Transcript Reconstructing HST Images of Asteroids A. Storrs, S. Bank, H. Gerhardt (Towson Univ.), K.

Reconstructing HST Images of
Asteroids
A. Storrs, S. Bank, H. Gerhardt (Towson Univ.), K. Makhoul (MIT)
WF/PC Images:
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. 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 nonobservation of a constriction or gap between the
components of 216 Kleopatra.
216-Kleopatra
8 - Flora
89 - Julia
WFPC-2 Images:
1 - Ceres
9-Metis
Raw
S-type
R=2.32 AU, D=1.49 AU
Projected size 222x182 km
Albedo 0.108
M-type
R=3.24 AU, D=2.38 AU
Projected size 238x121 km
Albedo 0.062
Raw
C – type
18-Melpomene
Restored
Ratio 1:2
Ratio 1:2 – F439W/F673N filterBlue Continuum Slope
Distance from Earth: 1.94 AU
Projected size 880 x 890 km
Apparent rotation of 216 Kleopatra during observations.
Shape: spherical
Albedo: 0.10
5 - Astraea
Visit #1
624-Hektor
Ratio 2:5
D-type
R=5.24 AU, D=4.33 AU
Projected size 363x207 km
Albedo 0.024
Raw
Restored
46 - Hestia
Raw
P – type
Restored
Dist. from Earth: 1.63
AU
Diameter: 131 km
S – type Visit #2
Apparent rotation of 624 Hektor during observations.
Dist. from Earth: 2.07 AU
Raw
C – type
Restored
Dist. from Earth: 1.99 AU
Diameter: 141 km
Diameter: 159 km
Shape: irregular
Shape: irregular
Albedo: 0.22
Albedo: 0.16
11 Parthenope
Restored
46 - Pales
Diameter: 148 km
Shape: spherical
Albedo: 0.22
Distance from Earth: 1.39 AU
Dist. from Earth: 1.20 AU
Diameter: 162 km
Diameter: 146 km
Shape: spherical
Shape: irregular
Albedo: 0.15
Albedo: 0.059
1220 - Crocus
121 - Hermione
Restored
Diameter: 154 km
Raw
C – type
Restored
Dist. from Earth: 2.31 AU
Diameter: 217 km
Shape: spherical
Shape: spherical
C – type
Dist. from Earth: 1.38 AU
Diameter: 120 km
Shape: spherical
Albedo: 0.058
Distance from Earth: 2.54 AU
Diameter: 429 km
51 - Nemausa
Restored
Unknown
Distance from Earth: 2.05 AU
Diameter: unknown
Shape: irregular
Shape: spherical
Albedo: unknown
Albedo: 0.075
29 - Amphitrite
54 - Alexandra
375 - Ursula
Raw
S – type
Restored
Raw
Restored
C – type
Dist. from Earth: 1.53 AU
Dist. from Earth: 2.49 AU
Diameter: 153 km
Diameter: Unknown
Shape: spherical
Shape: spherical
Albedo: 0.086
Albedo: 0.042
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.
Raw
Albedo: 0.042
Albedo: 0.051
Raw
G – type
Restored
Restored
C – type
38 - Leda
Raw
Restored
C – type
Raw
Dist. from Earth: 2.58 AU
Dist. from Earth: 1.87 AU
Raw
Albedo: 0.085
C – type
Restored
144 Vibilia
S – type
10 - Hygiea
Shape: spherical
Albedo: 0.046
Raw
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.
Distance from Earth: 1.34 AU
Diameter: 146 km
18 - Melpomene
Image Restoration:
Distance from Earth: 0.90 AU
93 - Minerva
Shape: spherical
Albedo: 0.14
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.
S – type
Diameter: 125 km
Raw
S – type
Restored
S – type
Raw
Shape: spherical
G-type
R=2.30 AU, D=1.53 AU
Projected size 225x205 km
Albedo 0.028
Raw
Ratio 2:5 – F673N /F791W filterWater of Hydration Map
S-type
R=2.22 AU, D=1.26 AU
Projected size 155x170 km
Albedo 0.155
19-Fortuna
Restored
Restored
Raw
Restored
C – type
Distance from Earth: 4.44 AU
Distance from Earth: 1.27 AU
Diameter: 219 km
Diameter: 171 km
Shape: irregular
Shape: spherical
Albedo: 0.16
Albedo: 0.050
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
Acknowledgements:
Support for this work provided by NASA through grant GO8583 from the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555.