PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM PRODUCER_ID = "ISAS/JAXA" LABEL_REVISION_NOTE = " 2017-04-06, K. McGouldrick, S. Murakami: Initial version; 2017-04-24, K. McGouldrick: cleaned to conform to PDS format requirements; 2017-05-10, S. Murakami: Revised; 2017-07-03, S. Murakami: Revised; 2018-05-02, S. Murakami: Revised; 2018-09-14, S. Murakami: Revised; " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VCO" INSTRUMENT_ID = "UVI" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "ULTRA VIOLET IMAGER" INSTRUMENT_TYPE = "CAMERA" INSTRUMENT_DESC = " This summary of the UVI camera is compiled primarily from [YAMAZAKIETAL2018]. Instrument Overview =================== The Ultra Violet Imager (UVI) on Akatsuki measures the wind with accuracy of ~1 m/s, planetary/gravity waves associated with momentum transfer from waves to general circulation, and meso-scale cloud structure near local noon as the energy source of atmosphere. The coupling between the waves/meso-scale cloud and global winds is also important to understand the dynamics of the Venus atmosphere. Specification of UVI -------------------- The specification of UVI are summarized as follows: - Observation target Solar radiation scattered at cloud top - Optics design Camera with off-axial catadioptric optics - Observational wavelength 283 nm and 365 nm - Field-of-view 12 degrees - Spatial resolution ~200 m (at periapsis) ~76 km (at 60 Rv) - Optics - F-number 16 - Focal length 63.3 mm - Aperture size 39.89 mm (hood entrance) - Filter parameters - Bandwidth - 283-nm filter 14 nm - 365-nm filter 14 nm - diffuser 90 nm with 335 nm of center wavelength - Detector - CCD Si-CCD (back illuminated and full-frame transfer) - Pixel number 1024 x 1024 pixels - CCD control - Exposure time 4 msec -- 11 sec - Data depth 12 bit - A/D frequency 2 MHz - Weight - UVI-S 2.9 kg - UVI-AE 1.2 kg - Size - UVI-S 199 mm x 206 mm x 376 mm - UVI-AE 220 mm x 220 mm x 50 mm - Power - Stand-by mode 17 W - FW movement mode 34 W - Observation mode 19 W Scientific Objectives ===================== The research targets by UVI are: (1) Large-scale (1,000-40,000 km) to meso-scale (1-1,000 km) cloud morphologies, (2) Three-dimensional haze distribution, (3) Interaction between the lower and the middle atmosphere, (4) Generation, propagation and dissipation of planetary waves and gravity waves, and their interaction with general circulation, (5) Generation of super-rotation, (6) Distribution of unidentified ultraviolet absorber, (7) Distribution of SO2, and the photochemical processes related to H2SO4 formation, and (8) Cloud aerosol microphysical properties. Measured Parameters =================== The UVI takes images of the ultraviolet solar radiation scattered at the altitude of the Venusian cloud top in two wavelength ranges at the center at 283 nm and 365 nm. There is the SO2 absorption band around the 283 nm wavelength, and the 283 nm images clarify the distribution of SO2. The 365 nm images are with a high contrast, which were taken by almost all of previous Venus missions, but the material associated with the contrasting density is still unknown. Subsystems ========== The UVI instrument consists of two parts of a sensor (UVI-S) and an analog electronics (UVI-AE). The UVI-S consists of band pass filters installed in a filter wheel turret, off-axial catadioptric optics, and the detector of a charge-coupled device (CCD) with a preamplifier circuit. The UVI-AE includes a power supply unit and a control and readout circuits of the CCD detector. The electric power is 19 W in the observation mode, and 34 W at the filter wheel rotation. Detectors ========= UVI-S has a full-frame transfer CCD of a back illuminated type with a UV sensitive coating to enhance its quantum efficiency up to 70% in the observational wavelengths range, and has a saturation level of 120,000 electrons per pixel. It has the pixel size of 13 um, and three areas of the image area of 1024 x 1024 pixels, storage area, and optical black area. The counts of the first two areas are used for the data processing at DE (Sensor Digital Electronics Unit), and the counts of the last area are used for the smear correction at the ground data processing. The angular resolution is 0.012 degrees, which corresponds to the spatial resolutions of ~200 m and ~76 km on the cloud top level in the observations from the altitudes of ~1,000 km at the periapsis and ~60 RV (the radius of Venus) at apoapsis, respectively. The noise count rate of the CCD detector