Characterization of the integrating sphere for the on-ground calibration of the SIMBIOSYS instrument for the BepiColombo ESA mission Vania Da Deppo*a,b, Elena Martellatoc,b, Guglielmo Rossid,j, Giampiero Nalettoe,a,b,f, Vincenzo Della Corteg, Fabrizio Capaccionig, Gianrico Filacchioneg, Michele Zusih, Pasquale Palumboi, Gianluca Aroldij, Marco Baronij, Donato Borrellij, Leonardo Tommasij, Michele Damij, Iacopo Ficai Veltronij, Enrico Flaminik, Gabriele Cremoneseb a
CNR-Institute for Photonics and Nanotechnologies UOS Padova LUXOR, Via Trasea 7, 35131 Padova, Italy b INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy c Department of Physics and Astronomy ‘G. Galilei’, University of Padova, Via Marzolo 8, 35131 Padova, Italy d Department of Earth Science, University of Florence, Via La Pira 4, 50121 Firenze, Italy e Dept. of Information Engineering, University of Padova, Via Gradenigo 6/B, 35131 Padova, Italy f CISAS G. Colombo, University of Padova, Via Venezia 15, 35131 Padova, Italy g INAF-IAPS, Via del Fosso del Cavaliere 100, 00133 Roma, Italy h INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy i Dip. Scienze Applicate, Universitá Parthenope, Centro Direzionale, Isola C4, 80143, Napoli, Italy j Selex ES, Via Einstein 35, 50013 Campi Bisenzio (FI), Italy k Agenzia Spaziale Italiana, Via del Politecnico, 00133 Roma, Italy ABSTRACT SIMBIOSYS is a highly integrated instrument suite that will be mounted on-board BepiColombo, which is the fifth cornerstone mission of the European Space Agency dedicated to the exploration of the planet Mercury and it is expected to be launched in 2016. The SIMBIOSYS instrument consists of three channels: the STereo imaging Channel (STC), with broad spectral bands in the 400–950 nm range and medium spatial resolution (up to 50 m/px); the High Resolution Imaging Channel (HRIC), with broad spectral bands in the 400–900 nm range and high spatial resolution (up to 5 m/px), and the Visible and nearInfrared Hyperspectral Imaging channel (VIHI), with high spectral resolution (up to 6 nm) in the 400–2000 nm range and spatial resolution up to 100 m/px. The on-ground calibration system has to cover the full spectral range of the instrument, i.e. from 400 to 2000 nm, and the emitted radiance has to vary over a range of four decades to account for both simulations of Mercury surface acquisition and star field observations. The methods and the results of the measurements done to calibrate the integrating sphere needed for the on-ground radiometric testing of the SIMBIOSYS instrument will be given and discussed. Temporal stability, both on short and long periods, spatial and spectral uniformity, and the emitted radiance for different lamp configurations and different shutter apertures have been measured. The results of the data analysis confirm that the performance of the integrating sphere is well suited for the radiometric calibration of all the three different channels of the SIMBIOSYS instrument. Keywords: calibrations, integrating spheres, radiometric calibration, radiometric model, optical testing, optical simulation *
[email protected]; phone +39-049 9815639; fax +39-049 774627 Space Telescopes and Instrumentation 2014: Optical, Infrared, and Millimeter Wave, edited by Jacobus M. Oschmann, Jr., Mark Clampin, Giovanni G. Fazio, Howard A. MacEwen, Proc. of SPIE Vol. 9143, 914344 · © 2014 SPIE CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2057349 Proc. of SPIE Vol. 9143 914344-1
1. INTRODUCTION BepiColombo is the fifth cornerstone mission of the European Space Agency (ESA) foreseen to be launched in July 2016 with the aim of studying in great detail Mercury, the innermost planet of the Solar System [1]. Mercury is very important from the point of view of testing and constraining the dynamical and compositional theories of planetary system formation. In fact, being in close proximity to the Sun, it has been subjected to the most extreme environmental conditions, such as high temperature and large diurnal variation, rotational state changes due to Sun induced tidal deformation, surface alteration during the cooling phase, and chemical surface composition modification by bombardment in early history. Mercury has been studied by the Mariner 10 spacecraft (S/C) in 1974-75, which imaged at low resolution (scale factor of about 1-2 km/px) less than half of the planet surface and discovered the presence of a magnetic field very similar to the terrestrial one. Since then, the only other S/C having reached Mercury is the NASA Messenger, which has been recently inserted in orbit around Mercury after three flybys with the planet itself in 2008-2009 [2][3]. The BepiColombo S/C will consist of two modules: the Mercury Planet Orbiter (MPO) [4][5], realized in Europe, carrying remote sensing and radio science experiments, and the Mercury Magnetospheric Orbiter (MMO) [6], realized by JAXA in Japan, carrying field and particle science instrumentation. These two complementary packages will allow to map the entire surface of the planet, to study the geological evolution of the body and its inner structure, i.e. the main MPO tasks, and to study the magnetosphere and its relation with the surface, the exosphere and the interplanetary medium, i.e. MMO targets. 