In-Flight Calibration Enhances Accuracy of Solar Orbiter’s STIX Instrument Using Solar Flares
Solar Orbiter’s Spectrometer/Telescope for Imaging X-rays (STIX) has recently undergone a significant in-flight recalibration, utilizing solar flares themselves to refine measurements of its tungsten grids’ X-ray transmission efficiency. Published on 10 September 2026 in Solar Physics (In-Flight Self-Calibration of the STIX Grid Transmission), this study systematically analyzed 91 carefully selected solar flare events out of an archive of approximately 25,000 recorded from January 2021 through February 2025. This new calibration notably adjusts STIX’s photometric scale, improving the accuracy of flare photon spectra and emission measures by roughly 13%, while maintaining previously established flare temperature estimates.
Why STIX Required In-Flight Grid Calibration
STIX is designed as an indirect hard X-ray imaging spectrometer, operating across an energy range of about 4 to 150 keV. It employs 30 pairs of tungsten grids that modulate incoming X-rays, allowing each detector-grid unit—called a sub-collimator—to sample specific Fourier components of solar X-ray sources. The 24 sub-collimators with coarser angular resolution use tungsten foils 33 or 50 micrometers thick, stacked to a nominal overall thickness of 400 micrometers.
Before launch, the grids underwent detailed optical characterization and X-ray testing. However, these pre-flight X-ray tests were not performed at the small incidence angles most relevant for observing solar flares, due in part to time and budget constraints. Optical characterization alone cannot fully capture subtle internal variations within the multilayer grid structure—such as small stacking misalignments or etching inconsistencies—that can modify effective slit widths and thus influence the amount of X-ray flux reaching detectors.
The new method leverages the STIX Coarse Flare Locator (CFL) as an internal reference. For flares positioned suitably, one or more large CFL pixels are fully illuminated, providing a reliable estimate of the incident total flux. Comparing this total flux to measurements from individual imaging sub-collimators enables an empirical determination of each sub-collimator’s effective transmission. This approach yielded an estimated CFL total-flux measurement accuracy of approximately 2.3%, accounting for both statistical and systematic uncertainties.
From 25,000 Flares to a Calibrated Sample of 91 Events
The initial dataset contained roughly 25,000 flare events recorded by STIX between January 2021 and February 2025. To ensure robust calibration, events were filtered to include only those with at least one fully illuminated large CFL pixel, reliable flare location data, and sufficient counts for statistical validity. This rigorous selection process narrowed the dataset to 91 flares.
Calibration focused on the 10–15 keV energy range, where the Solar Black coating covering STIX’s front entrance window transmits between 91% and 97% of incident radiation. This minimized the influence of coating inhomogeneities on inter-detector comparisons.
The new in-flight measurements revealed that effective grid transmission was lower than predicted by pre-flight optical models and exhibited less internal-shadowing variation with incidence angle than expected for idealized, perfectly aligned grids. Mechanical simulations incorporating random stacking and etching imperfections with a standard deviation of about 2 micrometers reproduced these observations, reducing the effective on-axis slit width by approximately 6 micrometers for sub-collimator 5, whose nominal slit width is 83 micrometers.
Independent Verification Confirms Calibration Improvements
To validate the new calibration, the team analyzed 25 additional flares recorded between March and December 2025 that were not part of the initial calibration set. Using the updated transmission model, normalized total-flux estimates from individual sub-collimators ranged narrowly from 0.98 to 1.02, with mean measurements within about 2% of the cross-detector average—demonstrating excellent inter-detector consistency.
Event-to-event variability also improved markedly: the standard deviation of normalized total flux fell below 3.3% for all detectors except sub-collimator 1b, which showed a slightly higher deviation of 6.7%. In contrast, the old calibration exhibited a wider spread, with normalized total-flux values ranging from 0.93 to 1.20 for coarse-resolution sub-collimators and 0.48 to 0.86 for the finest-resolution units.
Comparisons of fluxes calculated under the new versus old calibrations showed differences between 3% and 39% for coarse sub-collimators, and between 48% and 175% for the finest ones—highlighting the significant improvement in measurement reliability at lower energies.
A spectral test using an X2.2-class flare from 8 December 2024 further reinforced the enhancement. After excluding three coarse sub-collimators (3a, 5a, and 5b) due to energy-dependent discrepancies likely related to detector calibration rather than grid transmission, the standard deviation of fitted photon spectra between 10 and 15 keV dropped from 6.9% (old calibration) to 1.6% (new calibration). Extending the energy window to 8–20 keV yielded a spectral standard deviation of just 2.3% with the new calibration.
Importantly, while the absolute spectral scale shifted—photon spectra produced with the old calibration were on average 13.2% lower, and emission measures increased by 13.7% with the new calibration—fitted flare temperatures remained unchanged. This indicates that physical interpretations from prior STIX studies remain valid despite the revised photometric scaling.
Calibration Scope and Future Directions
The updated calibration is primarily applicable below approximately 20 keV and within offset angles of about −0.5 to +0.5 degrees. At higher energies, grid transparency effects modify the effective slit width, and insufficient CFL data counts limit reliable self-calibration. Additionally, below roughly 8 keV, increased spectral scatter is suspected to arise from inhomogeneities in the Solar Black coating, suggesting a target for future calibration efforts.
High-energy grid calibration remains an open challenge, crucial for studies investigating anisotropy in non-thermal hard X-ray emission from solar flares. Such studies require cross-calibration between instruments viewing the same flare from different vantage points, demanding calibration accuracy better than 10% to yield meaningful results.
The research also offers practical insights for future instrument design. Incorporating a dedicated total-flux monitor with sufficient collecting area would allow continuous calibration referencing for every flare and potentially extend calibration capabilities to higher energies. Notably, the Hard X-ray Imager aboard China’s Advanced Space-based Solar Observatory (ASO-S) already includes such total-flux monitors for calibration purposes.
For STIX, the authors plan to release the improved visibility-amplitude calibration in an upcoming software update. This advancement narrows internal inconsistencies in low-energy measurements across sub-collimators, enhancing data reliability while preserving the integrity of scientific conclusions drawn from previous flare analyses.
For more information, see the original study and related details Here.
