Description of the HST images, albedo maps, and shape of (1) Ceres bundle V1.0 ============================================================================== Bundle Generation Date: 2023-03-31 Discipline node: Small Bodies Node Content description for the HST images, albedo maps, and shape of (1) Ceres bundle ================================================================================== Note: This bundle was migrated to PDS4 from the PDS3 data set EAR-A-HSTACS-5-CERESHST-V1.0. For PDS3 data sets migrated to PDS4, the following text is taken verbatim from the data set description and confidence level note of the PDS3 data set catalog file. In these cases, some details may not be correct as a description of the PDS4 bundle This dataset contains three parts: 1) The photometrically calibrated images of HST ACS/HRC observations of asteroid (1) Ceres (HST GO-09748); 2) The albedo maps of Ceres; and 3) The shape of Ceres. 267 images through three ACS/HRC filters, F555W, F330W, and F220W, centered at 535 nm, 335 nm, and 223 nm, respectively, have been calibrated to the standard reflectance unit, I/F, where I is the intensity reflected off the surface of Ceres, and pi*F is the incident solar flux received at the surface of Ceres. The shape model of Ceres has been constructed from a subset of these images by (Thomas et al. 2005). The surface albedo maps of Ceres at above three wavelengths have been produced from these images and the shape model by Li et al. (2006). The albedo maps cover the surface of Ceres between +/-50 deg latitude. For detailed descriptions of the calibration and data processing, please refer to Thomas et al. (2005), and Li et al. (2006). Below is a brief summary for the photometric calibration. Observations ============ Three HST visits was performed on 12/28/2003, 12/30/2003, and 1/23-24/2004. The first visit consists of 6 HST orbits, providing a complete and almost evenly spaced coverage for one rotation of Ceres with a 9.075-h period at 6.2 deg phase angle. Other two short visits were at 5.4 deg and 7.5 deg phase angles, respectively. Totally 267 images were taken, of which 255 were in HRC-512 sub-frame for shape model and photometric study, and 12 were in HRC full frame for satellite search. Ceres was resolved to about 30 pixels across the disk. Both the sub-solar and the sub-Earth points on Ceres are close to its equator for all images. Processing ========== The HST images were initially processed through the standard HST data calibration pipeline, which applies CCD related corrections, bad pixel removal, bias correction, dark correction, and flat field (Pavlovsky et al. 2005). The calibration history can be found from the relevant history keywords in the images. Cosmic rays have not been removed in either the standard calibration pipeline or the following additional calibration, because Ceres is a moving and fast rotating object, for which cosmic ray rejection requires special processing. The images produced by the HST calibration pipeline were then corrected for geometric distortion, and the celestial North (J2000) was rotated up. The re-sampling is done by a nearest-neighbor transformation, and is thus none-reversible. We have stored all images in 1024x1024 frames, and filled inthe blank areas by zeros. The shape was produced by limb fitting from a subset of 217 images. The shape of Ceres was found to be an oblate spheroid. The planetocentric coordinate is defined based on the pole orientation of Ceres, and the prime meridian is defined on a bright spot at about 10 deg north latitude. For more details, please refer to Thomas et al. (2005) and Li et al. (2006). The photometric calibration was performed by calculating a calibration constant for each filter, to scale the images in DN/s to I/F unit directly, with the help of two HST standard photometry keywords, PHOTFLAM and PHOTZPT, as well as the helio- and geo-centric distances, and the pixel scale of ACS/HRC detector. The calibration constants are listed in Table 5 of Li et al. (2006). However, one more special step has to be applied for the images obtained through F220W filter, to correct for the substantial red leak, which was estimated for Ceres to be ~19.8% of the total flux measured through this filter. The calibrated I/F images of F220W filter were then scaled by a factor of 0.802. Finally, no correction for the charge transfer efficiency (CTE) of the HRC CCD detector has been made. But the effect of CTE has been estimated to be much less than 1% on the absolute calibration of these images. Starting from the calibrated images, the photometric properties of Ceres were modeled, and the surface albedo deviation maps of Ceres are constructed (Li et al. 2006). In the photometric modeling process, the areas too close to the edge of Ceres were disregarded. Therefore with the sub-solar and sub-Earth latitude of these observations close to the equator, the surface coverage of these maps can only reach +/-50 deg reliably. To avoid phase angle correction, we only used the images