1.1 The SIMBIOSYS suite The MPO module is carrying instruments which are devoted to the close range study of Mercury surface, to the investigation of the planet gravity field and to fundamental science and magnetometry. Imaging and spectral analysis are performed in the IR, visible and UV ranges. These optical observations are complemented by those of gamma-ray, X-ray and neutron spectrometers, which yield additional data about the elemental composition of the surface, and by those of a laser altimeter dedicated to high accuracy measurements of the surface figure, morphology and topography. MPO is characterized by an elliptical inertial polar orbit with periherm and apoherm altitudes of 400 km and 1500 km respectively, and a 2.3 hours orbital period, which is extremely challenging due to the thermal constraints on the S/C. These orbital properties are mainly determined by the need for the remote sensing instruments to have a relatively high spatial resolution that remains nearly the same all over the surface during the one year nominal mission lifetime. For a continuous observation of the planet surface during the mission, the S/C is 3-axis stabilized with the Z-axis, corresponding to payload boresight direction, pointing to nadir. The imaging and spectroscopic capability of the MPO module will be exploited by the Spectrometers and Imagers for MPO BepiColombo Integrated Observatory SYStem (SIMBIOSYS), an integrated system for imaging and spectroscopic investigation of the Mercury surface [7]. A highly integrated concept is adopted to maximize the scientific return while minimizing resources requirements, primarily mass and power. SIMBIOSYS incorporates capabilities to perform 50-200 m spatial resolution global mapping in stereo mode and color imaging in selected areas, high spatial resolution imaging (5 m/px scale factor at periherm) in panchromatic and broadband filters, and imaging spectroscopy in the 400 - 2000 nm spectral range. This global performance is reached using three independent channels: the STereoscopic imaging Channel, STC [8]; the High Resolution Imaging Channel, HRIC [9]; and the Visible and near-Infrared Hyperspectral Imager, VIHI [10]. Each of the channels of the SIMBIOSYS instrument has to be fully calibrated on-ground in order to characterize the geometrical, radiometrical, and spectral responses; for STC a dedicated stereo calibration procedure is foreseen too [11]. To carry out the calibration activities, an ad hoc Optical Ground Support Equipment has been conceived and tested at Selex ES premises. A dedicated mirror collimator has been built for HRIC calibration [12] and a dioptric one has been realized to be used both for STC and VIHI channels. For the radiometric calibration, the range of the fluxes needed to calibrate the system has been determined thanks to the radiometric models developed for each channel. The resulting expected radiance will vary over a range of four decades of magnitude since it accounts for two very different types of observations: acquisitions of the Mercury surface and of star fields. Indeed, calibrated stars are foreseen to be used as targets both for geometric and radiometric in-flight
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calibration. To accomplish the on-ground calibration of SIMBIOSYS, the chosen integrating sphere has four halogen lamps, and one of them is equipped with a shutter in order to span the full required radiance range. A complete set of measurements has been done to fully characterize the integrating sphere in all the expected spectral and radiance ranges. The radiance emitted by the sphere was characterized for different lamp configurations and different shutter apertures. The temporal stability on both short and long periods, and the spatial and spectral uniformity have also been measured. In this paper, by presenting the experimental set-up used for the calibration of the integrating sphere and the results of the data analysis, we aim to confirm that the performance of the integrating sphere is well suited for the radiometric calibration of all the three different channels of the SIMBIOSYS instrument.
