  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 4.2%/micron on slope
measurements over 0.8 to 2.4 micron. Air mass contributes to -0.92%/micron per
0.1 unit air mass difference between the asteroid and the solar analog, and
therefore for an overall 2.8%/micron slope variability in SMASS and MITHNEOS
designed to operate within 1.0 to 1.3 air mass. No additional observing
conditions (including 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.

Earth and Planetary Astrophysics