Reconstructing HST Images of Asteroids
A. Storrs, S. Bank, H. Gerhardt (Towson Univ.), K. Makhoul (MIT)
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.
Abstract:
We present reconstructions of images of 22 large main belt asteroids that were observed by Hubble Space Telescope with the Wide-Field/Planetary cameras. Allimages 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 canbe 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.
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
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 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 (about85% in this image) but does sharpen the image and lower thebackground level. Note companion star in WFPC-2 image.
11 11 ParthenopeParthenope
Raw Restored
S – type
Distance from Earth: 1.39 AU
Diameter: 162 km
Shape: spherical
Albedo: 0.15
144 144 VibiliaVibilia
Raw Restored
C – type
Dist. from Earth: 1.20 AU
Diameter: 146 km
Shape: irregular
Albedo: 0.059
89 - Julia89 - Julia
Raw RestoredS – type
Distance from Earth: 1.34 AU
Diameter: 159 km
Shape: irregular
Albedo: 0.16
8 - Flora8 - Flora
RestoredS – type
Distance from Earth: 0.90 AU
Diameter: 141 km
Shape: irregular
Albedo: 0.22
Raw
1220 - Crocus1220 - Crocus
Raw Restored
Unknown
Distance from Earth: 2.05 AU
Diameter: unknown
Shape: irregular
Albedo: unknown
10 - Hygiea10 - Hygiea
Raw RestoredC – type
Distance from Earth: 2.54 AU
Diameter: 429 km
Shape: spherical
Albedo: 0.075
29 - 29 - AmphitriteAmphitrite
Raw RestoredS – type
Distance from Earth: 4.44 AU
Diameter: 219 km
Shape: irregular
Albedo: 0.16
54 - Alexandra54 - Alexandra
Raw Restored
C – type
Distance from Earth: 1.27 AU
Diameter: 171 km
Shape: spherical
Albedo: 0.050
1 - Ceres1 - Ceres
C – type
Distance from Earth: 1.94 AU
Projected size 880 x 890 km
Shape: spherical Albedo: 0.10
Raw Restored Ratio 2:5 Ratio 1:2
Ratio 1:2 – F439W/F673N filter-Blue Continuum Slope
Ratio 2:5 – F673N /F791W filter-Water of Hydration Map
WFPC-2 Images:
5 - Astraea5 - AstraeaVisit #1
Raw Restored
Visit #2S – type
Dist. from Earth: 2.07 AU
Diameter: 125 km
Shape: spherical Albedo: 0.14
46 - Hestia46 - Hestia
Raw RestoredP – type
Dist. from Earth: 1.63 AU
Diameter: 131 km
Shape: spherical
Albedo: 0.046
93 - Minerva93 - Minerva
Raw RestoredC – type
Dist. from Earth: 1.99 AU
Diameter: 146 km
Shape: spherical
Albedo: 0.085
18 - Melpomene18 - Melpomene
Raw RestoredS – type
Dist. from Earth: 1.87 AU
Diameter: 148 km
Shape: spherical Albedo: 0.22
46 - Pales46 - Pales
Raw RestoredC – type
Dist. from Earth: 2.58 AU
Diameter: 154 km
Shape: spherical
Albedo: 0.051
Raw Restored
121 - Hermione121 - Hermione
C – type
Dist. from Earth: 2.31 AU
Diameter: 217 km
Shape: spherical
Albedo: 0.042
38 - Leda38 - Leda
Raw Restored
C – type
Dist. from Earth: 1.38 AU
Diameter: 120 km
Shape: spherical Albedo: 0.058
375 - Ursula375 - Ursula
Raw Restored
C – type
Dist. from Earth: 2.49 AU
Diameter: Unknown
Shape: spherical
Albedo: 0.042
51 - Nemausa51 - Nemausa
Raw RestoredG – type
Dist. from Earth: 1.53 AU
Diameter: 153 km
Shape: spherical
Albedo: 0.086
WF/PC 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.
9-Metis9-Metis
18-Melpomene18-Melpomene
19-Fortuna19-Fortuna
216-Kleopatra216-Kleopatra
624-Hektor624-Hektor
S-typeR=2.32 AU, =1.49 AUProjected size 222x182 kmAlbedo 0.108
S-typeR=2.22 AU, =1.26 AUProjected size 155x170 kmAlbedo 0.155
G-typeR=2.30 AU, =1.53 AUProjected size 225x205 kmAlbedo 0.028
M-typeR=3.24 AU, =2.38 AUProjected size 238x121 kmAlbedo 0.062
D-typeR=5.24 AU, =4.33 AUProjected size 363x207 kmAlbedo 0.024
Apparent rotation of 216 Kleopatra during observations.
Apparent rotation of 624 Hektor during observations.
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