2. EXPECTED IN-FLIGHT RADIANCE 2.1 Expected Mercury surface spectral radiance during regular observation For each of the SIMBIOSYS channels a radiometric model has been developed for simulation purposes in order to predict the expected in-flight input flux and integration time [13] [14][15]. This radiometric model allows to determine the expected instrument input radiance during the orbit around the planet and during the different phases of the mission. As a consequence of the planet high eccentricity orbit (0.206), the distance from the Sun varies from 0.31 AU at perihelion (true anomaly 0°) to 0.47 AU at aphelion (true anomaly 180°), thus the expected radiance of the light reflected by the Mercury surface varies from aphelion to perihelion by a factor of 2.3. In addition during the day side arc of the S/C orbit the radiance is varying with latitude. HRIC Latitude=30 deg
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c) Figure 1. Typical expected input radiance for the SIMBIOSYS channels: in a) HRIC; in b) STC, in c) VIHI.
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The radiometric model is designed to be as modular and flexible as possible to cope with the uncertainty of the input parameters. For example, the albedo of the planet can be assumed to be constant in wavelength, with a typical value of 0.12; or it can range between 0.08 and 0.45 respectively for smooth plain and bright craters. However the albedo can as well vary as a function of wavelength. The radiance of the Sun can be estimated through a black body curve at 5800 K or with a reference measured spectrum. Therefore, different scenarios have been simulated in order to set minimum and maximum expected radiance values. For each SIMBIOSYS channel different ranges of true anomaly (ta), depending on the observation strategy of each channel observation, have been considered and the latitude of the observation, i.e. typically periherm, 30° and 80°, has been taken into account as well in the simulation. In Figure 1 the expected typical radiance for each of the channel at 30° latitude and different ta is shown for reference. 2.2 Expected radiance during stellar calibration For the SIMBIOSYS instrument in-flight radiometric calibration verifications are foreseen through the observation of selected star fields [16] [17] [18]. Solar-type stars with 6≤mag≤8 will be considered for the in-flight calibration. Assuming that the counts due to the star will be distributed over a 2x2 pixel area, the estimated radiance at 600 nm for a mag=6 and a mag=8 star will be respectively of the order of 3 10-3 W/m2/µm/sr and 3 10-4 W/m2/µm/sr [19] for STC and 100 times higher for HRIC.
3. INTEGRATING SPHERE CHARACTERISTICS AND MEASUREMENT SET-UP 3.1 The integrating sphere For the on–ground radiometric calibration of SIMBIOSYS, the commercially available USS-2000-C integrating sphere (IS) produced by Labsphere is used. This sphere has an internal diameter of 50 cm and a 20 cm diameter output port. It is equipped with 4 tungsten halogen lamps; all the lamps are driven with constant current in order to have the same color temperature of 3000 K, but they have different powers. One of the lamp has 100 W power, and it’s equipped with a variable aperture shutter in order to modulate the output radiance, the other three are 35 W lamps. Each of the lamps can be power on and off independently from the others. Table 1. Summary of the specification of the IS Labsphere USS-2000-C. IS specification Inner diameter
20’’ (50 cm)
Aperture diameter
8’’ (20 cm)
Coating of inner wall Sphere coating reflectance
Spectraflect 98%
Lamp configuration (number and rated power)
three 35 W one 100 W(*)
Lamp type
Tungsten halogen
Color Temperature
3000 K
Current stability
3.07 A ±0.1% 8.33 A ±0.1%
(*)
Equipped with shutter
Peak Radiance (at 900 nm)
550 (W/m2/sr/µm)
Luminance Uniformity (at maximum radiance level)
>98%
Internal monitor detector
Silicon detector
In addition, on the internal surface of the sphere, a monitoring photodiode (MP) measures the integrated (over its spectral response curve) flux in order to gauge the stability of the emitted radiance. The MP is a standard silicon detector without
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filters. The coating of the sphere is Spectraflect. See Figure 2 for the typical reflectance of the coating and system configuration. The IS specifications are summarized in Table 1.
TYPICAL SP ECTRAFLECT° REFLECTANCE DATA
Typical Percent Reflectance vs. Wavelength of Spectraflect® 100 90 0
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40 250
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a)
b)
Figure 2. In (a) typical reflectance vs wavelength for the sphere coating [20], in (b) integrating sphere picture [21].
