Cool-Planet Imaging Coronagraph (CPI-C)

• Cool-Planet Imaging Coronagraph (CPI-C) is a high-contrast imaging instrument designed for direct observation of exoplanets and circumstellar disks using advanced step-transmission apodization and phase correction.
• It employs a kilo-actuator deformable mirror and a Shack-Hartmann wavefront sensor to achieve contrasts better than 10⁻⁸, targeting thousands of FGK-type stars within 40 parsecs.
• The instrument integrates detailed simulation frameworks and multi-roll observation strategies to optimize detection and characterization of cool exoplanets, debris disks, and polarimetric signals.

The Cool-Planet Imaging Coronagraph (CPI-C) is the high-contrast coronagraph on the China Space Station Telescope (CSST), designed for direct imaging and photometry of exoplanets and circumstellar disks at visible to near-infrared wavelengths. Its core scientific objective is to directly image cool planets around nearby solar-type stars within 40 pc, conduct high-contrast surveys of exoplanets ranging in size from Neptune-like to Jupiter-like at separations of 0.5 to 5 AU from their host stars, and perform systematic spectroscopic analysis through high-precision multi-band photometry (Collaboration et al., 7 Jul 2025, Dou et al., 12 Dec 2025).

1. Mission placement within CSST

CPI-C is one of the five scientific instruments on CSST, alongside the Multi-band Imaging and Slitless Spectroscopy Survey Camera, Multi-Channel Imager, Integral Field Spectrograph, and THz Spectrometer. Within that instrument suite, CPI-C is the dedicated high-contrast facility for exoplanets, circumstellar disks, and zodiacal dust. CSST descriptions emphasize the telescope’s large field of view, high image quality, and multi-band observation capabilities, while CPI-C operates in a substantially narrower, high-contrast regime oriented toward direct imaging close to bright stars (Collaboration et al., 7 Jul 2025).

The mission literature assigns CPI-C a survey role centered on nearby FGK-type or solar-type stars. One description states that CPI-C will survey thousands of FGK-type stars within 40 parsecs; another describes a prioritized survey of $\sim 700$ nearby bright ($V<7$ mag) solar-type stars within 40 pc. This suggests that the effective target set depends on selection criteria and observing allocation rather than on a single immutable catalog (Collaboration et al., 7 Jul 2025, Dou et al., 12 Dec 2025).

Operationally, CPI-C is described as a staring-mode instrument with telescope rotation about the optical axis between exposures to improve subtraction of quasi-static PSF systematics. Dedicated campaign windows of $\sim 4.5$ months during the 10-year mission are identified in one mission overview, and the instrument is also described as capable of parallel operation with other CSST instruments in some cases (Collaboration et al., 7 Jul 2025).

2. Coronagraphic architecture and performance envelope

CPI-C employs step-transmission apodization to suppress diffraction from the telescope pupil and precise phase correction to eliminate speckle noise due to imperfections of the optical surfaces. Mission papers describe it as a step-transmission apodized pupil coronagraph using a kilo-actuator deformable mirror for high-order phase correction and a Shack-Hartmann wavefront sensor equipped with an EMCCD detector for closed-loop low-order control (Dou et al., 12 Dec 2025).

Instrument-simulation work resolves that architecture into a transmission apodizing filter, a wavefront corrector, and a focal plane mask. In the CPISM optical model, the apodizer is implemented with 32-step strips in both directions, the wavefront corrector is a 952-actuator MEMS deformable mirror at the pupil plane, and the focal plane mask is a chromium-coated fused silica cross that blocks the star and cross-shaped diffraction from the apodizing filter while leaving dark holes for planet detection. The science camera is an EMCCD located at the $f/83$ focus (Zhao et al., 12 Nov 2025).

Across the mission and simulation papers, the principal performance target is contrast better than $10^{-8}$. The visible-band requirement is given as better than $10^{-8}$ at an inner working angle of $3\!-\!4\lambda/D$ in the 600–900 nm range, while simulation papers model dark-hole regions spanning $3\!-\!16\lambda/D$ or $4\!-\!16\lambda/D$, depending on the setup. One CSST overview also gives a high-contrast inner working angle of $\leq 0.55^{\prime\prime}$ at 633 nm. These statements are best read as mission-level requirements and simulation-specific realizations of the same high-contrast design space rather than as a single identical number in all contexts (Collaboration et al., 7 Jul 2025, Zhao et al., 12 Nov 2025, Dou et al., 12 Dec 2025).

