PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM PRODUCER_ID = "ISAS/JAXA" LABEL_REVISION_NOTE = " 2016-10-18, K. McGouldrick, S. Murakami: Initial version; 2017-02-09, K. McGouldrick: cleaned to conform to PDS format requirements; 2017-05-05, S. Murakami: Revised; 2017-06-26, S. Murakami: Revised; 2018-08-15, S. Murakami: Revised; " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VCO" INSTRUMENT_ID = "IR1" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "1-MICRON CAMERA" INSTRUMENT_TYPE = "CAMERA" INSTRUMENT_DESC = " This summary of the IR1 camera is compiled primarily from [IWAGAMIETAL2011]. Instrument Overview =================== IR1 [IWAGAMIETAL2011] was designed to image the dayside of Venus at 0.90 um wavelength and the nightside at 0.90, 0.97 and 1.01 um wavelengths, which are located in the atmospheric windows [TAYLORETAL1997]. These windows allow radiation to penetrate the whole atmosphere. Specification of IR1 -------------------- The specification of IR1 are summarized as follows: - Wavelength (bandwidth) - 0.90 um dayside filter 0.900 um (0.0091 um) for dayside - 0.90 um nightside filter 0.898 um (0.0289 um) for nightside - 0.97 um filter 0.969 um (0.0386 um) for nightside - 1.01 um filter 1.009 um (0.0391 um) for nightside - diffuser 0.750 um (0.4000 um) - Field of view 12 degrees x 12 degrees - IFOV (Pixel resolution) 0.20 mrad (0.012 degrees) - Optics (triplet optics) - F-number 8 - Focal length 84 mm - Detector Si-CSD/CCD - Number of pixels 1024 x 1024 - Pixel size 17 um x 17 um - Size 18 mm x 18 mm - Full well 700000 e- - Detector control - Exposure time 3 -- 100 sec - Dynamic range 14 bits - Noise level 0.77 mW cm**-2 sr**-1 um**-1 at 260 K (dayside) 1.3x10**-3 mW cm**-2 sr**-1 um**-1 at 260 K (nightside) - Weight - sensor 2.3 kg - electronics 3.7 kg (common to IR1 and IR2) - Size - sensor 51 cm x 28 cm x 21 cm - electronics 30 cm x 22 cm x 12 cm (common to IR1 and IR2) - Power - sensor 2.6 W (operation) - electronics 40.4 W (operation) (common to IR1 and IR2) Scientific Objectives ===================== The 1 micron camera named IR1 works both on the dayside and the nightside. On the dayside, it measures the 0.90 um solar radiation scattered by the clouds, and quantifies the horizontal wind vectors by using the cloud- tracking technique. By combining the information obtained at various heights by the other cameras and by the radio occultation, meteorological information such as the wind field and the distribution of eddy diffusion may be deduced. This will make it possible to investigate the generation mechanism of the super-rotation. On the nightside, it has three channels of 0.90, 0.97 and 1.01 um to detect thermal emission mostly from the surface and a little from the lowest atmosphere. The latter two channels are a differential absorption pair for measuring the surface H2O abundance with the 1.01 um channel as a reference. Although the center of the H2O band is located at 0.94 um, the 0.97 um channel is suitable for measuring the H2O abundance because of moderate absorption (it is too strong at the band center). H2O is one of the most important minor constituents in the lower atmosphere because of various reactions with surface minerals and a role for the greenhouse effects. Also it is related to the chemistry of the clouds, which are mostly made of H2SO4 and H2O. These nightside channels are also used to investigate the distribution of surface emissivity. Surface emissivities may be deduced from the measured radiances with a known surface temperature and a correction of the influence of overlying clouds (HASHIMOTO&SUGITA2003). Spatial variations of surface emissivities at near-infrared wavelengths were studied (e.g., HASHIMOTOETAL2008); however, little has been discussed about its wavelength dependence so far. Since each material shows each wavelength dependence of emissivity, observation of wavelength dependence will help us to constrain the surface material more confidently. Especially, the spectra of ferrous mineral such as olivine = (Mg,Fe)2SiO4 and ferric mineral such as hematite = Fe2O3 show different wavelength dependence each other in the 0.9 to 1.0 um region; it may be possible to discuss the redox state of Venus surface. Since