The optical blocking filter for the ATHENA wide field imager: ongoing activities towards the conceptual design

Abstract

ATHENA is the L2 mission selected by ESA to pursue the science theme "Hot and Energetic Universe" (launch scheduled in 2028). One of the key instruments of ATHENA is the Wide Field Imager (WFI) which will provide imaging in the 0.1-15 keV band over a 40'x40' large field of view, together with spectrally and time-resolved photon counting. The WFI camera, based on arrays of DEPFET active pixel sensors, is also sensitive to UV/Vis photons. Optically generated electron-hole pairs may degrade the spectral resolution as well as change the energy scale by introducing a signal offset. For this reason, the use of an X-ray transparent optical blocking filter is needed to allow the observation of all type of X-ray sources that present a UV/Visible bright counterpart.In this paper, we describe the main activities that we are carrying on for the conceptual design of the optical blocking filter, that will be mounted on the filter wheel, in order to satisfy the scientific requirements on optical load from bright UV/Vis astrophysical source, to maximize the X-ray transmission, and to withstand the severe acoustic and vibration loads foreseen during launch.

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

“…Figure 2 shows a schematic drawing of the WFI showing the position of the FW OBF in the context of the main instrument subsystems. [25], as well as investigations performed for LOFT [26], the use of a thin polyimide film 150 nm thick coated with 30 nm of aluminum has been proposed as additional OBF to be mounted on the WFI FW [11]. The combination of the on-chip OBF and the above FW OBF provides an average transmission in the UV/Vis lower than 10 -6 and is very efficient in attenuating the spectral range where the hot stars have their maximum emission.…”

“…Figure 4 shows the calculated number of electron-hole pairs per pixel within an integration time interval (1.3 ms) generated by the observation of early-type stars by the WFI large detector array (LDA) without (solid lines) and with the addition of the FW OBF (dashed line). The calculation is performed according to [11]. The main difference with respect to the previous calculation is due to a better modeling of the FW OBF transmission at wavelengths < 100 nm obtained by use of the refractive index of polyimide calculated from the atomic scattering factors in [15].…”

“…Given this very large size, and knowing that the filter will have to withstand severe acoustic and vibration loads during launch, the use of a reinforcing structure to mechanically support the thin aluminized polyimide film has been investigated. Based on the results of structural analysis under static load, we have chosen a mesh design with square pattern and wires parallel to the square frame diagonal [11]. The selected mesh thickness is 200 µm, the bar width is 100 µm, and the pitch is 6 mm.…”

“…Ideally, the WFI should not detect any optically generated electron-hole pair during a read-out integration time. An optical blocking filter (OBF) is thus needed in front of the detectors to observe Xray sources with UV/Vis counterparts [11].…”

Acoustic Simulation’s Verification of WFI ATHENA Filterwheel Assembly

“…The results will give the input parameters for the filter acoustic simulation, and help to decide if the launch of the module in atmospheric conditions is possible. As it was shown in previous papers (Barbera et al, 2015;Rataj et al, 2016), the filterwheel assembly can modify the sound pressure level in the filter position. It is especially dangerous for low frequencies, where the signal can be locally amplified by the assembly resonances, what results in large SPLs.…”