The SWAP EUV Imaging Telescope. Part II: In-flight Performance and Calibration (1210.3551v1)

Abstract: The Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope was launched on 2 November 2009 onboard the ESA PROBA2 technological mission and has acquired images of the solar corona every one - two minutes for more than two years. The most important technological developments included in SWAP are a radiation-resistant CMOS-APS detector and a novel onboard data-prioritization scheme. Although such detectors have been used previously in space, they have never been used for long-term scientific observations on orbit. Thus SWAP requires a careful calibration to guarantee the science return of the instrument. Since launch we have regularly monitored the evolution of SWAP detector response in-flight to characterize both its performance and degradation over the course of the mission. These measurements are also used to reduce detector noise in calibrated images (by subtracting dark-current). Since accurate measurements of detector dark-current require large telescope off-points, we have also monitored straylight levels in the instrument to ensure that these calibration measurements are not contaminated by residual signal from the Sun. Here we present the results of these tests, and examine the variation of instrumental response and noise as a function of both time and temperature throughout the mission.

An Analysis of the In-Flight Calibration and Performance of the SWAP EUV Imaging Telescope

The study presented in the paper offers a comprehensive evaluation of the in-flight performance and calibration of the Sun Watcher with Active Pixel System detector and Image Processing (SWAP) telescope, which was launched onboard the ESA PROBA2 mission. It continually captures images of the solar corona in the extreme ultraviolet (EUV) spectrum. This telescope features a radiation-resistant CMOS-APS detector with a unique onboard data-prioritization scheme, which demands regular in-flight calibration to ensure the scientific integrity.

Calibration and Noise Characterization

Detailed assessments of the SWAP detector's performance regarding dark-current accumulation, linearity, and noise factor, including shot noise, fixed pattern noise (FPN), and dark noise, are analyzed. The study highlights the variability of these noise factors due to passive cooling, which means the detector temperature is not consistently maintained. The dark-current, sensitive to temperature fluctuation, displayed a roughly 50% higher rate in-flight compared to pre-launch measurements, likely an artifact of early in-orbit aging.

Crucially, the study emphasizes a comprehensive empirical model developed for dark-current correction due to SWAP’s lack of a mechanical shutter and its variable temperature. This model informs dark-current removal in flight data, optimizing image fidelity despite fluctuating detector temperatures.

Detector Performance and Evolution

Regular calibrations confirmed a sub-2% annual decline in detector sensitivity, potentially linked to degradation of near-UV LEDs efficiently used for in-flight detector response monitoring. Importantly, comparative analyses with the Extreme-Ultraviolet Variability Experiment (EVE) on the Solar Dynamics Observatory suggested negligible degradation of SWAP's performance against this external reference.

The SWAP detector displayed some non-linearity, manifested as a 5% discrepancy between expected and actual signal levels over time. This evolution was consistent with typical performance changes of APS-CMOS sensors under constant exposure conditions.

Straylight and Hot Pixel Management

The research identifies both in-field and out-of-field straylight measurements, noting that the principal sources are likely the wings of the point-spread function (PSF) rather than internal reflections, supported by ray-tracing analyses. The out-of-field straylight level is a manageable fraction, under 1% of nominal signal, ensuring the clarity of off-point calibration sequences.

Hot pixels, exacerbated by dark-current dependencies, showed a slight increase annually, correlated with detector temperature variances, and were efficiently mitigated through calibration improvements.

Implications for Future Research

The integration of an APS-CMOS detector in long-term solar observation missions represents a valuable forebearer for future EUV instruments, such as the Extreme EUV Imager (EUI) aboard the ESA Solar Orbiter mission. Data from the SWAP mission provides key insights into maintaining scientific integrity through active in-flight calibration techniques, noise management, and image processing algorithms, suggesting APS-CMOS as a viable technology for long-duration space-based observation.

The study posits the potential for future enhancements in image-processing methodologies such as PSF deconvolution, which would improve the removal of straylight effects considerably. Continued evaluation of detector linearity and saturation behaviors will also contribute to enhanced calibration sequences. The findings assure a foundational baseline for adapting calibration strategies and improving collection methodologies in evolving spaceborne solar observing projects.