redox state of Venus' surface is likely related to the escape of water, the evolution of Venus would be inferred from IR1 observation. Also IR1 will search for an active volcanism (HASHIMOTO&IMAMUR2001). On Venus, there are lots of landforms, which are related to volcanic activity; however, no active one has been found so far. Although Venus Express has not found any sign of active volcanism (MUELLERETAL2008), IR1 still has a chance to find it because of its much wider coverage of the surface owing to the equatorial orbit. Detection of active volcanism on Venus has a huge impact on the theory of evolution of planetary interior. Measured Parameters =================== On the dayside, it measures the 0.90 um solar radiation scattered by the clouds. On the nightside, it has three channels of 0.90, 0.97 and 1.01 um to detect thermal emission mostly from the surface and a little from water vapor in the lowest atmosphere. Subsystems ========== IR1 camera is composed of a baffle, filters, lenses, a detector and electronics. The specifications of IR1 camera are listed in Table 1. The baffle is an important component because the solar direction is not far away from the line of sight during the nightside observation. It is designed to reduce the solar contamination less than 2 x 10**-6 of the initial when the solar direction is 26 degrees or more away from the line of sight and within plus or minus 8 degrees from the reference plane. Detectors ========= The detector is a 1024 x 1024 array of Si-CSD (charge sweeping device) / CCD cooled down to 260 K to achieve S/N ratios of 300 for the dayside measurements and of 100 for the nightside measurements. Those S/N ratios are from scientific requirements based on previous measurements. For example, the former comes from the dayside contrast of 3% found by the measurements on board Galileo (BELTONETAL1991B), and the latter from the nightside contrast of some tens percent found by the ground-based measurements (MEADOWS&CRISP1996). The readout scheme in the IR1 sensor is a combination of Charge-Sweep Device (CSD) and Charge-Coupled Device (CCD) transfers. Electrons in a pixel is firstly CSD transferred (vertically) to the upper or lower edge of the chip and then CCD transferred (horizontally) to the corner of the chip. The chip is electronically divided to 4 independent quadrants each of which is read through an amplifier at the corresponding corner. Because of this architecture, correction for the slight gain differences between quadrants is the primary purpose of the IR1 flat-fielding. Electronics =========== The control electronics IR-AE (common with IR2) digitizes the IR1 pixel data (number of electrons measured in the form of voltage) with a depth of 14 bits and a conversion factor of 70 electrons per ADU. The full well of the sensor, 10**6 electrons, corresponds to ~14,000 which is just below the largest 14-bit unsigned integer. Filters ======= The filter wheel has six positions on which four interference filters, a diffuser and a lid are placed. One of four filters is for the dayside measurement at 0.90 um. Other three filters are for the nightside measurements at 0.90, 0.97 and 1.01 um. They are synthesized from the measured transmissions and the wavelength shifts expected for their inclination of 7 degrees and the field of view of 6 degrees in half-cone- angle. Note that the peak transmission of the dayside filter is as small as 0.27% because the same detector is used for both the dayside and the nightside measurements. The inclination of 7 degrees aims at avoiding ghosts, which may be produced by unwanted multiple reflections between the optical components. Optics ====== The lens system is a triplet with a main wavelength at 0.90 um and a focal length of 84 mm. Although the main wavelength was 1.01 um at the earlier stage of the present project, it was changed to 0.90 um. This was because gradation of the image due to diffusion of charge in pixels was worried much more for the 1.01 um channel than for the 0.90 um channel. Later, the actual spot sizes were measured by using a collimator with a focal length of 90 cm. Their FWHMs (full width of a half maximum) were 1.5 pixel at 0.90 um and 2.5 pixel at 1.01 um. That is, the worry did not come