Solar Orbiter’s X-Ray Telescope Learns to Calibrate Itself Using the Sun’s Own Flares

Deep in the archive of the European Space Agency’s Solar Orbiter mission, a hard X-ray telescope has quietly taught itself a lesson that its engineers could not fully teach it on the ground. The Spectrometer/Telescope for Imaging X-rays, known as STIX, has spent years watching solar flares erupt from our nearest star, measuring the furious bursts of radiation produced by plasma heated to tens of millions of degrees and by electrons accelerated to near-light speeds. Now, a team led by Paolo Massa of the University of Applied Sciences and Arts Northwestern Switzerland has shown that the instrument can use the Sun itself as a calibration lamp, deriving a new, more accurate model of how much X-ray light actually passes through its intricate tungsten grids. The results, published in the journal Solar Physics, promise sharper measurements of flare physics and a cleaner path toward one of solar science’s most stubborn open questions.

STIX is not a conventional telescope. It is an indirect imaging spectrometer, a device that never forms a direct picture of the Sun. Instead, it observes the bremsstrahlung X-rays emitted by hot flare loops and accelerated electrons in the energy range from 4 to 150 kiloelectronvolts, using pixelated cadmium telluride detectors with an energy resolution of about 1 kiloelectronvolt. The imaging trick relies on 30 pairs of grids, each a stack of tungsten foils etched with fine slits, placed in front of the detectors. As X-rays stream through a grid pair, they cast a shifting Moiré pattern of light and shadow onto the detector behind it. By measuring the contrast and phase of that pattern, each sub-collimator, as a detector-grid unit is called, records one Fourier component of the X-ray source, much as a radio interferometer records a single visibility. Combine enough of these components and an image of the flare emerges from the mathematics.

The trouble is that every Fourier component depends on knowing precisely how transparent each grid pair really is. Before Solar Orbiter launched, engineers measured the grids optically, characterizing the pitch, slit width, and orientation of each one, and those measurements have underpinned the data analysis software ever since. But optical measurements have a fundamental blind spot: they can only see the top and bottom layers of a stack. The STIX grids for the 24 sub-collimators with the coarsest angular resolution are built from tungsten foils 33 or 50 micrometers thick, stacked to reach a total thickness of 400 micrometers, with the pattern etched into each layer. If the layers are not perfectly aligned, or if the etching leaves slits that vary slightly from layer to layer, the effective opening of the grid narrows in ways no optical inspection of the outer surfaces can reveal. Unlike its predecessors on Yohkoh, RHESSI, and the Chinese ASO-S mission, STIX also lacked a full pre-launch X-ray calibration at the small incident angles relevant to solar flare observations, a constraint of time and budget.

The new work turns that limitation into an opportunity. The key insight is that STIX carries a built-in flux monitor it was not entirely designed to be one: the Coarse Flare Locator, or CFL. This sub-collimator uses an H-shaped front grid and an empty rear grid, and for certain flare locations on the solar disk, the shadow cast by the front grid leaves up to three of the CFL’s large detector pixels fully illuminated. A fully illuminated pixel, by definition, sees the total incident X-ray flux, unmodulated by any grid. By comparing the flux recorded by the fully illuminated CFL pixels with the flux transmitted through each imaging sub-collimator, the team could compute the true effective transmission of every grid pair directly from flight data. The transmission is simply the ratio of the transmitted flux to the total flux, a beautifully direct measurement that sidesteps the uncertainties of the ground-based optical characterization.

Building the calibration dataset required patient mining of the STIX archive. From roughly 25,000 flares recorded between January 2021 and February 2025, the team selected events with at least one fully illuminated large CFL pixel, accurate flare locations, and enough counts to keep statistical errors at or below about 3 percent. The winnowing left 91 flares, a small fraction of the total, because only flares at particular solar latitudes illuminate the CFL pixels completely. The team focused on the 10 to 15 kiloelectronvolt band, where the transmission of the Solar Black coating on the spacecraft heatshield is high enough that any inhomogeneities introduce only second-order errors, and where photon statistics remain strong. Careful corrections for background counts, detector live time, and onboard energy channel binning were applied, and a self-consistency check of the CFL pixels themselves showed the total flux measurements are accurate to about 2.3 percent.

