Why a window ?
The EUV detector, sensitive in the 55nm – 155nm range, is equipped with a window in MgF2 (Magnesium fluoride) that can be opened in flight. Opening is done by activating a “one-shot” mechanism (i.e. can only be activated once; once opened, the window can only be closed manually).
The presence of this small window opening mechanism was imposed by the range of wavelengths covered by this detector. Indeed, below 100 nm, no material is sufficiently transparent to allow the window to be kept in place on the optical path (which would considerably reduce the amount of photons – already weak – that the detector could measure). A detector without a window is called an open detector. In addition, the detector surface is covered by a “photocathode” of Cesium Iodide (CsI) which improves the overall quantum yield of the detector in the range ~ 100 to 155nm. The performance of the CsI degrades in contact with humidity and thus the atmosphere, which requires keeping the detector closed and under vacuum during the ground activities (before launch).
Thus, from the assembly of the detector to the launch, the detector window is kept closed and the detector itself is pumped continuously thanks to the pumping system (see Focus on: the pumping unit).
The window opening mechanism
The mechanism consists essentially of the window, a torsion spring, a lock and a paraffin actuator (starsys).
In the “Closed” position the window is held by the latch, binding the spring. When activating the actuator, the paraffin it contains is heated and expands to transmit a translational motion to a piston that pushes the latch, releasing the window thanks to the potential energy contained in the constrained spring. A small “reed” (magnetic sensor) position sensor detects the “open” position of the window.
Activation of the actuator is ordered from the ground by sending a specific telecommand to the PHEBUS DPU (Data Processing unit). When receiving this telecommand (TC), the DPU manages the activation of the heater for the specified time. A temperature sensor attached to the actuator allows the evolution of the activation to be monitored.
In flight opening
The window was successfully opened Monday, December 3, 2018 during the second phase of the in-flight acceptance. The window opened after 130 seconds of activation while the initial temperature of the heater was-6 °c. The current consumed by the actuator has been detected as well as the opening of the window thanks to the reed sensor.
A few words about ground tests
In order to allow characterizations and performance tests of the detector and instrument, it is essential to open the EUV window to cover the full spectral range (55-155nm).
Thus, the tests with this detector must be carried out in a vacuum chamber. There are three reasons for constraint:
1. The window must be open to allow the UV radiation to pass through at wavelengths below 110nm, without degrading the moisture-sensitive CsI photocathode.
2. The atmosphere absorbs the luminous flux in the UV, below 200nm. Any “on the bench” measurement is therefore prohibited for these wavelengths: the detector would not receive any photon!
3. The detector is operated under high voltage (approx. 3600V for EUV). At these high voltage values, the risk of causing a discharge (an electric arc) is important when the window is closed.
At the end of the tests, the window must be able to be closed again in vacuum! An mechanism external to the instrument is therefore used: with the help of a motorized curved finger that enters the instrument through a hole provided for this purpose, it makes it possible to close the window while the whole thing is still inside the chamber and under vacuum!
These constraints mean that the detector has not been tested at satellite level. The latest activation of the EUV detector with high voltage ON was in January 2015 (PHEBUS was integrated on the satellite in April 2015)!