Restoration of HST Images of Asteroids

1
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 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 non-observation of a
constriction or gap between the components of 216
Kleopatra.
9-Metis
M-type R3.24 AU, D2.38 AU Projected size
238x121 km Albedo 0.062
S-type R2.32 AU, D1.49 AU Projected size
222x182 km Albedo 0.108
18-Melpomene
Apparent rotation of 216 Kleopatra during
observations.
624-Hektor
S-type R2.22 AU, D1.26 AU Projected size
155x170 km Albedo 0.155
19-Fortuna
D-type R5.24 AU, D4.33 AU Projected size
363x207 km Albedo 0.024
G-type R2.30 AU, D1.53 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 Users
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.