What the flares revealed was unambiguous. The measured transmission of the sub-collimators is systematically lower than the values predicted from the optical grid characterization, and the internal shadowing effect, the progressive narrowing of the effective slit width as photons arrive at increasing angles, is far weaker than a model of perfect grids would predict. Simple geometric simulations explain why. When the team modeled grids with random etching and stacking imperfections of a few micrometers, drawn from Gaussian distributions with a standard deviation of about 2 micrometers, the simulated on-axis transmission dropped and the shadowing effect flattened out, exactly matching the pattern in the flight data. For sub-collimator 5, with slits nominally 83 micrometers wide, imperfections of this scale reduce the effective slit width by roughly 6 micrometers. In some sub-collimators, such as 6b, a slight tilt of the stacked layers, plausibly caused by tilted alignment pins during assembly, produces a sloped transmission profile that the simulations reproduce as well.

The practical payoff is substantial. When the team applied the new transmission calibration to 25 independent flares observed between March and December 2025, the total flux estimates from the different sub-collimators agreed within about 2 percent, whereas the old calibration produced systematic biases ranging from 3 to 39 percent for the coarse-resolution sub-collimators and up to 175 percent for the finest ones. A second test on the powerful X2.2-class flare of December 8, 2024 drove the point home: photon spectra derived independently from the individual sub-collimators now agree within roughly 2 percent across the 8 to 20 kiloelectronvolt range, a dramatic improvement over the 6.9 percent scatter of the old calibration. The new calibration also means that derived photon fluxes are about 13 percent higher than previously computed, and fitted emission measures, a measure of the amount of hot plasma in the flare, increase by a similar 13 percent. Fitted flare temperatures, reassuringly, do not change, and the team emphasizes that conclusions from previously published STIX spectral analyses remain valid.

There are honest caveats. The calibration is valid only at low energies, below about 20 kiloelectronvolts, and only for offset angles between roughly minus 0.5 and plus 0.5 degrees, because at higher energies the tungsten grids become partially transparent and the CFL statistics grow too thin. Below 8 kiloelectronvolts, inhomogeneities in the Solar Black coating introduce discrepancies that the team hopes to self-calibrate in a future study using the dependence of transmission on flare location. Three sub-collimators, 3a, 5a, and 5b, showed energy-dependent discrepancies likely tied to detector rather than grid calibration and are recommended for exclusion from spectral analyses for now. The high-energy grid calibration, crucial for studying whether flare X-ray emission is anisotropic, will follow in a companion paper, and the team notes that cross-calibration better than 10 percent between Solar Orbiter and Earth-orbiting observatories is needed to settle that question.

Perhaps the most enduring lesson is a design philosophy. The authors strongly recommend that future indirect X-ray imagers include a dedicated total flux monitor, so that every recorded flare can contribute to self-calibration, as the Hard X-ray Imager aboard ASO-S already does. A flux monitor with sufficient effective area would even allow the same technique to be extended to high energies. For now, the new transmission model will be released in the upcoming version of the STIX analysis software, quietly improving every flare image and spectrum the mission produces. It is a striking demonstration that a spacecraft instrument, far from any laboratory, can diagnose its own imperfections using nothing more than the raw fury of the Sun, and that a few micrometers of tungsten misalignment, invisible to any optical microscope on Earth, can be measured from 100 million kilometers away by letting solar flares do the talking.

Subject of Research: In-flight self-calibration of the tungsten grid transmission of the STIX hard X-ray imaging spectrometer on board Solar Orbiter using solar flare observations

Article Title: In-Flight Self-Calibration of the STIX Grid Transmission

Article References: Massa, P., Volpara, A., Stiefel, M. Z., Bilgili, E., Degen, M., Limousin, O., Hurford, G. J., & Krucker, S. (2026). In-Flight Self-Calibration of the STIX Grid Transmission. Solar Physics, 301(9), Article 138. https://doi.org/10.1007/s11207-026-02739-6

Image Credits: AI Generated

DOI: 10.1007/s11207-026-02739-6

Keywords: Solar Orbiter, STIX, solar flares, X-ray imaging, instrument calibration, tungsten grids, Solar Physics, space instrumentation, hard X-rays, spectroscopy, ESA, helio physics