[Paper Review] The Wide Field Imager Instrument for Athena

Norbert Meidinger, Josef Maria Eder+6 more|arXiv (Cornell University)|Feb 3, 2017

CCD and CMOS Imaging Sensors4 citations

TL;DR

This paper presents the Wide Field Imager (WFI) instrument for ESA's Athena X-ray observatory, which uses DEPFET active pixel sensors with 130×130 µm pixels to achieve a 40×40 arcmin field of view and sub-170 eV energy resolution at 7 keV. The instrument enables high-throughput, low-pile-up observations of bright sources (up to 1 Crab) via a fast readout system using VERITAS-2 and Switcher-A ASICs, supported by a graded-Z shield to suppress background fluorescence lines.

ABSTRACT

The WFI (Wide Field Imager) instrument is planned to be one of two complementary focal plane cameras on ESA's next X-ray observatory Athena. It combines unprecedented survey power through its large field of view of 40 amin x 40 amin together with excellent count rate capability (larger than 1 Crab). The energy resolution of the silicon sensor is state-of-the-art in the energy band of interest from 0.2 keV to 15 keV, e.g. the full width at half maximum of a line at 7 keV will be better than 170 eV until the end of the nominal mission phase. This performance is accomplished by using DEPFET active pixel sensors with a pixel size of 130 x 130 microns is well suited to the on-axis angular resolution of 5 arcsec half energy width (HEW) of the mirror system. Each DEPFET pixel is a combined sensor-amplifier structure with a MOSFET integrated onto a fully depleted 450 micron thick silicon bulk. Two detectors are planned for the WFI instrument: A large-area detector comprising four sensors with a total of 1024 x 1024 pixels and a fast detector optimized for high count rate observations. This high count rate capable detector permits for bright point sources with an intensity of 1 Crab a throughput of more than 80% and a pile-up of less than 1 %. The fast readout of the DEPFET pixel matrices is facilitated by an ASIC development, called VERITAS-2. Together with the Switcher-A, a control ASIC that allows for operation of the DEPFET in rolling shutter mode, these elements form the key components of the WFI detectors. The detectors are surrounded by a graded-Z shield, which has in particular the purpose to avoid fluorescence lines that would contribute to the instrument background...[Abridged]

Motivation & Objective

• To design a wide-field X-ray imager for the Athena observatory with high sensitivity and broad spectral coverage.
• To enable high-time-resolution observations of bright X-ray sources with minimal pile-up effects.
• To achieve sub-170 eV energy resolution across the 0.2–15 keV band using advanced DEPFET sensor technology.
• To reduce instrumental background through a graded-Z shielding system that suppresses fluorescence lines.
• To develop a scalable, high-throughput readout system using VERITAS-2 and Switcher-A ASICs for DEPFET pixel matrices.

Proposed method

• The WFI employs two DEPFET-based detectors: a large-area sensor with 1024×1024 pixels and a fast-readout detector optimized for high count rates.
• Each DEPFET pixel integrates a MOSFET on a 450 µm thick, fully depleted silicon substrate, enabling high spatial and energy resolution.
• The VERITAS-2 ASIC enables fast, low-noise readout of the DEPFET matrices in rolling shutter mode, supporting high frame rates.
• The Switcher-A ASIC controls the DEPFET operation in rolling shutter mode, minimizing readout dead time and enabling high throughput.
• A graded-Z shield surrounds the detectors to absorb X-rays and reduce fluorescence-induced background lines.
• The instrument is designed to operate with Athena’s 5 arcsec HEW mirror system, matching its on-axis angular resolution.

Experimental results

Research questions

• RQ1 How can a wide-field X-ray imager achieve both high angular resolution and broad field of view in the 0.2–15 keV band?
• RQ2 What detector and readout architecture enables sub-170 eV energy resolution while maintaining high throughput for bright sources?
• RQ3 How can pile-up be minimized in high-count-rate environments, such as those with 1 Crab flux?
• RQ4 What shielding strategy effectively suppresses fluorescence background from detector materials?
• RQ5 How can DEPFET sensors be scaled and integrated into a space-qualified focal plane instrument for Athena?

Key findings

• The WFI achieves a full width at half maximum energy resolution of better than 170 eV at 7 keV, meeting the mission’s spectral performance requirements.
• The instrument supports a field of view of 40×40 arcminutes, enabling unprecedented survey power for X-ray astrophysics.
• For sources up to 1 Crab flux, the WFI maintains a throughput of over 80% with less than 1% pile-up, ensuring high-fidelity data collection.
• The VERITAS-2 ASIC enables fast readout of DEPFET matrices, supporting high frame rates essential for bright source observations.
• The graded-Z shield effectively suppresses fluorescence lines, reducing instrumental background contributions.
• The dual-detector design—featuring a large-area and a high-time-resolution detector—optimizes performance across diverse observational scenarios.