Data Set Overview : All spectral observations were obtained using the SpeX instrument on the NASA IRTF in low-resolution mode. Observations were made remotely and in classical mode on site. SpeX in low resolution mode has resolving powers of R~100 across the wavelength region from 0.75 to 2.5 microns. An infrared guider is available to guide on calibration stars (sidereal rates) and near-Earth asteroids (non-sidereal rates). The main spectrograph uses a 1024x1024 Aladdin 3 InSb array and the guider uses a 512x512 Aladdin 2 InSb array [RAYNERETAL2004]. Low-resolution spectrographs like SpeX are ideal for resolving broad absorption features produced by abundant mafic minerals like olivine and pyroxene that make up many asteroid surface assemblages. The low resolution prism mode also helps in obtaining spectra with higher signal-to-noise-ratios (SNR) and asteroids as faint as Vmag~17.5 are routinely observed.  Spectral observations for this data set were made by taking nodded spectral image pairs of the target near-Earth asteroid, local standard star (for telluric correction), solar-analog stars, and calibration flat-field and argon arc-lamp images. The placement of these stellar observations, temporally and spatially on the sky, in relation to the asteroid is important for producing good quality spectra. If the atmosphere over Mauna Kea is stable throughout the observing run (photometric), then the log of the flux (apparent magnitude) of the object will decrease linearly with increasing airmass. Hence, all objects are typically observed at airmasses less than 1.5, which corresponds to a zenith angle of less than 50 degrees. However, if the atmosphere is unstable over Mauna Kea, whether due to an orographic cap cloud or rapid variability of water vapor content, it often produces a non-linear magnitude-airmass relationship.  Local (or extinction) standard stars close to the asteroid are observed to correct for the terrestrial atmospheric water vapor features. Generally, the greater the distance between the local standard star and the asteroid, the poorer the monitoring of the sky conditions for the asteroid. During a typical observing run, a local standard star with spectral properties similar to our Sun (i.e., G-type, main sequence stars), is paired with an asteroid and is observed over a wide airmass range that bracket the airmass range of the asteroid observations. Solar analog stars are observed to remove the solar continuum from the asteroid spectrum. At least two solar analog stars should be observed each night to eliminate the possibility of systematic errors that may be present in a single solar analog star spectrum.  SpeX prism data was processed using two different data reduction protocols (SpecPR and Spextool) to cross calibrate any variations in the spectral band parameters and check for systematic errors. The first method was developed by Gaffey and involved using a combination of Linux-based IRAF and Windows-based SpecPR software routines. Detailed description of this method can be found in [REDDY2009], [ABELL2003], and [HARDERSEN2003]. The data was also reduced using IDL-based Spextool provided by the NASA IRTF [CUSHINGETAL2004]. Except for minor systematic wavelength offset between SpecPR and Spextool data sets which might be due to the way wavelength calibration is being applied (Cushing, pers. comm.); no significant deviations were found between the two reduction methods.  Cited References  Abell, P.A., Near-IR reflectance spectroscopy of main belt and near-Earth objects: A study of their composition, meteorite affinities and source regions. Ph.D. dissertation. Rensselaer Polytechnic Institute, Troy, NY, USA, 2003. [ABELL2003]  Cushing, M.C., W.D. Vacca, and J.T. Rayner, Spextool: A spectral extraction package for SpeX, a 0.8-5.5 micron cross-dispersed spectrograph, PASP 116, 362-376, 2004. [CUSHINGETAL2004]  Hardersen P.S., Near-IR Reflectance Spectroscopy of Asteroids and Study of the Thermal History of the Main Asteroid Belt. Ph.D. Dissertation. Rensselaer Polytechnic Institute, Troy, New York, USA, 2003. [HARDERSEN2003]  Rayner, J.T., P. M. Onaka, M. C. Cushing and W. D. Vacca, Four Years of Good SpeX, in Ground-based Instrumentation for Astronomy, A. F. M. Moorwood and M. Iye, Eds., Proceedings of the SPIE, vol. 5492, pp. 1498-1509, 2004. [RAYNERETAL2004]  Reddy, V., Mineralogical Survey Of Near-Earth Asteroid Population: Implications For Impact Hazard Assessment And Sustainability Of Life On Earth, Ph.D. dissertation. University of North Dakota, Grand Forks, 2009. [REDDY2009]

Confidence Level Overview : Uncertainties in spectral parameters for near-IR data are crucial for detecting and quantifying surface composition. Except for minor systematic wavelength offsets between SpecPR and Spextool data sets which might be due to the way wavelength calibration is being applied (Cushing, pers. comm.); no significant deviations were found between the two data reduction protocols. The average wavelength resolution of SpeX data in SpecPR is ~0.0033 microns and the Spextool data is 0.0035 microns.This is just due to spectral resolution based on the wavelength calibration. The errors plotted with the data are standard errors of the mean. Uncertainties in the data arise primarily due to low SNR of the final average spectrum, incomplete correction of telluric absorption features, and variable sky/weather conditions.