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Estimate on SOT light level in flight with
throughput measurements in SOT sun tests
T. Shimizu1, T. Tarbell2, Y. Suematsu3, M. Kubo1,
K. Ichimoto3, Y. Katsukawa3, M. Miyashita3,
M. Noguchi3, M. Nakagiri3, S. Tsuneta3,
D. Elmore4, B. Lites4 and SOT team
1. ISAS/JAXA, 2. LMSAL, 3. NAOJ, 4. HAO/NCAR
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Abstract


The solar light into the telescope penetrates through many optical
elements located in OTA and FPP before illuminating CCDs.
Natural solar light was fed to the integrated SOT flight model in two
sun-test opportunities for verifying various optical aspects. One of
important verification items is to confirm light throughput.
– CCD exposures provide the number of photons accumulated in an
exposure-duration in clean room test condition.
– A pinhole-PSD (position sensitive detector) sensor (525 nm band) was
used to monitor the light level simultaneously, giving the “absolute” light
level.
– The PSD sensor was pre-calibrated with continuous monitoring the solar
light level in a day long under a clear constant sky condition, giving what is
the voltage for one solar light level.
– Transmissivity of heliostat two flat mirrors plus clean-room entrance
window glass was also measured as a function of wavelength.

This throughput measurement with solar light has confirmed the light
level in flight experimentally.
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1. Solar Optical Telescope (SOT)

Litrow Mirror consists of optical telescope
Solar-B SOT (solar optical telescope)
Folding Mirror
(OTA) and focal plane instruments (FPP). Polarizing BS
256 x 1024 CCD
X3 Mag lens
Polarizing BS
Folding Mirror
Slit
Grating
Preslit
Folding Mirror
Shutter
Field lens
Field lens
X2 Mag lens
Shutter
Filterwheel
Telecentric
lenses
Beam Distributor
Birefringent Filter
Field Mask
2048 x 4096 CCD
Secondary
Filterwheel
50 x 50 CCD
Folding Mirror
Reimaging Lens
Demag lens
Image Offset Prisms
Folding Mirror
Polarization
Modulator
Primary
CLU
Tip Tilt Mirror
Optical layout
OTA
Common Optics
CT
NFI
BFI
SP
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2. Measurements (1)




Throughput measurements were
conducted in two SOT sun tests (2004
August and 2005 June) in NAOJ clean
room.
Natural solar light was fed to the
integrated SOT by the heliostat on the
roof, as shown in the figure
The solar light illuminated the full aperture
of the OTA (See photo).
With this configuration, FG, SP, and CT
CCD images were obtained for all of
wavelengths with several different solar
light levels.
* CCD exposures provide the number of
photons accumulated in an exposureduration in this test condition.
* Dark frames were also obtained to
subtract dark signals from the exposed
CCD data.
Natural sun light
Heliostat flat
mirrors
Calibrated
PSD sensor
Entrance
window
NAOJ clean room
OTA
FPP
OBU
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Test configuration
2. Measurements(2)
NAOJ Heliostat on the
roof of clean room
Sun light illuminated
OTA full aperture
Integrated SOT
flight model
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2. Measurements (3)



A pinhole-PSD sensor continuously monitored the light level on the roof
during the measurements.
The sensor consists of ND filter, a band pass filter, a pinhole and
HAMAMATSU position sensitive detector.
The band pass filter is the same type of the filter used in NSAS and
UFSS sun sensors onboard Solar-B, which wavelength is centered at
525 nm with bandwidth of 60nm.
Pinhole-PSD sensor
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3. “Absolute” light level
at measurements

The pinhole-PDS sensor allows us to estimate what the “absolute”
solar intensity level is at each of CCD exposures by the equation:
I / I0  (V / 816
. )Tatmos ()Theliostat ,
where
V
Tatmos(l)
Theliostat
the voltage output from PSD sensor
coefficient for correcting wavelength dependence of
the atmospheric absorption
the transmission of the heliostat mirrors and window glass.
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3.1. Calibration as standard sensor (1)


The purpose of the calibration for the pinhole-PSD sensor is to estimate the
sensor output (voltage) at one solar light level, which is the flight condition
without earth atmosphere attenuation.
The PSD sensor was pre-calibrated with continuous monitoring the solar light
level in a day long under a clear constant sky condition, giving what is the
voltage for one solar light level..
The measurements
were made a few times
in May – June 2004 on
the roof of NAOJ clean
room building.

