| DATA_SET_DESCRIPTION |
Data Set Overview : The Small Main-belt Asteroid Spectrographic Survey (SMASS) was initiated in 1990 with the goal of obtaining spectra over the visual wavelength region for a substantial number of small (D < 20 km) main-belt asteroids. Observations were primarily made using the 2.4 m Hiltner telescope, located at the Michigan-Dartmouth-MIT (MDM) Observatory on the southwest ridge of Kitt Peak in Arizona. The Mark III spectrograph, equipped with a TI 4849 CCD (398 x 598 pixels) was used with a low resolution grism (150 line/mm, blazed at 7300 A) to cover the wavelength range from 4000 to 10000 A in a single exposure, with a dispersion of ~10 A/pixel. A Wratten 12 filter (cutoff < 6000 A) was placed over one-half of the CCD dewar window (the red spectral half) in order to block the second order spectral image. This setup has the ability to image the entire spectrum in a single exposure and has many advantages over piecing together the different spectral regions from separate images. In particular, the effects resulting from the asteroid's rotational lightcurve can be neglected when the entire spectral range is imaged simultaneously. While the typical seeing at MDM gives images with a FWHM of less than or equal to 2 arcsec, a 4.7 arcsec wide slit, oriented in the N-S direction, was regularly used. This slit width results in a spectral resolution of abut 50 A. A total of 316 asteroids were included in the survey. The data have been published in Xu, et al. (Icarus 115, 1-35, 1995) [XUETAL1995]. SMASS is an ongoing survey and data will be added to this archive yearly as they are published. Parameters : The parameters included for each asteroid are scaled reflectance as a function of wavelength (in SMASS.TAB), and the first and second principal components of the principal component analysis carried out on the reflectance spectra (in SMASSPC.TAB). Data : The scaled reflectances as a function of wavelength are in the file SMASS.TAB. The data on the 316 different asteroids are concatenated in this file. SMASSNDX.TAB is an index to the data table, and gives the starting record, ending record, and number of records in SMASS.TAB for each asteroid spectrum. SMASSPC.TAB gives the first and second principal components for each of 315 of the asteroids, 5145 Pholus having been omitted from the principal component analysis. Ancillary Data : Principal component analysis was carried out for 315 of the asteroid spectra (asteroid 5145 Pholus was omitted from the analysis due to its very unusual spectrum). SMASSPC.TAB gives the first and second principal components for each of the 315 asteroids. For information on the principal component analysis technique, see [CHATFIELDETAL1980]. The eigenvectors for the first two principal components are tabulated in SMASSEIG.TAB. The principal components PC1 and PC2 for each asteroid are calculated from the observed reflectances (normalized to unity at 5500 A), at 50 equally spaced wavelengths from 4800 - 9700 A. At the ith wavelength: lambda(i) : Wavelength E1(i) : ith Eigenvector coefficient for PC1 E2(i) : ith Eigenvector coefficient for PC2 R(i) : Asteroid reflectance at lambda(i) (normalized to unity at 5500A) m(i) : mean value of R(i) for the entire set of asteroid spectra s(i) : standard deviation of R(i) for the entire set of asteroid spectra PC1 and PC2 are calculated as follows: 50 PC1 : SUM E1(i)*{[R(i)-m(i)]/s(i)} i:1 50 PC2 : SUM E2(i)*{[R(i)-m(i)]/s(i)} i:1 (where 'SUM' represents the standard mathematical summation notation) Modification History : For Version 2.0, separate spectra files for each asteroid were created by extracting the appropriate lines from the 'smass.tab' main data file, trimming off the object identification, and providing individual labels. The original data files were updated slightly to include catalog numbers for those asteroids which received numbers after the data were published. Data were not re- sorted for these numbers, however, which all occur at the end of the file. The provisional designations used to identify objects in the original data file have been retained for cross-reference.
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