XUV Photometer Configuration
The 12 XUV Photometers (XPs) are grouped into 3 sets. Each set of 4 XPs is arranged in a circle for use with the filter wheel mechanism. The filter wheel, which has 3 different rings of filters for the 3 sets of XPs, has 8 positions: 4 blocked for dark measurements, 2 clear for solar XUV measurements, and 2 with fused silica windows for solar visible background measurements. An observation run is a sequence of measurements from 5 consecutive filter wheel positions, normally starting and ending with dark measurements. XP #1-4 are grouped in the inner ring. XP #5-8 are grouped in the middle ring. XP #9-12 are grouped in the outer ring. XP #1, 5, and 10 are redundant XPs as part of the in-flight calibration plan. XP #4, 8, and 12 are bare photodiodes for measuring the fused silica window transmission during each observation run.
The electronics for each photodiode are simple and include only a current amplifier and a voltage-to-frequency (VTF) converter. The VTF converter was chosen over an analog-to-digital (A/D) converter because the VTF converter provides a much wider dynamic range with significantly lower power consumption. The interface from the VTF converters to a microprocessor is a 32-bit digital counter for each photodiode.
There are several suitable metals for use as filters, and the use of multiple coatings on the same diode provides a way of narrowing the bandpass of each diode. Powell et al.  discuss thin film filters suitable for this wavelength range. In addition to selecting a filter for its bandpass, one must also select a filter that can block the solar visible radiation by a factor of 1E10 or better; otherwise, the XUV Photometer signal would be dominated by the solar visible radiation instead of the XUV radiation. Previous LASP projects have flown the following XUV photodiode filters: Ti, Ti/Zr/C, Al/Sc/C, Al/C, Sn, Al/Sn, Ti/Zr/Au, Ti/Zr/Si, Al/Cr, Ti/Pd, Al/Nb/C, and Al/Mn. The detector sensitivities for the silicon photodiodes of the five filters planned for SORCE are shown in the following plot. These five filters are designed to have a bandpass of about 7 nm over the range 1 to 34 nm, a visible light blockage of at least 10 to 12 orders of magnitude, and a solar XUV signal between 1 and 10 nÅ. The XUV photometer with the Ti filter measures the irradiance below 6 nm. The photometer with the Ti/Zr/Au filter measures the 6 to 12 nm region. The photometer with the Ti/Zr/Si filter measures the 12 to 20 nm region. The Al/SC/C filter is designed to eliminate the bright solar He II 304 Å emission so that the 17 to 25 nm region can be measured. Finally, the photometer with the Al/Cr filter measures the 25 to 34 nm solar irradiance. The additional filtered photometer is a bare XUV photodiode with Acton Lyman-alpha (121.5 nm) filters for a redundant measurement of the important Lyman-alpha irradiance.