through the readout electronics tends to enlarge rapidly at the CCD temperature over -10 degrees Celsius, and has below 150 counts/sec at the temperature below a few degrees, which is achieved by being cooled with the radiator under the normal observation mode. The nominal exposure time of the Venus observation is decided as the time when the signal level becomes the half saturation level of the CCD device. The S/N ratio achieves over 10 for the raw image and over 100 after the onboard smear correction at the normal observation, which satisfies the scientific requirement. Electronics =========== The output count of the CCD has the data depth of 12 bits by the analog- to-digital converter. The data processing at the DE equipment, such as median filtering, subtraction of dark current, and desmearing, is performed as a 16 bits data to make one image with the 16-bits data depth. The data size of an image is diminished to several hundred KByte by the compression at the DE. Filters ======= The filter wheel has four positions. Two interference band pass filters and one diffuser are installed in the filter wheel of UVI. The interference filters select the observational wavelengths of 283 nm and 365 nm both with a bandpass width of 14 nm. The 283 nm images are enabling detection of SO2 absorptions [STEWARTETAL1979]. The 365 nm images reveal the absorptions of the unknown UV absorbers, whose strong contrasts enable us to track the cloud morphology easily, as all previous Venus orbiters observations had reported. The diffuser is used for the onboard calibration such as measurement of flat field and modification of relative sensitivity between pixels. The shutter position is used to obtain the noise counts. The wheel positions are adjusted by a stepper motor and two hall sensors to know the wheel rotation angle, and they are also used to automatically return the wheel to the shutter position. The transmittances of two interference filters and one diffuser installed in the filter wheel were measured before the launch. The measurement errors are within the size of marks in the figures. The 365-nm and 283-nm interference filters have the effective bandwidth of 14 nm and 14 nm, and the transmittance of 60% and 30% at the each observational wavelength, respectively. The diffuser has a broad bandpass of 90 nm including these two observational wavelengths with the transmittance of a few %. Filters specification are summarized as follows. Channel Wavelength [nm] Bandwidth [nm] Transmittance [%] -------- --------------- -------------- ----------------- shutter N/A N/A 0.0 283 nm 283 14 30 diffuser 335 90 2.8 365 nm 365 14 60 Optics ====== The UVI-S has an off-axial catadioptric optics that consists of two lenses and two reflecting surfaces. The optics has a composite focal length of 63.3 mm and an F-number of 16. The 12 degrees x 12 degrees field-of-view (FOV) can capture the whole Venus disk during 97% of one orbit, except ~8 hours near a periapsis. Onboard Data processing ======================= The exposure time is determined from 0.004 to 11 sec with 24 steps by a command. The chosen exposure times are 0.25 (or 0.50 after 6 June, 2016) sec and 0.046 sec at the Venus dayside observations of the 283- and 365-nm wavelengths, respectively, from ground-level calibration results. The CCD detector with no electrical cooler system is thermally in contact with a cooling radiator on the outside panel of the spacecraft body to reduce the dark current. The radiator has area to make the CCD temperature less than 9 degrees Celsius in the observation mode. The signal-to-noise ratio of raw images keeps over 10 with this temperature before image data processing. The CCD has no mechanical shutter, so a smear noise in transferring from the image area to the storage area on the chip degrades the signal-to- noise ratio of the obtained image, especially in the case of the short exposure operation. In the nominal operation of UVI, 18 images (6 Venus and 12 shutter images) are used for the smearing correction in the onboard data processing at the DE equipment. Venus images consist of 6 images taken with the 0-second and the chosen exposure time three times. Shutter images consist of 12 images taken in the same way before and after the Venus shots. Four images (Venus and shutter images with the 0-second and the chosen exposure time) are created form the each 3 images under the identical condition by using median filter. The Venus and shutter images with the same exposure time are