from the first visit to construct the maps. Since the Hapke single-scattering albedo (SSA) averaged over the surface of Ceres is only about 7% at V-band, and less for shorter wavelengths, the ratio maps of the original images to the modeled images were considered as the albedo deviation maps from the global average. Because the normal reflectance of the dark surface is also proportional to its SSA, these ratio maps are also the deviation maps of the normal reflectance. Finally the ratio maps covering different areas on Ceres were projected to a simple cylindric planetocentric longitude-latitude system, and combined to construct the albedo maps. Data ==== The photometrically calibrated images contain all the FITS headers propagated from the original HST images. Several additional keywords were added as well, including: PHOTIOF0 : Calibration constants applied to each image REDLEAK : The fraction of red leak flux REDUCMAG : The total magnitude of Ceres at both helio- and geo-centric distances at 1 AU CERSUND : Ceres-Sun distance in (AU) CEREARD : Ceres-Earth distance in (AU) PHASE : Solar phase angle (deg) NORAZ : North pole clock angle in the image (deg) CENTERX : row number of Ceres body center in the image CENTERY : line number of Ceres body center in the image SUBSLON : Sub-solar east longitude (deg) SUBSLAT : Sub-solar latitude (deg) SUBELON : Sub-Earth east longitude (deg) SUBELAT : Sub-Earth latitude (deg) The reduced magnitude and the later seven keywords above about Ceres coordinates were only calculated for a subset of 217 images. The geometry keywords were calculated from the most recently developed shape model and the corresponding planetocentric coordinate (Thomas et al. 2005). A table listing the aspect data of these images can be found from this dataset. The shape of Ceres is expressed in an ASCII table by its equatorial and polar radii, as well as the pole orientation and prime meridian argument in J2000 frame. The albedo deviation maps are expressed as 720x361 images, with the horizontal axis corresponding to east longitude, and the vertical axis corresponding to the latitude. The longitude starts from 0 deg at the left-most column of each map, increasing eastward, with each pixel corresponding to 0.5 deg increment, until 359.5 deg at the right-most column. The latitude starts from -90 deg at the bottom row, increasing upward, with each pixel corresponding to 0.5 deg increment, until the +90 deg at the top row. The pixel values in each map represent the deviations at the location from the global average at the wavelength of that map. For example, in the 330 nm map, if a pixel value is 0.96, it means the single-scattering albedo at that position is 96% of the average value at this wavelength, so is the normal reflectance. The values of the global average Hapke SSA and normal reflectance are listed in the FITS headers of these maps. Also included with these maps are the corresponding relative uncertainty maps. Having the same structure as the albedo maps, the uncertainty maps represent the standard deviations of each corresponding pixel in the albedo maps, as calculated from all images that cover that longitude-latitude position. Note that for positions that only have one or a couple of data points from different images, the standard deviations do not have a definitive meaning. This is the case for some points at high latitude in the uncertainty map of 223 nm albedo map, where we did not have as many images to construct the albedo map as for other two wavelengths. Note that these uncertainty maps do not include either the absolute photometric calibration uncertainties, or the modeling uncertainties for the global average Hapke's parameter. However, neither of them affects the relative uncertainties of these albedo maps. The CTE uncertainties and HST imaging noise are not included in the uncertainty maps either, but they are both very small and negligible. Caveats to the data user ======================== The uncertainties of the absolute photometric calibrations are ~2% for F555W images, ~3% for F330W images, and ~8% for F220W images, respectively. The uncertainties for the modeled single-scattering albedo are ~3% at 535 nm, ~4% at 335 nm, and ~9% at 223 nm, respectively. See Li et al. (2006) for more details. The shape and pole orientation of Ceres have been determined very accurately, with 5 deg in the pole orientation, and 1.8 km in the semi-axes (Thomas et al. 2005). The uncertainty for the argument of prime meridian is not applicable because this parameter defines the starting point of the longitude system, and is not a measurement. It needs to be noted that cosmic rays have not been rejected for these images, and the very small effect of CTE has not been removed, either. The Ceres albedo maps are defined under the most recently defined body-fixed coordinate system as indicated in the maps. Please refer to Thomas et al. (2005) for the definition of the coordinate system, and please note that east longitude is used in these maps.