Reconstructing HST Images of Asteroids

1. 1 - Ceres Raw Restored Ratio 1:2 Ratio 2:5 8 - Flora 89 - Julia C – type Distance from Earth: 1.94 AU Projected size 880 x 890 km Shape: spherical Albedo: 0.10 Ratio 1:2 – F439W/F673N filter- Blue Continuum Slope Ratio 2:5 – F673N /F791W filter- Water of Hydration Map Raw Restored Raw Restored S – type Distance from Earth: 0.90 AU Diameter: 141 km Shape: irregular Albedo: 0.22 S – type Distance from Earth: 1.34 AU Diameter: 159 km Shape: irregular Albedo: 0.16 5 - Astraea Visit #1 46 - Hestia 93 - Minerva 11 Parthenope 144 Vibilia Raw Restored Raw Restored Raw Restored C – type Dist. from Earth: 1.99 AU Diameter: 146 km Shape: spherical Albedo: 0.085 P – type Dist. from Earth: 1.63 AU Diameter: 131 km Shape: spherical Albedo: 0.046 Raw Restored Raw Restored Visit #2 S – type Distance from Earth: 1.39 AU Diameter: 162 km Shape: spherical Albedo: 0.15 C – type Dist. from Earth: 1.20 AU Diameter: 146 km Shape: irregular Albedo: 0.059 S – type Dist. from Earth: 2.07 AU Diameter: 125 km Shape: spherical Albedo: 0.14 121 - Hermione 46 - Pales 18 - Melpomene 10 - Hygiea 1220 - Crocus Raw Restored C – type Dist. from Earth: 2.31 AU Diameter: 217 km Shape: spherical Albedo: 0.042 Raw Restored Raw Restored Raw Restored C – type Dist. from Earth: 2.58 AU Diameter: 154 km Shape: spherical Albedo: 0.051 Raw Restored C – type Distance from Earth: 2.54 AU Diameter: 429 km Shape: spherical Albedo: 0.075 Unknown Distance from Earth: 2.05 AU Diameter: unknown Shape: irregular Albedo: unknown S – type Dist. from Earth: 1.87 AU Diameter: 148 km Shape: spherical Albedo: 0.22 375 - Ursula 54 - Alexandra 38 - Leda 29 - Amphitrite Raw Restored Raw Restored Raw Restored C – type Distance from Earth: 1.27 AU Diameter: 171 km Shape: spherical Albedo: 0.050 C – type Dist. from Earth: 2.49 AU Diameter: Unknown Shape: spherical Albedo: 0.042 Raw Restored C – type Dist. from Earth: 1.38 AU Diameter: 120 km Shape: spherical Albedo: 0.058 S – type Distance from Earth: 4.44 AU Diameter: 219 km Shape: irregular Albedo: 0.16 Reconstructing HST Images of Asteroids A. Storrs, S. Bank, H. Gerhardt (Towson Univ.), K. Makhoul (MIT) 216-Kleopatra WFPC-2 Images: WF/PC Images: Abstract: We present reconstructions of images of 22 large main beltasteroids that were observed by Hubble Space Telescope with the Wide-Field/Planetary cameras. All images were restored with the MISTRAL program (Mugnier, Fusco, and Conan 2003) at enhanced spatial resolution. This is possible thanks to the well-studied and stable point spread function (PSF) on HST. We present some modeling of this process and determine that the Strehl ratio for WF/PC (aberrated) images can be improved to 130%, while WFPC-2 images can achieve a Strehl ratio over 80% after reconstruction. We report sizes, shapes, and albedos for these objects, as well as any surface features. Images taken with the WFPC-2 instrument were made in a variety of filters so that it should be possible to investigate changes in mineralogy across the surface of the larger asteroids in a manner similar to that done on 4 Vesta by Binzel et al. (1997). Of particular interest are a possible water of hydration feature on 1 Ceres (see figure), and the non-observation of a constriction or gap between the components of 216 Kleopatra. 9-Metis M-type R=3.24 AU, D=2.38 AU Projected size 238x121 km Albedo 0.062 S-type R=2.32 AU, D=1.49 AU Projected size 222x182 km Albedo 0.108 18-Melpomene Apparent rotation of 216 Kleopatra during observations. 624-Hektor S-type R=2.22 AU, D=1.26 AU Projected size 155x170 km Albedo 0.155 19-Fortuna D-type R=5.24 AU, D=4.33 AU Projected size 363x207 km Albedo 0.024 G-type R=2.30 AU, D=1.53 AU Projected size 225x205 km Albedo 0.028 Apparent rotation of 624 Hektor during observations. 51 - Nemausa Raw Restored G – type Dist. from Earth: 1.53 AU Diameter: 153 km Shape: spherical Albedo: 0.086 References: Mugnier, L.M., T. Fusco, and J.-M. Conan, 2003. JOSA A (submitted) Binzel, R.P., Gaffey, M.J., Thomas, P.C., Zellner, B.H., Storrs, A.D., and Wells, E.N. 1997. Icarus 128 pp. 95-103 Krist, J. 1993. The Tiny Tim User’s Manual, Space Telescope Science Institute The above images demonstrate the technique. The top row contains the unrestored images, and the second row the results of optimal MISTRAL restoration. Both log stretched and oblique surface plots of each image are shown. The left group is an image of a standard star taken with the WF/PC (aberrated) camera, and the right group is taken with the WFPC-2 camera (F439W filter). The Strehl ratio for the WF/PC images is 50% before correction, and 180% after. For WFPC-2, the reconstruction process does not improve the Strehl ratio (about 85% in this image) but does sharpen the image and lower the background level. Note companion star in WFPC-2 image. Image Restoration: Normal astronomical deconvolution processes do not work well on extended objects with sharp brightness variations, such as asteroids. These deconvolution processes will tend to over-enhance the edges of such sources, and so here we have used the MISTRAL routine (Mugnier et al. 2003) to avoid this problem. WFPC-2 images of the asteroids were restored with a theoretical (TinyTim, Krist 1993) PSF. The resultant images have a four times resolution improvement over the unrestored images. The figure to the right shows the results of running MISTRAL on an extended image. The test image is on the left, convolved with WF/PC (aberrated) PSF in the center, and after restoration with MISTRAL on the right. Acknowledgements: Support for this work provided by NASA through grant GO-8583 from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.