used to remove the dark and noise count. The image without noise count with the 0-second exposure time is removed from the image with the chosen exposure time, which is a desmearing process. As a result of the data processing, the signal-to- noise ratio of the UVI image improves to over 100 in the case of the observation that Venus moves slowly in viewing from Akatsuki. Flow chart of ``taking median filtered image with smear correction'' -------------------------------------------------------------------- operation/computation obtained image ========================== ================================= +------------------------+ | take 3 images with | | zero-exposure time |-----------------+ +------------------------+ | V +---------------------------+ | (1,2,3) 3 images | +--------------| with 0-exposure time | | +---------------------------+ V +------------------------+ | take median of (1,2,3) |-----------------+ +------------------------+ | V +---------------------------+ | (a) median-filtered image | | with 0-exposure time |---+ +---------------------------+ | V +------------------------+ | | take 3 images with | | | finite exposure time |-----------------+ | +------------------------+ | | V | +---------------------------+ | | (4,5,6) 3 images with | | +--------------| finite exposure time | | | +---------------------------+ | V | +------------------------+ | | take median of (4,5,6) |-----------------+ V +------------------------+ | | V | +---------------------------+ | | (b) median-filtered image | | | with finite exposure time | | +---------------------------+ | | | +-------------------------------+ | | +----------------------------------------------+ | | V V +------------------------+ | subtract (a) from (b) |-----------------+ +------------------------+ | V +---------------------------+ | (c) median-filtered, | | smear corrected image | +---------------------------+ Flow chart of ``taking Venus image with smear correction, with median filter'' ------------------------------------------------------------------------ operation/computation obtained image ========================== ============================= +------------------------+ | take median-filtered | | dark image with |--------------+ | smear correction | | +------------------------+ V +---------------------+ | (c-1) dark image |--+ +---------------------+ | +------------------------+ | | take median-filtered | | | Venus image with |--------------+ | | smear correction | | | +------------------------+ V | +---------------------+ | | (c-2) Venus image |-----+ +---------------------+ | | +------------------------+ | | | take median-filtered | | | | dark image with |--------------+ | | | smear correction | | | | +------------------------+ V | | +---------------------+ | | +------------------| (c-3) dark image | | | | +---------------------+ | | | | | | +---------------------------------------+ | | | | V V | +------------------------+ | | take mean of | | | the two dark images,|--------------+ | | (c-1) and (c-3) | | | +------------------------+ V | +---------------------+ | +------------------| (d) mean dark image | | | +---------------------+ | | | | +------------------------------------------+ | | V V +------------------------+ | subtract dark image, | | (d), from Venus |--------------+ | image, (c-2). | | +------------------------+ V +---------------------------+ | (e) processed Venus image | +---------------------------+ When Venus is moving with a very fast speed as is observed around the periapsis time, the above method of the smear correction is not performed effectively. In this case the count in an optical black area, which is mechanically masked outside the imaging area, of CCD is used to make the smear correction at the ground. Calibration =========== The flat field image, the distortion image, and the spatial frequency response (SFR) were obtained as the total optical performance check of UVI-S. The image of a surface light source at the wavelength of 365 nm was obtained using the integrating sphere at the optical facility in the Earth Observation Research Center (EORC) at JAXA. The system is absolutely calibrated with the nonuniformity of the area light source of below 1%. The count rate derives the sensitivity of UVI throughout the filter, optical lens, and CCD detector before the launch. It is noted that the angular diameter of the integrating sphere aperture viewed from UVI was 12 degrees circle, and that the FOV of UVI is 12 degrees square. The count rate