The sphere output spectral radiance and the photopic mapping uniformity at the exit port have been calibrated by the producer for the configuration in which all the lamps are switch-on and the shutter is completely open. See Figure 3 for measured spectral radiance curve and uniformity map.
Integrating sphere radiance
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Figure 3. In (a) IS spectral radiance at full illumination; in (b) photopic mapping of the exit port.
In order to adopt this IS as the source for the SIMBIOSYS radiometric calibration, the spectral radiance and the uniformity of the IS for different lamp configurations and different aperture of the shutter have to be accurately known. Therefore a dedicated set-up has been conceived to: a) measure the spectral radiance at the center of the output port in different lamp configurations and at different shutter apertures;
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b) measure the time, after switch on, needed for the sphere to reach a good stability of the output radiance (long term stability); c) measure the radiance stability over a short period of time when the IS is supposed to be already stable (short term stability); d) measure the spatial uniformity at the exit port for different lamp configurations and at the same time verify the spectral spatial uniformity in different points. 3.2 Measurement equipment and set-up The instruments used in the laboratory for the measurements described above are: a calibrated spectroradiometer (FieldSpec Pro (FS)) and a calibrated external photodiode (UDT radiometer). The FS is able to perform absolute, or relative, measurements of spectral radiance and irradiance. It covers the 350-2500 nm spectral range using two different techniques and equipments for the ranges 350-1050 nm (VNIR) and 950-2500 nm (SWIR). The spectral sampling in the VNIR and SWIR regions are respectively: 1.4 nm with 3 nm of spectral resolution and 2 nm with 10-12 nm spectral resolution. A silicon detector is used in VNIR, two InGaAs are used in the SWIR. The radiometric calibration accuracy of the FS is ±5% from 400 to 900 nm (UV/VNIR), raising to ±8% at 2200 nm over a 10 to 30°C temperature range. These values have been determined by viewing a stable, NIST traceable, radiance source with the FS and from the radiometric accuracy of the calibration source quoted by its manufacturer. FS field of view is 25°, but for these measurement it is coupled to a foreoptics having 1° FoV and 1” aperture. The typical response of the UDT is depicted in Figure 4a. The measurement accuracy is of the order of 0.2% or 1.2% +1 count depending on the range used. Typical UDT responsivity 0.6
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Figure 4. In (a) typical UDT response with wavelength. In (b) projected area of the UDT radiometer over the IS exit port.
The UDT is mounted with a foreoptics having 1.5° FoV and it has been positioned at a distance from the IS such that the analyzed area is of about 4 cm in diameter. The size of projected area over the IS exit port can be verify through the use of a service led lamp mounted in place of the UDT (see Figure 4b). FS and UDT have been placed at approximately 60 cm from the IS output port and as perpendicular as possible to the exit aperture. Through a translation XYZ stage FS and UDT can be moved, with mm resolution. Both instruments can be moved along the x axis approximately perpendicular to the IS exit port and in the plane yz orthogonal to it. Since the FS and UDT can be mounted together, the movement in the yz plane allows to map all the exit port emission characteristics both in terms of spectral radiance and radiance uniformity. The drawback is that FS and UDT are mounted few cm apart and so they are sampling slightly different regions on the exit port when they are acquiring simultaneously. A PC is set-up to control the IS lamps and shutter and also to acquire data from the external and internal photodiodes.
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4. DATA ACQUISITION AND RESULTS 4.1 Spectral radiance characterization The spectral radiance characterization at the IS exit port was fulfilled through the FS, which has been aligned with the center of the IS exit port with an accuracy of about ±5 mm. During all the measurements, the illumination stability of the sphere has been maintained by a stabilized current source for the lamps, and it was monitored via MP. In particular, the MP provides a numerical value related to the IS internal radiance; in fact the internal radiance is integrated by MP in a solid angle of 2π, over its sensible area and weighted by its responsivity. In addition, the MP allows to measure the relative luminous flux variation as a function of the shutter opening. A sketch of the radiance measurement set-up is depicted in Figure 5.
Surface I
Integrating
El
G
Sphere
-60 cm
FS
Al Figure 5. Sketch of the measurement setup used for the radiance characterization. The translation stage is placed on the track along xaxis with index 75 cm, which corresponds to a distance of about 60 cm between the FS optics and the emitting surface of IS.
