Solar Optical Telescope (SOT)The Solar-B Solar Optical Telescope (SOT) will be the largest telescope with highest
performance ever to observe the sun from space. The telescope itself (the so-called OpticalTelescope Assembly, OTA), along with its focal plane package (FPP), is optimized formeasurement of the vector magnetic field and associated dynamics in the solar photosphere.
SOT will obtain a continuous series of diffraction-limited images (0.2−0.3 arcsec) in 388–668
nm range, which is achieved by 50-cm-diameter aperture from space while being free from theconditions of atmospheric seeing. The images will be acquired under very stable condition(<0.09 arcsec) archived by the image stabilization system, in which a piezo-driven tip-tilt
mirror (CTM) is controlled by using a displacement error estimated from correlation trackingof solar granules (correlation tracker, CT) in order to minimize jitter in solar images on thefocal plane CCDs. Thanks to the sun-synchronous orbit of Solar-B, the observations will bepossible for 24 hours for about 8 months per year.
The broadband filter imager (BFI) produces photometric images with broad spectral
resolution in 6 bands (CN band, Ca II H line, G band, 3 continuum bands) at the highest
possible spatial resolution (0.0541 arcsec/pixel) and cadence (<10 sec typical) over the full
field of view (218 × 109 arcsec). This will allow accurate measurements be made of thehorizontal flows and temperature of the solar surface, and to identify sites of strong magnetic
fields.
The narrowband filter imager (NFI) provides intensity, Doppler, and vector-polarimetric
imaging with moderate spectral resolution (0.08 arcsec/pixel) in 11 spectral lines (including
Fe lines with different Lande g factor, MgIb, NaD lines, and Hα) over the full field of view
(328× 164 arcsec). The spectral lines cover the lower photosphere through the chromosphere.
Dopplergram and longitudinal magnetogram are obtained in cadence of ~20 sec or less.
Shutterless exposure operations provide high cadence (1.6–4.8 sec) of vector magnetogram(Stokes IQUV), although the field of view is restricted; 5.3×164 arcsec with 0.08 arcsec/pixel,
14.7×164 arcsec with 0.16 arcsec/pixel, and 35×164 arcsec with 0.32 arcsec/pixel.
The spectro-polarimter (SP) provides line profiles in all Stokes parameters with high spectral
resolution (30 mÅ) in two magnetically sensitive lines of Fe I at 630.15 and 630.25 nm.
Normal mapping observation produces polarimetric accuracy of 0.1%. The field of view along
the slit is 164 arcsec with 0.16 arcsec resolution. The slit position can be moved step by step
(0.16 arcsec), allowing the spatial coverage of ±164 arcsec at maximum. To make a map of 160
arcsec-wide area covering a moderate-sized active region, it takes 83 min in normal mapping
mode and 30 min in fast mapping mode. For narrow 1.6 arcsec-wide area, it takes 50 sec and18 sec, respectively.
Details of the instrument are summarized in the following tables.

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Littrow mirror

SP CCD
FG CCD

shutter

FPP
Scan mirror

gratingslit

Filter wheel

shutter
FOV mask wheel
Universal birefringent filter
The SunCT CCD

Filter wheel
Reimaging lenswedge wheel

Secondary mirror

CLUPMUOTA
Tip-tilt mirror

Primary mirror

Figure. 1 SOT Optical Layout
Table. 1 Optical Telescope Assembly (OTA) Overview
Telescope
Optics type
Aplanatic Gregorian with heat dump mirror
Primary mirror
50cmφ aperture, light-weighted ULE
Primary-secondary mirror length
1.5 m
Central obscuration ratio
0.344 in radius
Effective F ratio
9.055 at secondary focus
Collimator Lens (CLU)
Exit pupil size
3 cmφ, collimated in air
Polarization Modulator (PMU)
Rotation speed
Continuous, 1.6 sec/rotation
Tip-tilt mirror for image stabilizer (CTM)
Signal used for closed loop control Residual signal from correlation trackerActuator
3 Piezo actuators
Tilt range
10.5 arcsec in radius on the sky
Control crossover frequency
14 Hz (nominal gain)
Stability<0.02 arcsec (in laboratory environment)48