directed to the center area of the integrating sphere aperture is correct in order to derive UVI's total sensitivity. Therefore the direction of UVI field-of-view was changed into 3 x 3 directions to obtain images of the integrating sphere and the sensitivity of 9 areas of UVI field-of-view was independently calibrated. The averaged image created from the 9 images derived the sensitivity of the whole field-of-view. The flat field pattern was also obtained as Venus images taken by using the diffuser in the orbit around Venus. The diffuser images of Venus are used to create the calibration flat pattern for the data processing on the ground. The UVI image data are corrected by the flat pattern and did not have the clear nonuniformity caused from the sensitivity difference of CCD detector pixel by pixel. However, the flat pattern is not a perfect correction for photometry analysis which requires careful treatment of the brightness. It is because the brightness gradient of one image due to vignetting slightly remains. Therefore the factor (a flat conversion factor), which is based on the pixel sensitivity estimated from the calibration results using the integrating sphere before the launch, is also prepared in the data set. It is recommended that the image in the data set is used basically for morphology analysis and that the flat conversion factors are used for photometry studies. The product of the conversion factors and the released data for morphology analysis serves as the absolutely-calibrated brightness for photometry analysis. The distortion was measured by using black-light lamps with two different sizes of rectangle masks. One pattern has three 3.6-cm and one 10-cm wide masks. Four patterns were set up at one time at 20.2-m distance from UVI-S with 36-cm interval. UVI-S is mounted on a tilt-and-swivel base to change its line-of-sight direction to 39 positions (3 elevation angles x 13 azimuth angles). 39 images are superimposed to create one image. The image reveals that the distortion is -0.3% at the direction of 5.7 degrees angle away from the optical axis. Several images of a test chart were taken to estimate an index of the optical performance, SFR. The measured SFR indicates that the limiting resolution of UVI is 650 LW/PH with SFR of 5%. The total sensitivity of UVI is calibrated from the results of the ground experiments before the launch. The values of the 283- and 365-nm channels, which convert pixel count rate to radiance, are 9227 and 5020 [W/m**2/sr/m/count rate], respectively. The onboard calibration is performed based on the star observations after the launch. Star fields of Sagittarius and Scorpius were measured during cruising (Oct. 2010; before Venus orbit insertion) and in orbit (Feb. and Sept. 2016). The observed star flux is compared to a known value, so a calibration factor beta can be derived as beta = F_exp / F_obs where F_obs is the observed star flux by UVI, and F_exp is the known star flux. F_exp is calculated as Integral[ T(lambda) F_star(lambda), dlambda ] F_exp = ----------------------------------------------- Integral[ T(lambda), dlambda ] where lambda is wavelength, T is the transmittance profile of the 365-nm filter, and F_star is a star flux spectrum, which is taken from Pulkovo Spectrophotometric Catalog ([ALEKSEEVAETAL1996], the data was downloaded from http://cdsarc.u-strasbg.fr). The observed radiance [W/m**2/sr/um] of stars is converted to flux [W/m**2/um], multiplying a solid angle of one pixel, Omega_pix = (0.00021)**2 square radians. Then, the star flux F_obs has been calculated using the aperture photometry technique which is widely used in star flux calculations for ground-based observations [MIGHELL1999, LAHERETAL2012]. The 283-nm channel is also measured in the same method using data obtained by International Ultraviolet Explorer (IUE). The table below shows the summary of measured calibration factor beta for both of 365- and 283-nm channels. Date measured calibration factor (beta) at the 365 nm channel at the 283 nm channel ------------ --------------------- --------------------- 2010-10-08 1.63 +- 0.078 2016-02-08 1.49 +- 0.24 2016-09-08,9 1.58 +- 0.19 1.94 +- 0.16 " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "ALEKSEEVAETAL1996" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "LAHERETAL2012" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MIGHELL1999" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "YAMAZAKIETAL2018" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END