New Instruments To Calibrate Atmospheric Transmission

Changing atmospheric transmission accounts for the largest systematic errors limiting photometric measurement precision and accuracy for ground-based telescopes. While considerable resources have been devoted to correcting the effects of the atmosphere on image resolution, the effects on precision photometry have largely been ignored. To correct for the transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the same direction and time that the supported photometric telescope is acquiring its data.We describe a multi-wavelength lidar, the Facility Lidar for Astronomical Measurement of Extinction (FLAME) that observes the stable upper stratosphere, and the Astronomical Extinction Spectrophotometer (AESoP), a spectrophotometer that creates and maintains NIST absolute standard stars. The combination of these two instruments enables high photometric precision of both the stellar spectra and atmospheric transmission. The throughput of both FLAME and AESoP are calibrated to NIST radiometric standards.This inexpensive and replicable instrument suite provides the lidar-determined monochromatic transmission of Earth's atmosphere at visible and near-infrared wavelengths to better than 0.25% per airmass and the wavelength-dependent transparency to better than 1% uncertainty per minute. These atmospheric data are merged to create a metadata stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere and ultimately limit ground-based all-sky photometry to 1% - 2% precision.