ATHENA WFI optical blocking filters development status toward the end of the instrument phase-A

Abstract

The Wide Field Imager (WFI) is one of the two instruments of the ATHENA astrophysics space mission approved by ESA as the second large mission in the Cosmic Vision 2015-2025 Science Programme. The WFI, based on a large array of depleted field effect transistors (DEPFET), will provide imaging in the 0.2-15 keV band over a 40'x40' field of view, simultaneously with spectrally and time resolved photon counting.The WFI detector is also sensitive to UV/Vis photons, with an electron-hole pair production efficiency in the UV/VIS larger than that for X-ray photons. Optically generated photo-electrons may degrade the spectral resolution as well as change the energy scale by introducing a signal offset. For this reason, the use of X-ray transparent optical blocking filters (OBFs) are needed to allow the observation of X-ray sources that present a UV/Vis bright counterpart.The OBFs design is challenging since one of the two required filters is quite large (~ 160 mm  160 mm), very thin (< 200 nm), and shall survive the mechanical load during the launch.In this paper, we review the main results of modeling and characterization tests of OBF partially representative samples, performed during the phase A study, to identify the suitable materials, optimize the design, prove that the filters can be launched in atmospheric pressure, and thus demonstrate that the chosen technology can reach the proper technical readiness before mission adoption.

Design and acoustic tests of the ATHENA WFI filter wheel assembly development model towards TRL5

“…This result strongly suggests that the damage is related to a local filter defect more than to a poor design or technology—probably not fully glued surface interface between filter membrane and mesh. More details on the investigation of the observed damage and failure modeling will be discussed elsewhere 11 …”

“…The WFI requires filters to reduce visible and ultraviolet light exposure of the DEPFET detector. Separate filters will be mounted on a filter wheel (FW), two thin optical blocking filters (OBF) one for the LDA and one for the FD, and one thick x-ray blocking filter (XBF) for the FD, specifically designed to attenuate the soft emission (E<2 keV) from x-ray bright sources 4 . The FW will have seven functional positions, namely: (1) both detectors closed, (2) LDA thin OBF, (3) FD thick XBF, (4) both detectors open, (5) LDA calibration (Fe55 radioactive source), (6) FD calibration (Fe55 radioactive source), and (7) FD thin OBF.…”

DEPFET Active Pixel Sensors

“…It provides the following functionalities. First, optical blocking filters made of aluminium deposited on polyimide [3]. Second, an onboard calibration source based on a radioactive 55 Fe source.…”

Elastic characterization of nanometer-thick polymeric film for astrophysics application with an experimental-numerical method

“…The x-ray astrophysics future missions require an increase in the in-band transparency of the optical blocking filters (OBF) to fully exploit new detectors’ sensitivity 1 – 3 This implies the adoption of increasingly thin membranes. The OBFs are required to block infrared to ultraviolet light and molecular contamination and to attenuate the electromagnetic interference originated by the telemetry system and the instrument electronics.…”