An Assessment of the LEISA Spectrometer for Extragalactic Background Light Measurements

The extragalactic background light (EBL) is the summed emission from all sources outside of the Milky Way Galaxy. An accurate measurement of the EBL can be used as a benchmark to test whether there are any components in excess of the integrated light from galaxies. Any discrepancies would imply the presence of new sources of diffuse extragalactic emission, which is predicted to include the aggregate light from the first galaxies in the universe. We have studied archival data from the Linear Etalon Imaging Spectral Array (LEISA) aboard the New Horizons spacecraft to determine whether it could be used to measure the EBL at wavelengths between 1.2 to 2.5 μm. We have empirically determined LEISA’s sensitivity to diffuse brightness to be 2x10⁶ nW m^-2 sr^-1 in 1.5 second exposures, which essentially precludes the possibility of reaching EBL signals at the level of 10 nW m^-2 sr^-1. Further, static image artifacts from the PICNIC detector and readout contaminate the diffuse signal with correlated spatial structures several tens of pixels wide at the level of 3x10⁶ nW m^-2 sr^-1 and we have not found an operable mitigation. These studies have allowed us to conclude that LEISA is not even capable of generating useful upper limits on the amplitude of the EBL at near-infrared wavelengths.