Diamonds: measurements
Solid curve: fitted
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3.1. Calibration as standard sensor (2)
The attenuation by the earth atmosphere is proportional to the length of the
atmospheric layer along the light path from the sun to the ground, and it is
approximately represented as a function of 1/cosθ in the zenith angle (θ) up to
30 deg.
The light level measured on the ground Y is expressed by


Y= A0*[1A1/cosθ],
where A0 is the one solar light level and A1 is the atmospheric absorption
coefficient.

The 5-June-2004 data gives
sensor output at one solar light A0 = 8.16 ±0.07 V
absorption coefficient A1= 0.201±0.003
which is good agreement with a value at 500nm shown in Astrophysical Quantities
(Allen, 1973).
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3.2. Transmission of heliostat mirrors and window
Theliostat
0.7
NAOJ Clean Room Heliostat - 2004/5/11, 8/2, 9/21 -
0.6
0.5
0.4
0.3
Average 2004/5/11
0.2
2004/9/21
0.1
0
400 450 500
550 600
Wavelength (nm)
0.6
0.5
0.4
0.3
0.2
2005/7/27
0.1
Estimated from 525 nm measuremet
(2004/8/2)
350
NAOJ heliostat 2005/7
0.7
Transmissivity (Ratio)

Multiple numbers of band pass interference filters were used to measure the
solar light levels both inside the clean room and on the building roof, giving
how much percent of the light is transmitted into the clean room.
The transmitted percentage is 35~45% at the shorter wavelength and 50~60%
at the longer wavelength. Note that the major source of attenuation is the thick
entrance window, rather than the mirrors’ reflectivity.
Transmissivity (Ratio)

650 700 750
0
350
400
450
500
550
600
650
700
750
wavelength (nm)
Measured transmission of heliostat mirrors and window glass
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3.3. Wavelength dependence Tatmos(l)
It is known that the atmospheric transmission changes as a function of
wavelength, as shown in left panel (Allen 2000, Astrophysical Quantities Table
11.25).

Since the solar light level is
measured in 525nm band, a
correction is made for the data
in other wavelengths,
according to the right panel.

Wavelength dependence of atmospheric transmission
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4. Results (1)


Photon signals recorded in the exposed CCD data were plotted as a function of
estimated solar light level.
Extrapolation to the 1 solar level gives the expected photons in flight.
Nearby continuum
2x2summing
Nearby continuum
1x1summing
examples
SP
FG/NFI 5250
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4. Results (2)
Summary of photon level in flight for all the wavelengths
Optical
path

(nm)
Gain
(e/DN)
CT
SP (left)
(right)
BFI
630.0
630.2
48
100
388.3
396.9
430.5
450.5
555.0
668.4
517.3
525.0
557.6
589.6
630.2
656.3
64
NFI
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Estimated photons at 1 solar
light (1x1 sum)
Photon
Exposure
count (DN)
time (msec)
873
frame
26922
48 frames
22811
48 frames
1245
100ms exp
2555
1000ms exp
4439
100ms exp
2431
100ms exp
664
100ms exp
1100
100ms exp
113
100ms exp
382
100ms exp
391
100ms exp
480
100ms exp
580
100ms exp
515
100ms exp
CCD well
depth
(%)
Longest exposure without
saturation (worst)
1x1 sum
2x2 sum
(msec)
(msec)
42%
40%
34%
144
703
40
74
271
163
1590
470
460
374
310
349
Note) Estimated photons for SP and NFI are for nearby continuum near the spectral line of
interest. The number of photons inside the spectral line is smaller than the values in the table.
75
353
24
38
136
082
620
228
228
200
162
180
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4. Results (3)
From throughput measurements of the flight model integrated SOT with natural
sun light, we have confirmed the light level in flight experimentally
Spectro-Polarimeter (SP)
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SP data will have suitable number of photons in flight.
The photon accumulated in each exposure (0.1sec) is 34-40% of the CCD full
well.
The signal-to-noise achieved with 4.8 sec (48 frame) accumulation is 1500
(0.07%). S/N with 3.2sec (32 frame) accumulation is 1235 (0.08%).
Broadband/Narrowband Filter Imagers (BFI/NFI)


In most of wavelengths, suitable exposure duration (100~500msec) can be
used to have suitable number of photons.
However, G-band (430.5nm) and blue continuum (450.5nm) may have
saturated pixels for bright features, even if the shortest exposure is used. We
are currently working to have additional ND filter before flight.
Correlation Tracker (CT)


CT data will have suitable number of photons in flight.
The expected photon level in flight is about 42% of the CCD full well.
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