The wavelength coverage is likewise described at two levels. Mission-planning and target-simulation papers specify four optical bands centered at 520 nm, 662 nm, 720 nm, and 850 nm, together with four infrared bands centered at 940 nm, 1265 nm, 1425 nm, and 1542 nm. The instrument-simulation paper focuses on the four visible bands F520, F662, F720, and F850, because its scope is the high-contrast imaging optical system and the visible-band science camera (Zhu et al., 13 Nov 2025, Zhao et al., 12 Nov 2025).

3. Wavefront sensing, detector physics, and calibration

Wavefront stability is central to CPI-C’s contrast budget. In the CPISM high-contrast model, systematic phase and amplitude aberrations limit raw contrast to roughly $V<7$0 before correction. A $V<7$1 contrast dark hole region, consistent with design specifications, is then simulated using Electric Field Conjugation (EFC). The modeled control law builds a Jacobian from small deformable-mirror voltage perturbations and iteratively updates the mirror through

$V<7$2

with Tikhonov regularization used in the pseudo-inverse to avoid ill-conditioning (Zhao et al., 12 Nov 2025).

The visible science camera is modeled as an EMCCD with explicit treatment of photon collection, charge transfer, electron multiplication, and readout. The simulation includes dark current, charge transfer efficiency, clock-induced charge, multiplication noise factor, striping, drift, overscan, blooming, bad columns, and cosmic-ray effects. A detailed EM-register model is constructed from $V<7$3 stages, and an improved statistical model for the EM process is proposed to enhance simulation efficiency while maintaining fidelity at low gains (Zhao et al., 12 Nov 2025).

A separate calibration study addresses the EMCCD used in the wavefront sensor. Through a multi-stage screening protocol, it identifies an EMCCD chip with high resolution and low noise, and then characterizes gain, readout noise, single-photon sensitivity, and inter-channel non-uniformity. At $V<7$4C, the detector achieves a gain range of $V<7$5 with inter-channel gain non-uniformity of $V<7$6, and the minimum noise-equivalent photon count is $V<7$7. The study also introduces an iterative robust correction algorithm for channel-response non-uniformity and reports that the optimized detector meets CPI-C’s initial application requirements (Dou et al., 25 Nov 2025).

Taken together, these results define two complementary detector regimes within CPI-C: a science EMCCD whose behavior is modeled end-to-end for realistic image synthesis, and a wavefront-sensing EMCCD whose operating range and non-uniformity are calibrated experimentally. A plausible implication is that CPI-C’s contrast performance depends as much on detector calibration and stability as on coronagraphic optics alone.

4. Simulation framework, synthetic observations, and observing strategy

CPISM is the principal end-to-end simulation environment described for CPI-C. It is a modular, Python-based program intended to support data-processing pipeline development, scientific performance assessment, target selection, and observation-strategy optimization. Its architecture includes target simulation, imaging simulation, observational effects, camera simulation, and data-product generation modules, and it can generate Level 0 FITS files formatted according to CSST conventions (Zhu et al., 13 Nov 2025).

The imaging module models the optical system with Fourier optics and incorporates the apodizer, deformable-mirror correction, focal-plane mask, and dark hole. The science simulation paper additionally states that CPISM integrates telescope optics, detector noise, cosmic rays, and speckle noise, and that it produces simulated images closely similar to the expected real observational data. For broadband observations, the simulator subdivides each filter into sub-bands and combines monochromatic PSFs using SED-based weights,

$V<7$8

so that chromatic changes in speckle morphology are represented explicitly (Zhao et al., 12 Nov 2025).

Target and scene modeling are correspondingly detailed. Stellar SEDs may be taken from the Castelli and Kurucz 2004 grid or supplied by the user, while planetary spectra may use reflection albedo models such as Batalha et al. 2018. Planetary reflected-light contrast is modeled as

$V<7$9

and cosmic-ray counts are drawn from

$\sim 4.5$0

The same framework is used to examine trade-offs among exposure time, EM gain, saturation, blooming, and cosmic-ray contamination (Zhu et al., 13 Nov 2025).

For actual observing concepts, CPI-C studies emphasize differential strategies adapted to a stable space platform. Mission descriptions highlight telescope roll between exposures; science simulations for the $\sim 4.5$1 Eridani debris disk use a multi-roll strategy with eight roll angles at $\sim 4.5$2 intervals because each exposure covers only part of the disk owing to the dark-zone geometry; and target-planning studies state that advanced post-processing will include ADI/LOCI, O-IRS, and G-RDI (Collaboration et al., 7 Jul 2025, Bao et al., 20 Sep 2025, Dou et al., 12 Dec 2025).

5. Science cases: cool exoplanets, debris disks, and polarimetry

The primary science case is direct imaging and characterization of cool exoplanets around nearby solar-type stars. Mission descriptions state that CPI-C targets mature Jupiter analogs, super-Earths, and planets from Neptune-like to Jupiter-like, at orbital separations from 0.5 or 0.8 AU out to 5 AU, with reflected-light imaging in the visible intended to constrain effective temperature, surface gravity, radius, mass, clouds, and atmospheric composition through multi-band photometry and spectroscopy (Collaboration et al., 7 Jul 2025, Dou et al., 12 Dec 2025).

