2020ApJS..247...73M

We examined two decades of SpeX/NASA Infrared Telescope Facility observations from the Small Main-Belt Asteroid Spectroscopic Survey (SMASS) and the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS) to investigate uncertainties and systematic errors in reflectance spectral slope measurements of asteroids. From 628 spectra of 11 solar analogs used for calibration of the asteroid spectra, we derived an uncertainty of σ_s^′^=4.2 % µm^–1 on slope measurements over 0.8-2.4 µm. Air mass contributes to -0.92% µm^–1 per 0.1 unit air mass difference between the asteroid and the solar analog and therefore for an overall 2.8% µm^–1 slope variability in SMASS and MITHNEOS designed to operate within 1.0-1.3 air mass. No additional observing conditions (including the parallactic angle, seeing, and humidity) were found to contribute systematically to slope change. We discuss implications for asteroid taxonomic classification works. Uncertainties provided in this study should be accounted for in future compositional investigation of small bodies to distinguish intrinsic heterogeneities from possible instrumental effects.