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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 elem

ents located in OTA and FPP before illuminating CCDs. Natural solar light was fed to the integrated SOT flight model in two su

n-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 exposur

e-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) Solar-B SOT (solar optical telescope) consists of optical

telescope (OTA) and focal plane instruments (FPP).

Secondary

Primary

CLU

Polarization Modulator

Tip Tilt Mirror

Reimaging Lens

Beam Distributor

Folding Mirror

2048 x 4096 CCD

Polarizing BS

Birefringent Filter

Filterwheel

Field Mask

Field lens

Shutter

X2 Mag lens

Folding Mirror

Folding Mirror

Telecentric lenses

X3 Mag lensShutter

Field lens

Filterwheel

Litrow Mirror

256 x 1024 CCD

Polarizing BSFolding Mirror

Slit

PreslitGrating

Folding MirrorImage Offset Prisms

Demag lens

50 x 50 CCD

OTACommon OpticsCTNFIBFISP

Optical layout

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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 exposure-duration in this test condition.

* Dark frames were also obtained to subtract dark signals from the exposed CCD data.

OBU

OTA

FPP

Calibrated PSD sensor

Heliostat flat mirrors

Natural sun light

NAOJ clean room

Entrance window

OBU

OTA

FPP

Calibrated PSD sensor

Heliostat flat mirrors

Natural sun light

NAOJ clean room

Entrance window

Test configuration

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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 I V T Tatmos heliostat/ ( / . ) ( )0 816 ,

where V the voltage output from PSD sensor Tatmos(l) coefficient for correcting wavelength dependence of

the atmospheric absorption Theliostat 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..

Diamonds: measurementsSolid curve: fitted

The measurements were made a few times in May – June 2004 on the roof of NAOJ clean room building.

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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±0.003which 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 Theliost

at 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.

NAOJ heliostat 2005/7

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

350 400 450 500 550 600 650 700 750

wavelength (nm)

Tran

smiss

ivity

(Rat

io)

2005/7/27

NAOJ Clean Room Heliostat - 2004/ 5/ 11, 8/ 2, 9/ 21 -

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

350 400 450 500 550 600 650 700 750Wavelength (nm)

Tran

smis

sivi

ty (R

atio

)

Average 2004/ 5/ 11

2004/ 9/ 21

Estimated from 525 nm measuremet(2004/ 8/ 2)

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).

Wavelength dependence of atmospheric transmission

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.

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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.

SP FG/NFI 5250

2x2summing

1x1summing

Nearby continuum

Nearby continuum

examples

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4. Results (2)Summary of photon level in flight for all the wavelengths

Estimated photons at 1 solar light (1x1 sum)

Longest exposure without saturation (worst)

Optical path

(nm)

Gain (e/DN)

Photon count (DN)

Exposure time (msec)

CCD well depth (%) 1x1 sum

(msec) 2x2 sum (msec)

CT 630.0 48 873 frame 42% 26922 48 frames 40% SP (left)

(right) 630.2 100

22811 48 frames 34%

388.3 1245 100ms exp 144 75 396.9 2555 1000ms exp 703 353 430.5 4439 100ms exp 40 24 450.5 2431 100ms exp 74 38 555.0 664 100ms exp 271 136

BFI

668.4

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1100 100ms exp 163 082 517.3 113 100ms exp 1590 620 525.0 382 100ms exp 470 228 557.6 391 100ms exp 460 228 589.6 480 100ms exp 374 200 630.2 580 100ms exp 310 162

NFI

656.3

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515 100ms exp

349 180

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.

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4. Results (3)

Spectro-Polarimeter (SP) 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%).

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.

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.

From throughput measurements of the flight model integrated SOT with natural sun light, we have confirmed the light level in flight experimentally