Scientific characterization and calibration strategies of the EnVisS instrument for the Comet Interceptor mission

Conceived for the Comet Interceptor mission, the EnVisS (Entire Visible Sky) camera will investigate a yet-to-be-identified dynamically new comet.

Mounted on the spin-stabilized Probe B2, the instrument employs a rotational push-frame imaging concept, enabling the acquisition of the entire sky surrounding the probe during a 24-hour flyby.

1 - Introduction

Comet Interceptor [1] is the first ‘Fast’ mission of the European Space Agency (ESA), developed in collaboration with JAXA. The primary goal is interception and in situ study of a dynamically new comet. These bodies contain pristine material unaltered by solar heating, thus providing an unprecedented opportunity to understand the earliest stages of Solar System formation.

Targeted to launch in 2028, the mission architecture consists of a main spacecraft (Spacecraft A) and two smaller probes (Probe B1 and Probe B2) that will separate prior to the flyby to perform simultaneous, multi-point measurements of the cometary environment. The EnVisS (Entire Visible Sky) instrument is integrated aboard Probe B2, a spin-stabilized module with a nominal rotation frequency of approximately 15 rpm (a spin period of ~4 seconds).

2 – EnVisS Scientific Objectives

EnVisS (see Figure 1) is a wide-angle camera designed to map the entire celestial sphere (360° x 180°) from within the comet coma during its flyby passage. This close-up observation geometry aims to reconstruct the three-dimensional structure of the dust emitted by the nucleus.

The key scientific targets are the determination of the coma morphology, including a global-scale study of the spatial distribution of dust and potential jets, and the investigation of the physical properties of the dust grains (size distribution, porosity, and shape) through the analysis of scattered sunlight at various phase angles.

3 - EnVisS Configuration

To maximize scientific return while minimizing structural complexity (avoiding moving parts), EnVisS adopts a push-frame imaging technique. As Probe B2 rotates, the sensor acquires high-frequency frame sequences that are subsequently stitched together to generate a continuous panoramic map.

The instrument operates in the 550–800 nm spectral band and features a 2kx2k commercial sensor on the focal plane with a pixel pitch of 5.5 microns [2]. A filter strip assembly (FSA) [3] is mounted in front of the detector. The FSA is divided into three sections:

• A central broadband filter.
• Two linear polarizing filters with orientation angles set at 0° and 45° with respect to the reference axis.

Figure 1: Left: EnVisS CAD model. Right: Optical head and camera components highlighted [4].

4 - EnVisS on-ground calibration

On-ground full calibration activities are challenging the EnVisS camera. Testing the entire field of view (180° x 45°) is required to verify the instrument optical performance (MTF, PSF, distortion,…) and perform radiometric and polarization calibration. Measurements are to be carried out in vacuum including checks at different wavelengths and in the whole operative temperature range.

An ad-hoc set-up has been devised, procured and integrated. The OGSE setup consists of:

• A collimation system equipped with led sources in the operative wavelength range of EnVisS (550-800 nm) and targets (pinholes).
• Polarization module, consisting of polarizers and quarter wave plates, providing controlled polarization input to measure the polarization response.
• A rotation system to allow the collimator and polarization module to span the FoV.

5 – Conclusions

The EnVisS wide-angle camera for the Comet Interceptor mission features a rotational push-frame technique to map the entire celestial sphere. Thanks to its unique filter strip assembly, the instrument will simultaneously provide intensity and polarization data to characterize the physical properties of cometary dust.

The implementation of a dedicated on-ground calibration setup is essential for verifying optical performance and ensuring high-quality scientific results.

Acknowledgements

This work is the result of a collaborative effort of an international consortium including different institutions (CNR-IFN (Italy), IAA-CSIC (Spain), INAF-OACN (Italy), University of Naples “Parthenope” (Italy), Aalto University (Finland), ASI and ESA) and industries (Leonardo S.p.A. (Italy), Sener (Spain), Huld (Finland)).

It has been funded: by the Italian Space Agency (ASI) through contracts to the Istituto Nazionale di Astrofisica (2023-14-HH.0 and 2023-14-HH.1.2025) and to Leonardo SpA (2024-61-I.0); by the European Space Agency (ESA) under a Contract to the Italian National Research Council (CNR) (Contract n. 4000136673/21/NL/IB/ig); by the Spanish Ministerio de Ciencia e Innovación (MCIN) through ESA PRODEX and the Spanish National Plan Ref PID2021-126365NB-C21.

References

• [1] G. Jones, et al., “The Comet Interceptor Mission”, Space Sci Rev 220, 9 (2024).
• [2] B. Tofani, et al., ”Design of the EnVisS instrument optical head”, SPIE Proc. 12777, International Conference on Space Optics — ICSO 2022; 127772P (2023).
• [3] C. Naletto et al., “Characterization measurements for the realization of the filter strip assembly of the EnVisS camera”, Proc. SPIE 13699, International Conference on Space Optics — ICSO 2024, 136994D (2025).
• [4]  V. Da Deppo, et al., “The EnVisS fish-eye camera for the Comet Interceptor ESA mission: design and performance”, Proc. SPIE 13699, International Conference on Space Optics — ICSO 2024, 136995Q (2025).