A second major science case is circumstellar material. CPI-C is explicitly intended for high-contrast imaging and studies of circumstellar disks, debris, and zodiacal dust. The $\sim 4.5$3 Eridani study provides the most developed example: using MCFOST to generate synthetic scattered-light images and spectral energy distributions for three inner-disk models, it finds that CPI-C can resolve disk structures down to $\sim 4.5$4 au, recover inclinations and radial extents close to the input models, and distinguish narrow rings from broader continuous emission. In that simulation, the inner working angle at 600 nm is expressed as $\sim 4.5$5, corresponding to $\sim 4.5$6 mas for the $\sim 4.5$7 working zone, or $\sim 4.5$8 au at the distance of $\sim 4.5$9 Eri (Bao et al., 20 Sep 2025).

The same $f/83$0 Eridani analysis also treats polarimetry as a distinct measurement mode rather than merely an adjunct to total-intensity imaging. Planetary polarization is simulated with PyMieDAP, instrument-level polarization leakage is taken as $f/83$1, and dual-polarization imaging at $f/83$2 and $f/83$3 is used in the forward model. The cold Jupiter-like planet $f/83$4 Eri b is not detected in total intensity in those simulations, but the reflected-light contrast peaks at $f/83$5, and polarimetric imaging is identified as a potential route to improved planet-to-speckle contrast, with at least $f/83$6 s integration required for a viable detection under the stated assumptions (Bao et al., 20 Sep 2025).

These science cases place CPI-C in the regime of reflected-light imaging of cold planets and close-in dust structures rather than only the young, self-luminous, wide-separation planets that have dominated earlier direct-imaging samples. The literature therefore treats CPI-C as both a survey instrument and a characterization instrument.

6. Engineering qualification, validation, and methodological caveats

CPI-C development includes dedicated structural engineering for its scientific probe module. The mechanical test platform for that module is optimized using finite-element sensitivity analysis, Central Composite Design, and a third-order response surface model fitted by least squares. The optimized structure is then subjected to modal analysis and to sine, random, and swept-frequency vibration simulations. Swept-frequency testing on an 18 t vibration table yields a measured fundamental frequency of $f/83$7 Hz, above the $f/83$8 Hz design index, with good agreement between the response-surface fitting algorithm and experiment; the average total error is reported as $f/83$9 (Kong et al., 20 Nov 2025).

The same engineering study reports a simulated first modal frequency of $10^{-8}$0 Hz and additional modes at 638.2, 804.85, 940.9, 981.05, and 1025.2 Hz. Under random vibration, the maximum stress is $10^{-8}$1 MPa, far below the $10^{-8}$2 MPa yield value cited for the material, and the minimum safety factor is $10^{-8}$3. The measured and simulated behavior in $10^{-8}$4 differs more noticeably than in $10^{-8}$5 and $10^{-8}$6, which the study attributes to unmodeled fixture and screw conditions and practical limitations. In this part of the CPI-C literature, structural stiffness, vibration response, and detector calibration are treated as enabling conditions for optical performance rather than as separate concerns (Kong et al., 20 Nov 2025).

A methodological caveat emerges when CPI-C is placed in the broader high-contrast imaging literature. Direct-imaging reviews emphasize that coronagraphic performance is set not only by the focal-plane mask and apodization, but also by wavefront aberrations, stellar speckles, low-order stability, post-coronagraphic wavefront control, and post-processing strategy (Galicher et al., 2023). Similarly, the study of coronagraphy combined with spectral and angular differential imaging shows that SDI does not systematically improve sensitivity because its gain depends on the spectral properties of the companion and on self-subtraction. For CPI-C, this is relevant because its planning papers emphasize multi-roll observations, reference-based subtraction, and polarimetric methods rather than assuming that any single differential technique is uniformly optimal (Maire et al., 2014, Zhu et al., 13 Nov 2025, Bao et al., 20 Sep 2025).

In comparative context, Roman CGI is described as a visible-light space coronagraph technology demonstration with raw contrast goals of $10^{-8}$7 to $10^{-8}$8, multiple internal coronagraph types, and active wavefront sensing and control (Kasdin et al., 2021). This suggests that CPI-C belongs to the same broader class of actively controlled space coronagraphs, while remaining distinctive in the CSST literature for its integration into a survey telescope and its explicit focus on cool planets around nearby solar-type stars (Collaboration et al., 7 Jul 2025, Dou et al., 12 Dec 2025).