Estimate on SOT light level in flight with throughput measurements in SOT sun tests

1. 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

2. 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.

3. 1. Solar Optical Telescope (SOT) Litrow Mirror Folding Mirror Polarizing BS 256 x 1024 CCD X3 Mag lens Polarizing BS Folding Mirror Folding Mirror Shutter Slit • Solar-B SOT (solar optical telescope) consists of optical telescope (OTA) and focal plane instruments (FPP). Grating Field lens Field lens Preslit X2 Mag lens Shutter Filterwheel Birefringent Filter 2048 x 4096 CCD FieldMask Telecentric lenses Secondary Beam Distributor Filterwheel 50 x 50 CCD Folding Mirror Demag lens Reimaging Lens Image Offset Prisms Folding Mirror Polarization Modulator OTA CommonOptics CT NFI BFI SP Primary CLU Tip Tilt Mirror Optical layout

4. 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. Test configuration

5. 2. Measurements(2) NAOJ Heliostat on the roof of clean room Sun light illuminated OTA full aperture Integrated SOT flight model

6. 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

7. 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: 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.

8. 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

9. 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 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).

10. 3.2. Transmission of heliostat mirrors and window Theliostat • 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. Measured transmission of heliostat mirrors and window glass

11. 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

12. 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

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

14. 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) • 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.