The radiance measurement was performed at different luminous conditions of the IS: 1) 2) 3) 4)
4L - all the lamps on (three 35 W and one 100 W) 3L - three lamps on (two 35 W and one 100 W), 2L - two lamps on (one 35 W and one 100 W) 1L - one lamp on (100 W)
Also intermediate radiance configurations, reached by moving the shutter by steps of 10 or 20%, have been tested. For the configurations with shutter at 100%, the measurements were done acquiring the signal 30 times, otherwise, for the shutter characterization, measurements were repeated 10 times. Each acquisition of the FS is actually a mean over 100 acquisitions. 4.1.1 Spectral radiance results The data acquired for the different cases have been analysed. The mean and the standard deviation of the measurements done with the FS have been computed. For the configuration with all the lamps on and the shutter completely open, the measured radiance is in good agreement with the expected results (see Figure 6a). In Figure 6b the radiance measured for all the different lamp configurations (4L, 3L, 2L, 1L) is shown. Taken the configuration 4L as reference, the levels of the radiance are approximately 0.77 for 3L, 0.55 for 2L and 0.30 for 1L.
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Figure 6. In a) FS measured radiance is compared with the expected one for the configuration with all the lamps on. In b) the measured radiance for all the lamp configurations is shown. 1.0005
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Ratio of SMARTS ........ Irradiance @ 1.800 km to Irradiance 01.801 km for 3 atm -cm 1120 0.9975 (5 nm resolution)
MODT RAN dires .coon putation of
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Figure 7. In a) FS radiance measurement standard deviation. In b) the radiance ratio for the case with 4 lamps on is compared with water vapor absorption bands in the range 800-2000 nm [22].
In Figure 7a the standard deviation of the measurements is drawn. It is clearly visible that there are wavelength ranges in which the dispersion of the measurements is greater than in other ones. The peaks of the standard deviation can be due either to changes of the FS configuration, or to water vapor (or other atmospheric gases) absorption bands. In particular, the peaks at 1000 and 1830 nm are due to a detector change, whereas, the peaks at 940, 1120, 1400 and 1900 nm are perfectly matching the expected water absorption bands. In Figure 7b the radiance ratio for the configuration 4L is directly compared with water vapor expected absorption bands. The water vapor absorption bands are clearly visible, in particular those at 940 nm, 1120 nm, 1400 nm and 1900 nm. In the range 400-1000 nm the maximum variation of the radiance is 0.25%; in the range 1000-1300 nm maximum variation of 0.25% is at the water band 1120 nm, outside the variation is 0.15%; in the range 1300-1600 nm maximum variation is
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0.85% at the water band (1350-1450 nm) outside it’s 0.06%; in the range 1600-2000 nm maximum variation is 1.2% at the water band (1900 nm) outside, below 1800 nm, it’s 0.05% [22]. 3L/4L 2L/4L 1L/4L
1.06
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1.04 1.02 1.00 0.98 0.96 0.94 0.92 0.90 500
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Figure 8. Normalized radiance ratio for the different lamp configurations (4L, 3L, 2L, 1L) considered for spectral radiance measurements.
Taking the configuration 4L (all the lamps on and shutter completely open) as reference, the normalized radiance ratio for the other configurations, i.e. 3L, 2L and 1L, can be calculated. Figure 8 shows that the spectral behavior is stable within ±2% within the range that will be used for the calibration (400-2000 nm) except for the water band at 1900 nm. A nitrogen flux is foreseen for the IS during VIHI calibration to reduce the water vapor absorption in the IR and the distance between the IS and the instrument should be maintained as short as possible. 4.2 Stability characterization The stability of the IS lamp light flux has been characterized on a long and a short period of time. Test consisted in 2 types of data collection: long-term, over a period of about 2 hours, and short-term over 15 minutes. Each test was done with all the lamps on in configuration 4L. Long- and short- data were collected using both the UDT and MP. A scheme of the set-up used is depicted in Figure 9.
X Y
Emitting
-, Integrating
\
Sphere
Surface G
a
`NN
y Figure 9. Sketch of the measurement setup used for the time stability characterization. The frame is placed on the track along x-axis with index 75 cm, which corresponds to a distance of about 60 cm between the UDT optics and the emitting surface of IS.
