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The weekly pyrheliometer files contain intensity data from a pyrheliometer as a dedicated cloud sensor for the NEID project.
This is an instrument commonly used in photovoltaic science to monitor solar flux (specifically direct normal irradiance, DNI)
over a large wavelength range, but for our purposes, it fulfills the role of directional cloud sensor, since obscuration of
the solar disk will cause a drop in flux.
Uneven extinction of the solar disk--for example, due to passing clouds--can cause large spurious RV shifts. Because the Sun
is a resolved source, heterogeneous obscuration of the solar disk will preferentially block either redshifted or blueshifted
light, distorting the observed RVs.
The NEID pyrheliometer is model MS-57 from EKO Instruments, which measures the total solar direct normal irradiance (DNI) from
200 to 4000 nm in a 5-degree field of view around the Sun--for our purposes, it is effectively a directional bolometer. A
thermopile within the pyrheliometer reads out an analog voltage to a LabJack data acquisition module, and the voltage can be
converted to an intensity via a factory-calibrated conversion factor. We set the readout rate of the pyrheliometer to 1 Hz
to match the cadence of the exposure meter, as faster cadences offer no significant additional benefit for detecting clouds
during solar exposures.
The columns in the pyrheliometer files are:
- Date (UTC-ISOT)
- Pyroheliometer_Photocell_Output (Volts)
- Solar_Irradiance (W/m^2)
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