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.