The stability is analyzed on the data acquired with UDT, whereas those acquired with MP were mainly used as monitoring. For the long period stability characterization, the photodiodes (UDT and MP) readings are acquired with 1 s sampling rate after the switching on of the lamps. Short term stability was characterized through the data of both UDT
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and MP using a subsample of the measures taken during the long term stability campaign and considering data taken at least 15 minutes after lamp switch on. 4.2.1 Long term stability results For the long term measurements the sphere lamps were powered on and the data collection began immediately. The detector current was read every 1 s for UDT and every about 1.2 s for MP. Both on UDT and MP readings it’s clearly visible an exponential trend (see Figure 10), suggesting that while the lamps of the sphere are warming up, the emitted flux is increasing. It increases very rapidly in the warming phase approximately 15-20 min from switch on; it continues to slightly increase even afterwards. Between 15 and 30 min after switch on, the signal acquired with UDT is increasing of about 0.1% and on the whole time span between 15 min and the end of the measurement (2h 15min) the UDT signal is increasing of about 0.3%. 5.00x10° -4
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Figure 10. Data acquired during the long term stability test starting from the switch on of the system. In a) UDT acquired data plot; in b) MP one.
The test has been repeated four times showing the same behavior each time. 4.2.2 Short term stability results The short term stability can be tested at different times after switch on. We considered 3 different 15 min intervals, the first one is between 15 and 30 min after switch on, the other two are after one and two hours from switch on. Data were collected every second with UDT, and monitored every 1.2 s with MP.
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Figure 11. Short term stability analysis. The relative deviation from the mean over 15 min of acquisitions is plotted. In a) deviation measured with UDT just after warm up (between 15 and 30 min from switch on). In b) deviation calculated 1 hour after switch on both from UDT and MP data.
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Between 15 and 30 minutes after switch on the relative deviation from the mean measured with UDT is about 0.1% (see Figure 11a). One hour after switch on the relative deviation from the mean measured with the UDT is of about 0.04%, slightly higher with MP 0.1%, anyhow extremely small (see Figure 11b). Analysis of the data taken 2 hours after switch on shows relative deviation from the mean less than 0.02% both for UDT and MP acquired data. 4.3 Spatial uniformity To assess the characterization of the IS spatial uniformity, the UDT, mounted on the translation stage at approximately 60 cm from the sphere output port, was methodically moved by horizontal and vertical prefixed steps in order to explore different zones of the emitting surface (see Figure 12a). Twenty-one different points have been used to map the output port, for each considered point the configurations tested are 5: the same configurations described for the radiance characterization, i.e. 4L, 3L, 2L, 1L with shutter at the maximum aperture (100%), and the 1L configuration with the shutter at 25% aperture (1L_25). For each of the points, after lamps switch off they are switched on in the same following order: 1, 2, 4, and each time the measures are done after 5 min to account for lamp stabilization; the emitted flux is constantly monitored with MP. The acquisition time for UDT was 1 min for each position, with 1 s sampling. The IS radiance was simultaneously acquired also with FS.
1.008
1 il& 41111 mir,w.Famrri
...... IkirTiFIWWW
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Figure 12. In a) a sketch of the sampling of the IS emitting surface. The diameter of the IS emitting surface is 20 cm (blue big circle). The sampling area at each UDT measurement is 4 cm in diameter (white small circle). The four yellow circles represent the lamp positions. In b) c) d) e) f) uniformity map for the different configurations 4L, 3L, 2L, 1L and 1L_25 (with shutter at 25%). All the values are normalized with respect to the central value.
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Results of the analyzed data are reported in Figure 12b)-f). The 4L and 3L configurations have a spatial uniformity of 99.2%, 2L of 99.1%, whereas the 1L and 1L_25 configurations are less uniform having values of about 97.9% and 91.5% respectively. 4.4 Shutter characterization As already stated, one of the lamps is equipped with a shutter in order to have different output radiance levels. The spectral radiance and the integrated radiance have been measured with FS and MP for different configurations of the IS lamps and shutter openings. As an example, the relative variation of the emitted radiance versus shutter opening for the 1L configuration is depicted in Figure 13. 1.0
Relative response
0.8
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0.0 0
20
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Figure 13. Shutter relative response. Relative variation of the emitted radiance vs shutter aperture for the 1L configuration.
