Description of the OSIRIS-REx Mt. Bigelow Ground Based Bennu Observations bundle V1.0 ===================================================================================== Bundle Generation Date: 2020-09-17 Peer Review: 2018a Asteroid Review Discipline node: Small Bodies Node Content description for the OSIRIS-REx Mt. Bigelow Ground Based Bennu Observations bundle ========================================================================================= All data were reduced with the IRAF software package. The images were bias-subtracted and flat-fielded with twilight and night sky flat images using tasks in the CCDRED package. The APPHOT package was used to perform aperture photometry of Bennu and photometric standard stars. In order to compensate for variable seeing and maximize signal-to-noise, the average FWHM was measured for each image and the photometric aperture was set to a radius of 2 * FWHM (Howell 1989). Sky background was measured with a circular ring aperture with an inner radius of 20 pixels and outer radius of 30 pixels. The sky aperture was centered on the position of the measured source (Hergenrother et al., 2013). Data ==== Bennu ECAS survey data are divided into two sub-directories, image and lightcurve. Images of Bennu and standard stars from the observation are contained in the 'image' sub-directory. Light curve data derived from the images are contained in the 'lightcurve' sub-directory. The 'ccd21' camera was used to obtain these data. This camera consisted of an NSF llick3 L4-W7-(1,0) device with 2048x2048 15-micron pixels. A binning of 3x3 pixels was used for an effective plate scale of 0.435 arcseconds per pixel. The field-of-view was 297x297 arcseconds (5.1x5.1 arcminutes). Orientation is north to the bottom and east to the right for all images. The filters conform to the eight-color asteroid system (ECAS) standard as defined in Tedesco et al. (1982). Each filter has the following peak wavelength and FWHM in microns: u (0.320, 0.133), b (0.430, 0.110), v (0.545, 0.055), w (0.705, 0.068), x (0.860, 0.080), and p (0.955, 0.083). The following ECAS standard stars were observed at different airmasses: SA71-07 (ubvwxp), SA71-20 (ubvwxp), and SA114-223 (w). Tedesco et al. (1982) contain the v magnitude and color indices relative to v for each star. The magnitude in each filter for each star was produced by correcting the v magnitude by the color index. As a result, each standard star filter magnitude is transformed to v and corrected for the color of the Sun. For example, a star with a v magnitude of 12.00 and v-w color index of +0.05 would have a solar color corrected w magnitude of 11.95. 101955 Bennu was imaged 339 times on the nights of September 14-17, 2005. Observations were taken with the ECAS b,v, and w filters on the night of the 14th. ECAS filters u,b,v,w,x, and p were used on the following nights. All Bennu observations are stored as FITS files that contain observational information in the image headers. Standard star observations were also made each night. These observations are also stored as FITS format files. Again, headers contain specific observational parameters. Image files are named according to the following convention: DsepYY_TARGET_Filter_ImageNumber.fit for example 5sep14_101955Bennu_b_0285.fits 5sep14_sa7101_b_0193.fits The data collected during this observation campaign was used to calculate light curves and ECAS photometry for Bennu. The full results of this study can be found in Hergenrother et al., 2013. Derived light curve data can be found in the 'lightcurve' sub-directory. Light curve files are named according to the following convention: Bennu_ECAS_Filter.txt for example Bennu_ECAS_w.txt References ========== Hergenrother, C.W., M.C. Nolan, R.P. Binzel, E.A. Cloutis, M.A. Barucci, and 11 others. Lightcurve, Phase and Color Photometry of the OSIRIS-REx Target Asteroid (101955) Bennu. Icarus, 226, 663-670, doi:10.1016/j.icarus.2013.05.044, 2013. Howell, S.B. (1989. Publications of the Astronomical Society of the Pacific 101, 616, https://doi.org/10.1086/132477. Tedesco, E. F., D. J. Tholen, B. Zellner. (1982). The eight-color asteroid survey: standard stars. Astronomical Journal, Vol. 87, p. 1585-1592, doi:10.1086/113248. Caveats to the data user ======================== The Bennu photometric data were obtained over the course of four nights between 2005 September 14 and 17. ECAS standard stars were observed at multiple airmasses for each of the six ECAS filters used. The conditions were measured to be photometric on all four nights. Seeing was variable and ranged from 2.9 arcseconds to 4.7 arcseconds on Sep. 14, 2.5 arcseconds to 6.3 arcseconds on Sep. 15, 2.9 arcseconds to 4.7 arcseconds on Sep. 16, and 2.2 arcseconds to 5.8 arcseconds on Sep. 17. A variable photometric aperture of 2 * FWHM of seeing was used to compensate for the changing seeing. Photometric measurements that were compromised by the close proximity of a star or galaxy were removed from the archived photometry. Upon preparing this dataset for PDS submission, it became known that the legend in Figure 1 of Hergenrother et al. (2013) is incorrect. The blue data points affiliated with Sep. 14 in the Figure are actually for Sep. 17. The green data points affiliated with Sep. 15 in the Figure are for Sep. 16. The red data points affiliated with Sep. 16 in the Figure are for Sep. 15. The black data points affiliated with Sep. 17 in the Figure are for Sep. 14. The error only affects the linkage between the data points and their date of acquisition in Figure 1 and not any other results in Hergenrother et al. (2013). It also became known that the instrument used for the Bennu data acquisition was ccd21 rather than the previously indicated ccd32.