Table. 2(a) Focal Plane Package (FPP) Overview I: Filter Observations.
Broadband Filter Imager (BFI)
Field of view
218 × 109 arcsec (full FOV)
CCD
4K × 2K pixel (full FOV), shared with NFISpatial sampling
0.0541 arcsec/pixel (full resolution)
Spectral coverage
Center (nm) Width (nm)Line of interest Purpose
388.350.7
CN I
Magnetic network imaging
396.85
0.3
Ca II H
Chromospheric heating430.500.8
CH I
Magnetic elements
450.45
0.4
Blue continuum, temperature
555.05
0.4
Green continuum, temperature
668.400.4
Red continuum, temperature
Exposure time
0.03–0.8 sec (typical)
Narrowband Filter Imager (NFI)
Field of view
328 × 164 arcsec (unvignetted 264 × 164 arcsec)
CCD
4K × 2K pixel (full FOV), shared with BFISpatial sampling
0.08 arcsec/pixel (full resolution)
Spectral resolution
0.009 nm (90 mÅ ) at 630 nm
Spectral band (tunable filter)
Center (nm) Width (nm)Lines of interest Purpose
517.2
0.6Mg I b 517.27
Chromospheric dopplergrams
and magnetograms
525.0
0.6
Fe I 524.71
Photospheric magnetograms
Fe I 525.02
Fe I 525.06
557.6
0.6
Fe I 557.61
Photospheric dopplergrams
589.6
0.6
Na D 589.6
Very weak fields (scatteringpolarization), Chromosphericfields.
630.2
0.6
Fe I 630.15
Photospheric magnetograms
Fe I 630.25
Ti I 630.38
Umbral magnetograms
656.3
0.6
H I 656.28
Chromospheric structure
Exposure time
0.1–1.6 sec (typical)
Standard observable examples for filter observations
Filtergram
A signal exposure for each spectral coverage
Frame size
4K×2K, 2K×2K, 1K×1K, or 0.5K×0.5K
Summing
1×1, 2×2, or 4×4 pixel
Readout time
3.4sec (1×1 summing), 1.7sec (2×2), 0.9sec (4×4)
Partial readout for faster cadence
Reconfigure time
<2.5 sec (for changing filter wheels etc)
Dopplergram
Image of the Doppler shift of a spectral line derived from narrowband
filtergrams at several wavelengths
Frame size
2K×1K, 1K×1K, or 0.5K×0.5K
Summing
1×1, 2×2, or 4×4 pixel
Duration
12.8sec (4 images, 2×2 summing, 0.8sec exposure)Longitudinal
Stokes V/I images converted onboard from narrowband filtergrams
magnetogram Frame size
2K×1K, 1K×1K, or 0.5K×0.5K
Summing
1×1, 2×2, or 4×4 pixel
Duration
8 images (4 wavelengths) are taken.
12.8 sec for 1K×1K and ~21 sec for 2K×1K

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Stokes IQUV
I/Q/U/V images made onboard from narrowband filtergrams at(for vector
different polarization modulator positions
magnetogram) Shuttered exposures
Frame size
2K×1K,1K×1K, or 0.5K×0.5K
Summing
1×1, 2×2, or 4×4 pixel
Frame size
144×1K, or 64×1K
Shutterless exposures
Summing
1×1, 2×2, or 4×4 pixel
Duration
1.6–4.8 sec(1–3 waveplate rotations)
Table. 2(b) Focal Plane Package (FPP) Overview II: Spectro-Polarimeter Observations.
Spectro-Polarimeter (SP)- Spectra of two Fe lines at 630.15 and 630.25 nm and nearby continuum- Raw spectra are added and subtracted onboard to demodulate, forming Stokes
parameters I, Q, U, and V.
Field of view along slit
164 arcsec (NS direction)
Spatial scan range± 164 arcsec (transverse to slit)
Spatial sampling (slit)
0.16 arcsec
Spectral coverage
630.08 nm to 630.32 nm
Spectral resolution/sampling
30 mÅ / 21.5 mÅ
Measurement of polarization
Stokes I, Q, U, V simultaneously with dual beam(orthogonal linear components)
Polarization signal to noise
103 (normal map)
Standard observable (mapping mode) examples for SP
Normal mapping
Time per position
4.8 sec (3 rotations of waveplate)
Polarimetric accuracy 0.001
FOV along slit
164 arcsec
Sampling along slit
0.16 arcsec
Data size
918K pixels in 4.8 sec or 191K pixel/s
Slit-scan sampling
0.16 arcsec
Time for map area
50 sec for 1.6 arcsec wide
83 min for 160 arcsec wide
Fast mapping
Time per position
One rotation for the 1st slit position andanother rotation for the 2nd slit position
FOV along slit
164 arcsec
Sampling along slit
0.32 arcsec
Data size
459K pixels in 3.6 sec or 127K pixel/s
Slit-scan sampling
0.32 arcsec
Time for map area
18 sec for 1.6 arcsec wide
30 min for 160 arcsec wide
Dynamics
Time per position
1.6 sec (One rotation)
FOV along slit
32 arcsec (to reduce data size)
Sampling along slit
0.16 arcsec
Data size
179K pixels in 1.6sec or 120K pixel/s
Slit-scan sampling
0.16 arcsec
Time for map area
18 sec for 1.6 arcsec wide50

Table. 2(c) Focal Plane Package (FPP) Overview III: Correlation Tracker
Correlation Tracker (CT)- Producing displacement error for feeding back to CTM tip-tilt mirror control
CCD
50×50 pixels, 0.22 arcsec/pixel
Frame rate
580Hz
Spectral range
629−634nm
Displacement Range+/− 5 pixelsError signal accuracy~ 0.01 arcsec
Table. 3 SOT observation controls and data handling
Observation controls- Managed by observing tables in Mission Data Processor (MDP)- An observing table exists for filter observation and SP observation, respectively.
- Macro-commands for taking observables and instrument commands is listed withinterval in the table.
Data handling
Effective process speed in MDP 832K pixels/sec (maximum, for FPP data)
Bit compression in MDP
16bit data compressed to 12 bit, 8 lookup tables
Image compression in MDP
12bit DPCM (lossless)
6−8bits/pixel(expected compression ratio)
12bit JPEG(DCT) (lossy) <~3bits/pixel for filters~1.5bits/pixel for SP# Compression rate depends on images and required image quality
Allocated telemetry rate (max) ~1.3Mbps (nominal)for SOT~1.8Mbps (SOT dominant)Data rate (after compression)~300Kbps, assuming 15 downlink stations in a day
averaged per day

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