true fortunately. They correspond to 24 and 40 km, respectively, horizontal resolution on the Venus surface seen from the apoapsis (13 Venus radii = 79000 km above the surface). The 40 km spatial resolution of the 1.01 um nightside channel is not so disappointing because of the smearing effect due to the clouds (HASHIMOTO&IMAMUR2001); a point light source on the surface appears as a spot of 100 km in diameter when it is seen through the clouds. Onboard Data processing ======================= The nominal ``dayside'' observing sequence is to acquire 3 Dark images (``pre'' Darks), 3 Venus images at 0.90 um, and 3 Dark images (``post'' Darks). Each set of 3 consecutive images is median processed and the resultant ``pre'' and ``post'' Darks are averaged for subtraction from the Venus image. These are done on the spacecraft and ``pre'' and ``post'' Darks as well as the Venus image are stored on the data recorder (DR) for downlink to the ground. The nominal ``night-side'' observing sequence is divided to 2 steps. In the first step, 3 ``pre'' Darks, 3 Venus images at 0.97 um, 3 Venus images at 1.01 um, and 3 ``post'' Darks are acquired. The image processing is essentially the same as the ``day-side'' sequence. The second step of the ``night-side'' observing sequence is to acquire 3 ``pre'' Darks, 3 Venus images at 0.90 um, and 3 ``post'' Darks. Calibration =========== The flat-field measurement and determination of the absolute sensitivity were carried out at Tsukuba Space Center by using a one meter integration sphere with a known radiance. IR1 has a diffuser to obtain a flat-field with a spatial scale of one degree or so during the flight with the Venus's dayside as a light source. However, a flat-field with a larger spatial scale of several degrees is difficult to be obtained inflight, and is measured with the integration sphere. The sensitivity fluctuations measured are about 3% peak-to-peak in the main part of the field of view. For the 0.90 um dayside channel, input radiance of 37 mW cm**-2 um**-1 sr**-1 (77% of the nominal input) resulted an output of 633 ADU/s (analog-to-digital conversion unit) = 44300 e-/s (65% of the nominal output); that is, the measured sensitivity was proved to be 84% of the designed. For the 0.90 um nightside channel, an input radiance of 55 x 10**-3 mW cm**-2 um**-1 sr**-1 (37 times as large as the nominal input) resulted an output of 565 ADU/s = 39600 e-/s (27 times as large as the nominal output); that is, the measured sensitivity was proved to be 73% of the designed. The nominal exposure durations are 3 -- 6 s and 10 -- 30 s, respectively, for the dayside and nightside measurements. The thermal noise measured is 130 ADU/s at 290 K, which will be reduced to 1/10 when the detector is cooled down to 260 K. The depth of the well is measured to be around 10000 ADU = 700000 e-. The S/N ratio of the actual measurements is mostly determined by the statistical noise. The thermal noise of 15 ADU/s at 260 K is usually unimportant. The expected S/N ratio for the measurements may be estimated as follows: Since the expected dayside output is 1000 ADU/s = 70000 e-/s, the expected S/N ratio due to statistical noise is 0.2%; this satisfies the scientific requirement of S/N = 300 for the 0.90 um dayside measurements. The value 1000 ADU/s is based on the expected dayside radiance and the measured sensitivity noted above. Since the expected nightside output is 21 ADU/s = 1500 e-/s, the expected S/N ratio due to statistical noise is 0.8%; this satisfies the scientific requirement of S/N = 100 for the 0.90 um nightside measurements. The value 21 ADU/s is also based on the expected nightside radiance and the measured sensitivity. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "BELTONETAL1991B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HASHIMOTO&IMAMUR2001" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HASHIMOTO&SUGITA2003" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HASHIMOTOETAL2008" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "IWAGAMIETAL2011" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MEADOWS&CRISP1996" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "TAYLORETAL1997" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END