| DATA_SET_DESCRIPTION |
Data Set Overview
=================
All observations were obtained with SpeX, the low- to medium-resolution
near-IR spectrograph and imager at the NASA Infrared Telescope Facility
(IRTF) on Mauna Kea, Hawaii. This instrument uses prism cross-dispersers
and gratings to achieve resolving powers up to R~2500 across the wavelength
region from 0.8 to 5.5 microns. An infrared slit-viewer allows for object
acquisition and guiding. The SpeX spectrograph employs a 1024x1024 Aladdin
3 InSb array and the slit-viewing imager uses a 512x512 Aladdin 2 InSb
array. [RAYNERETAL2004]. Of specific importance to asteroid studies, SpeX
provides a low-resolution (R~100 when using the nominal 0.8 arcsecond slit)
'prism' mode covering the wavelength interval 0.8-2.5 microns. In prism
mode, asteroids as faint as Vmag~16.5 are routinely observed, with even
fainter magnitude limits being reached when good sky conditions permit.
Asteroid observations are normally obtained by taking nodded spectral image
pairs of the target asteroid, solar-analog stars, and (possibly) telluric
standard stars, followed by calibration flat-field and arc-lamp images. The
image pairs are taken by shifting the asteroid (or star) position on the
slit (nodding the telescope position) by a fixed amount. In this 'A-B'
image pair, the background sky spectrum is being recorded simultaneously
with the target, allowing the sky to be subtracted in both the A-B and B-A
pairs. After flat fielding the individual SpeX images and carrying out the
pairwise sky subtraction, multiple images of the target or standard star can
be stacked, and a 1-dimensional spectrum extracted, wavelength calibrated
and re-binned as appropriate. Calibration of each asteroid spectrum in
dimensionless units of relative reflectance is achieved by dividing the
asteroid spectrum by that of a solar analog star.
Because this data set contains results derived by several different research
groups, there are some variations in observing strategies and reduction
procedures. This is particularly the case in the final stages of spectral
calibration where effects of the Earth's atmosphere (telluric absorptions)
are fitted and removed. Different techniques for telluric correction
include: 1) the modeling of atmospheric absorption bands using ATRAN
[LORD1992] which is based on the HITRAN database of molecular absorption
lines (for convenience, a .pdf copy of Lord 1992, NASA Technical Memorandum
103957 is included in the document directory), 2) using the method included
in SpeXTool [VACCAETAL2003] where observations of an A0 (telluric standard)
star are convolved with a high-resolution model of Vega to construct a
telluric correction spectrum, and 3) the use of SpecPR, where
wavelength-dependent extinction coefficients are derived from nightly
standard star observations. While the application of any of these telluric
calibration methods leads to very similar results, users of this data set
are encouraged to refer to the original papers for specific details about
the data reduction and calibration procedures used.
Papers for which data are archived are:
Binzel et al. (2001) M&PS 36, 1167-1172 [BINZELETAL2001]
Clark et al. (2004) AJ 128, 3070-3081 [CLARKETAL2004]
Rivkin et al. (2005) Icarus 175, 175-180 [RIVKINETAL2005]
Sunshine et al. (2004) M&PS 39, 1343-1357 [SUNSHINEETAL2004]
Sunshine et al. (2007) M&PS 42, 155-170 [SUNSHINEETAL2007B]
Sunshine et al. (2008) Science 320, 514-517 [SUNSHINEETAL2008]
Data File Description
=====================
Data file names have the format: AAAA_YYMMDDTHHMMSS.tab, where AAAA is the
permanent number or provisional designation of the asteroid, YYMMDD is the
UT start date, and HHMMSS the UT start time of the observation. This naming
convention is particularly useful when a study involved rotationally
resolved spectra, where multiple spectral files are being archived for a
single asteroid.
To determine what asteroid spectra have been archived, and to help navigate
to specific files, a science index (science_index.tab) is included in the
upper level of the data directory. This file lists all spectra in order of
asteroid designation, and includes the data path, UT start and end times of
the first and last asteroid observations used in creating the spectrum, the
apparent magnitude, phase angle and heliocentric distance of the asteroid at
the time(s) of the observations(s), and a listing of the solar analog stars
used in calibration.
Each spectrum file contains 3 columns: wavelength (in microns), relative
reflectance, and error in relative reflectance. For those spectra reduced
using IRAF, the output spectrum is usually re-binned to a consistent
dispersion, which allows different spectra to be easily compared and
combined. During the various reduction and calibration steps, if a
particular spectral data point is identified as discrepant, that point is
flagged and removed from the final spectrum file. All spectra have been
scaled by fitting a polynomial over the region centered on the J-band (1.215
microns), and then normalizing the fitted value at 1.215 microns to 1.00.
Thumbnail plots of all spectra archived in this data set are provided (pdf
files) in the document directory. These plots are organized by publication
(one file per publication), using the same naming scheme as the data
directory structure.
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