Note that the curve is not linear due to the mechanical configuration of the shutter blade. The shutter is able to reduce of a factor of 103, or even less, the radiance at the full lamp configuration, and this is extremely useful in order to simulate dim light levels similar to those that the instrument will experience during stellar calibration.
5. CONCLUSIONS The methods and the results of the measurements done to calibrate the integrating sphere needed for the on-ground radiometric testing of the SIMBIOSYS instrument for the BepiColombo ESA mission have been discussed. Emitted radiance for different lamp configurations, temporal stability, both on long and short periods of time, spatial and spectral uniformity, and their behavior at different shutter apertures have been measured. The spectral radiance of the sphere is confirmed to be the one expected from the manufacturer in the nominal configuration (all 4 lamps on and shutter completely open) with a peak at about 900 nm of 520 W/m2/µm/sr, but the spectral radiance for the different lamp configurations has also been measured and proved to be stable. Particular attention should be given in the IR region to avoid radiance variability due to water band absorption. The temporal stability over long period of time has been checked with an external photodiode (UDT) and confirmed with the IS internal one. The sphere radiance is stable within 0.5% after 15 min from switch on. A greater stability, within 0.1%, is reached after one hour of warming up, whereas after two hours it is better than 0.02%. As for the short term stability, over 10-15 min period of time, in the immediate period after warming-up the signal is stable within 0.1%, and after one hour, or two, the stability is always better than 0.1%. The uniformity of the sphere output has been verified to be better than 99.2% for the configuration with all the lamps on and slightly decreasing with powering off of the different lamps. Some measurements with different shutter openings allow to determine the minimum radiance level available with the sphere.
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The results of the data analysis confirm that the performance of the integrating sphere is well suited for the radiometric calibration of all the three different channels of the SIMBIOSYS instrument, in particular the maximum radiance emitted from the sphere with all the lamps on is matching the radiance level expected typically from the Mercury surface. The presence of the shutter on one of the lamps allows to reduce of 3-4 orders of magnitude the radiance level to simulate the radiance expected for the stellar calibration.
ACKNOWLEDGMENTS This activity has been realized under the BepiColombo Agenzia Spaziale Italiana (ASI) contract to the Istituto Nazionale di Astrofisica (INAF I/022/10/0 and with the support of Selex ES (Campi di Bisenzio (Fi) – Italy).
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[14] Epifani Mazzotta, E., Palumbo, P. and L., Colangeli, "HRIC_Requirement-on-QExFF", BC-SIM-OACUPATN-071, Internal Report (2007). [15] Capaccioni, F., Filacchione, G., Piccioni, G., Dami, M., Tommasi, L., Barbis, A. and Ficai-Veltroni, I., "Prelaunch calibrations of the Vis-IR Hyperspectral Imager (VIHI) onboard BepiColombo, the ESA mission to Mercury", Proc. SPIE 8867, Infrared Remote Sensing and Instrumentation XXI, 886704 (2013). [16] Naletto, G., Da Deppo, V. and Cremonese, G., "STC calibration plan", BC-SIM-OPD-TN-011, Internal Report (2008). [17] Epifani Mazzotta, E. and Palumbo, P., "HRIC (High Resolution Imaging Channel) Input for the SteCM (Stellar Calibration Model)", BC-SIM-OACPUA-TN-110, Internal Report (2012). [18] Martellato, E., and Cremonese, G., "Stellar fields database for the inflight calibration of STC", BC-SIM-OPDTN-009, Internal Report (2008). [19] Da Deppo, V., Fornasier, S., Naletto, G., and Cremonese, G., "Signal to noise estimate for star observed with STC", BC-SIM-OPD-TN-007, Internal Report (2007). [20] Labsphere, "Spectraflect Reflectance coating", http://www.labsphere.com/uploads/datasheets/spectraflect.pdf. [21] Labsphere, "Intagrating Sphere Manual", AQ-00273-000Rev13Table-TopUniformSourceSystems.pdf. [22] Capaccioni, F., "Integrating Sphere Temporal Stability 4L_100", Private Communication (2013).
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