EnVisS Signal to Noise Ratio estimates

By the end of 2029, the European Space Agency (ESA) will launch its first Fast-class mission, Comet Interceptor (Jones et al., 2024). The mission is designed to study and explore a dynamically new comet, with the aim of deepening our understanding of formation and evolution of comets and, more in general, of the Solar System.

To achieve this goal, the mission is composed of several instruments dedicated to both in situ measurements and remote sensing. One of the instruments that operates in the latter mode is the Entire Visible Sky camera, EnVisS, which is designed to measure the radiance and polarization characteristics of the light scattered by the dust particles in the cometary coma (Fulle et al., 2000).

The fish-eye camera of EnVisS works in push-frame mode, taking images of the entire coma within the visible wavelength range. The main components are an optical head (composed of ten lenses), a Filter Strip Assembly, and a CMOS detector.

The best performance of the instrument is achieved when the images show the highest Signal to Noise Ratio (SNR), which, as per requirements, has to be larger than 10 for the broadband images (Da Deppo et al., 2022). For this reason, one of the analysis that must be performed consists of defining the most suitable observational strategy in terms of the detector binning and exposure time while getting closer to the target. With this goal in mind, a simulator of a comet and its coma has been developed and the SNR of simulated EnVisS images has been estimated. In Figure 1 the building blocks of the general simulation environment are shown.

Figure 1: Building blocks of the general simulation environment used for the EnVisS camera to estimate the Signal to Noise Ratio.

The simulator considers a pure equidistant fish-eye model for the camera, an attenuation by optics and filter transmission, as well as the appropriate quantum efficiency curve of the detector working with 12 bits configuration. Furthermore, only broadband observations are taken into account and the geometric information of the flyby is given considering the spice kernel provided by ESA [4], which consists of a flyby around comet 8P/Tuttle, with a close approach (CA) distance of 250 km, and a spin period of the probe of 4 s.

The main problem that had to be faced to estimate SNR is the uncertainty derived from the fact that the target, its activity level, and its coma characteristics are unknown, thus, several assumptions have then been made. In particular, a spherical and symmetric coma with dust particles following a power law size distribution (r ^−a, where r is the particle size and a the exponent) has been taken into account. The activity of the target has been defined by a water production rate of 1e30 molec/sec and a dust production defined in terms of dust-to-gas ratio. The dust size distribution ranges from 0.1 μm to 1 mm divided in 50 bins. Furthermore, Mie scattering with refractive index n = 1.8+0.1i and effective wavelength of 656 nm have been used. Only after testing the validity of these assumptions, the estimate of the SNR of EnVisS observations has been computed.

For the analysis, several different cases have been studied. In particular, the impact of the slope of the dust size distribution, a, and the total dust production rate, Q_d, of the comet have been analyzed for different binnings and exposure times.

The simulation results show that, maintaining the same detector parameters, the signal detected by the camera sensor is extremely different for the studied cases. Consequently, different observational strategies must be adopted from case to case.

To define the best strategy for each instance, all the data collected from the previous tests have also been quantitatively described in terms of the saturated and useful fractions of the phase curve, and of how those fractions evolve with time.

This analysis has shown that to have the maximum useful fraction, a combination of different binning and exposure times is necessary during the whole flyby period. Furthermore, a combination of two different images taken with different binnings or same binning but different exposure times might be a possible solution to achieve the maximum SNR possible and recreate almost the entire phase function.

For this study, the suitable binnings and exposure times to obtain the best coverage of the phase function with the highest possible SNR as a function of time have been considered for the combinations Q_dust/Q_gas = 3 and three different slopes (a = 3.1, 3.5, 4.1), and two dust to gas ratios (Q_dust/Q_gas = 1, 6) and a = 3.5.

The final best observational strategy is summarized in the table in Figure 2.

Figure 2: Summary of the binning and exposure time to obtain the best coverage of the phase function as a function of the time to close approach (or distance) for the test performed considering the uncertainty in the slope of the dust size distribution.

ACKNOWLEDGMENTS

This activity has been funded by the Italian Space Agency (ASI) via contract 2023-14-HH.0 to the Istituto Nazionale di Astrofisica (INAF).

REFERENCES

[1] Jones, G., Snodgrass, C., and Tubiana, C., e. a., “The comet interceptor mission.,” Space Sci Rev 220, 9 (2024).

[2] Fulle, M., Levasseur-Regourd, A. C., McBride, N., and Hadamcik, E., “In situ dust measurements from within the coma of 1p/halley: First-order approximation with a dust dynamical model,” The Astronomical Journal 119, 1968 (apr 2000).

[3] Da Deppo, V., Della Corte, V., Zuppella, P., Nordera, S., Pernechele, C., Lara, L. M., Castro, J. M., Jiménez, J., Martinez, I., Praks, J., and the EnVisS Team, “The entire visible sky (enviss) imager for the comet interceptor esa mission,” Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-273 (2022).

[4] https://s2e2.cosmos.esa.int/bitbucket/projects/SPICE_KERNELS/repos/comet-interceptor/browse