, however 10s–20 s was more typical) over a 218″×109″ field of view. Exposure times were typically between 0.03–0.8 s, however, the instrument was capable of longer exposure times when needed. The BFI provided accurate measurements of horizontal flows and temperature in the photosphere, and measurements in the short wavelength bands permitted identification of sites of strong magnetic field.

The NFI provided intensity, Doppler, magnetic, and full Stokes polarimetric imaging at high spatial resolution (0.08″ per pixel) in any of ten spectral lines (including a range of Fe lines having various sensitivities to the Zeeman effect, Mg I b, Na D lines, and Hα) over the full 328″×164″ field of view. These spectral lines span the photosphere to the lower chromosphere, and enabled diagnoses of dynamical behavior of magnetic and velocity fields within the lower atmosphere. The passband of the Lyot filter was approximately 90 mÅ (depending slightly on wavelength) and the wavelength center was tunable to several positions in a spectral line and its nearby continuum. The NFI could be operated in synchronous mode with the polarization modulator of SOT in order to take Stokes IQUV images. The edges of the full field of view were slightly vignetted due to the limited size of the optical elements of the tunable filter. The unvignetted area was 264″ in diameter. Exposure times were typically 0.1s–1.6s, however the instrument was capable of longer exposure times when needed.

A schematic of the light paths within the SOT Optical Telescope Assembly (OTA), CT, NFI, BFI, and SP is shown in the image below, which is adapted from Figure 5 of Tsuneta et al. (2008).

SOT schematic