Calibrations for high precision differential astrometry onboard Theia and HWO

Scientific Impact Assessment

Core Contribution. This is an instrumentation status/progress paper (SPIE-proceedings style) that bundles three developments in service of sub-micro-arcsecond space astrometry for two future missions (the ESA M8 candidate Theia and a proposed astrometric mode for NASA's HWO). The three threads are: (1) an independent laboratory characterization of the GIGAPYX-4600 back-side-illuminated CMOS detector against manufacturer specifications; (2) an interferometric (Young's-fringes) method to measure pixel centroid displacements for inter-pixel response mapping, presented via simulation and a testbed; and (3) an "astro-calibration" framework that jointly solves for stellar astrometry, telescope attitude, plate scale, and optical distortion, modeled after the Gaia core solution (Lindegren et al. 2012). Of these, only the detector characterization is presented as a completed, self-contained result; the other two are explicitly works-in-progress, with substantive detail deferred to companion papers (Rousset et al. 2026; Lizzana et al. 2026).

Methodological Rigor. The detector characterization is the strongest and most convincing element. It follows the EMVA 1288 standard, uses an integrating sphere for flat-fielding, employs the photon-transfer-curve method for gain, and reports a thorough parameter set (gain, linearity, readout noise, dark current, PRNU, six-category defective-pixel taxonomy, IPC). The side-by-side comparison to Pyxalis datasheet values (Table 1) constitutes a genuine independent verification, and the paper is appropriately candid about the one discrepancy (dark current: 36 vs. 24 e⁻/s). This part could stand on its own as evidence. By contrast, the interferometric method section largely summarizes prior testbed results (JPL, Crouzier et al. 2016) and points to a companion paper for its own simulation and hardware results; no new quantitative performance figures are reported here. The astro-calibration section presents the mathematical formulation (cost function, block-iterative Levenberg–Marquardt for nonlinear s, a; weighted least squares for linear c) and a single illustrative convergence plot on an 11-star synthetic field, but explicitly states "no robust results are currently available." So two of three contributions lack the evidence to support strong claims — appropriate for a proceedings paper, but limiting for impact.

Potential Impact. The audience is a fairly narrow instrumentation community: teams developing focal-plane hardware and calibration pipelines for ultra-high-precision astrometry. Within that niche, the GIGAPYX-4600 characterization is genuinely useful — it provides an independent, standards-based data point on a candidate detector for gigapixel focal planes, of direct interest to Theia/HWO instrument teams and to Pyxalis as a commercial vendor. The IPC deconvolution-correction demonstration is practically relevant since IPC directly biases centroids. The interferometric and astro-calibration methods, if matured, could feed into mission calibration strategies, but their current stage limits near-term influence. The broader astronomical community (exoplanets, dark matter, compact objects) is a beneficiary only indirectly and contingently on these missions being selected/flown.

Timeliness & Relevance. Very timely: Theia was submitted to ESA's M8 call in May 2025, and HWO technology maturation is active. Detector and calibration technology development is a recognized bottleneck for sub-µas astrometry, so the topic addresses a real current need. However, the paper's contingency on mission selection is a risk to its long-term citation trajectory.

Strengths & Limitations. Key strengths: a rigorous, standards-compliant independent detector characterization with an exceptionally low measured defect rate (44 pixels / 46 Mpx, 0.0004%); honest reporting of discrepancies; and a coherent framing that connects hardware, pixel-level metrology, and system-level astrometric calibration. The reference to radiation-tolerance results adds relevance for spaceflight qualification. Key limitations: the paper is largely a bundling/overview of work published (or to be published) elsewhere — the two more novel methodological contributions are preliminary and delegate their evidence to companion papers, reducing this document's standalone contribution. Novelty is moderate: the interferometric method extends established JPL/French testbed approaches, and the astro-calibration is an adaptation of the Gaia solution. There are minor writing issues (typos: "insulated" for "illustrated," "at ones," "calle"). Reproducibility of the two method sections is weak as presented; the detector section is more reproducible in principle but also defers to reference [4].

Other observations. The strongest reusable asset is the detector benchmark data, which functions partly as an independent replication of vendor specifications — valuable for the community deciding on focal-plane hardware. The astro-calibration framework, if completed and validated, has the most potential to become a reusable building block, but it is not yet at that stage. Commercial/translational relevance is real given the industry partner (Pyxalis) and spaceflight hardware context.

Overall, this is a competent, timely, well-motivated instrumentation progress report whose completed component (detector characterization) is solid and useful within a narrow subfield, but whose more ambitious methodological contributions are still immature and outsourced to companion papers, capping its independent scientific impact.