The SORCE XPS is a package of twelve silicon XUV photodiodes for measuring the XUV and EUV irradiance from 1 to 34 nm. Each photodiode has a thin-film filter to provide an approximate 7 nm spectral bandpass. These thin film filters are deposited directly on the photodiode to avoid using delicate metal foil filters which are difficult to handle, prone to develop pin holes, and degrade with time.
This set of twelve XUV photometers is packaged together with a common filter wheel mechanism, which can rotate a closed aperture, a fused silica window, or an open aperture in front of any given photometer. The fused silica windows on this filter wheel permit accurate subtraction of the background signal from visible and near UV light. While nine of the XUV photometers have thin film filters for making solar XUV irradiance measurements, the other three photometers are bare silicon photodiodes to be used in tracking the transmission of the fused silica windows. Of the nine filtered XUV photometers, one photodiode is a bare XUV photodiode with Acton Lyman-alpha filters for a redundant measurement of the important Lyman-alpha irradiance. A typical measurement cycle for the XUV photometers is to measure the dark signal with a closed aperture, to measure the background signal with the window, and then to measure the solar XUV radiation with an open aperture. For in-flight calibration purposes, three of the nine XUV photometers are redundant and are used with a lower duty cycle (approximately once per week) for tracking instrument degradation.
The XPS has had one flight anomaly associated with its filter wheel mechanism. On December 19, 2005, the XPS filter mechanism became stuck in position 0 and then several hours later, the XPS filter mechanism began to operate normally. When discovered, the XPS was first activated to safe mode and then following an initial review, the XPS filter wheel was moved to position 6 to maximize its science return. In this fixed filter wheel position, the XPS can measure 2/3 of its original spectral range, gains higher time cadence for its solar observations (1-min instead of 5-min), but loses redundant channel calibrations. From the discussions at the XPS flight anomaly reviews, the root cause is thought to be debris in the motor's planetary gear system and that the debris has either been ground down to smaller size or has worked itself out of the gear system. Although the filter mechanism has worked well ever since, we took the conservative approach to only operate the XPS filter wheel mechanism occasionally (about monthly) for calibration purposes. Only two days of solar observations by XPS have been lost due to this anomaly.