The Filters and Calibration Wheel (FCW)

The filter and the calibration wheel (FCW) has been designed to monitor, during the observational life of the mission, the performances of the detector. In particular we intend to check the low and high energy modulation factor and the level of absence of spurious modulation. Also we monitor the gain that depends both on temperature and on charging.

IXPE-DU — (a) Deconstructed diagram of IXPE DU, showing the position of the FCW on top of the GPD; (b) picture of the FCS installed onto the wheel in the IAPS clean room.

The FCW is placed on the top lid of the DU, as shown in the figure above. By rotating around its central axis, the FCW allows the placement of one of the four calibration sources or a gray filter in front of the GPD, in addition to the open and closed position, depending on the observational requirements.

IXPE-FCW — Filter and Calibration Wheel (FCW): Cal A, Bragg-reflected polarized 2.98-keV (Ag-La fluorescence) and 5.89-keV (Mn-Ka) illumination; Cal B, unpolarized 5.89-keV spot; Cal C, unpolarized 5.89-keV flood; and Cal D, unpolarized 1.74-keV (Si-Ka fluorescence) flood.

The FCW hosts the filters and the calibration sources to calibrate the detector during flight or to perform peculiar observations. The FCW has 7 positions commanded by the DSU. The positions correspond to open position, closed position, gray filter and calibration source A, B, C and D. The stepper motor is a Phytron phySPACE^TM 42-2. It is a COTS component with 25 years of space flight heritage. The phySPACE^TM series is developed and built to resist vacuum, vibrations, low/high temperature and radiation while maintaining high performance, precise positioning long life.

The motor pinion is made of Vespel®. The double bearing system that connects the fixed part to the rotating one is made of stainless steel. The positioning is performed and monitored by two different systems. One system is based on the use of Hall sensors as non-contact switches to stop the wheel rotation to the requested position. A combination of three hall sensors allows for setting and monitoring the 7 wheel positions. In order to have an independent measurement of the wheel position a second, analog, device, a Novotechnik PRS65/S152 potentiometer, is implemented. A qualification campaign allowed for declaring this component as flight compatible.

The FCW positioning can be set digitally or on a step-by-step base allowing, for example for spanning the entire sensitive area for measuring the gain and modulation factor at different positions from the center. The calibration set is composed of the X-ray sources and of their holders. It comprises the items which can be put in front of the GPD by rotating the FCW.

Updated on September 7, 2021

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