Solar Radial Velocity Archive

Pyrheliometer Data

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)