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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
Restoration of HST WFPC2 Images in Gyro­Hold Mode
J. Mo and R. J. Hanisch
Space Telescope Science Institute 1 , 3700 San Martin Drive, Baltimore,
MD 21218
Abstract. In gyro­hold tracking mode, HST is stabilized only with its
gyros, and the Fine Guidance Sensors are not used to maintain pointing.
With gyro drift rates of order 0: 00 002 per second of time, a WFPC 2
Planetary Camera image with a typical exposure time of 100 s can be
blurred by as much as five pixels. Image restoration techniques developed
for use on aberrated HST images have been adapted for use to remove
this motion blur. Experiments have been done on WFPC 2 images of
NGC 330 and NGC 422 (having exposure times of 80 and 100 s in gyro­
hold mode) with complete success.
1. Introduction
Since the first servicing mission in 1993 December, HST 's optical performance
has been corrected to nearly the original design specifications. As a result,
however, spacecraft stability and the guiding mode used by the Fine Guidance
Sensors (FGSs) have become much more significant factors in determining overall
image quality. The FGSs now have only two guiding modes: fine lock and
gyro­hold. In order to utilize gaps in the HST observing program that would
otherwise be unused, about 200--300 ``snapshot'' observations are placed on the
program in each observing cycle. In order to minimize spacecraft overhead,
snapshot observations are taken without guiding, using gyro pointing control
only (e.g., Bond 1994). Gyro­hold pointing drifts at a rate ah high as 1:4 \Sigma 0:7
mas s \Gamma1 . This motion causes a significant blur in WFPC 2 images, even for
exposures as short as 100 s. Given our experience in restoring the aberrated
images from WFPC 1, we decided to explore the possibilities of removing the
motion blur from images taken in gyro­hold pointing mode with WFPC 2.
Two WFPC 2 snapshot observations of NGC 422 and NGC 330 (kindly pro­
vided by M. Shara) have been restored successfully using the image restoration
techniques which have been implemented in the STSDAS package.
2. Motion Blurred WFPC 2 Images
An NGC 422 WFPC 2 WC2 image and an NGC 330 WFPC 2 PC1 image were
taken in mode FSGLOCK = GYROS in 1994 January. A synopsis of these images
1 Operated by AURA, Inc., for NASA
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is given in Table 1. In the table, the column ``Image Section'' indicates the
pixel coordinates of the detector in the sequence (x min : x max ; y min : y max ). The
restorations have each been done on a 256 \Theta 256 pixel subimage.
Table 1. WFPC 2 Images
Object Camera Filter Exp Time Date Image Section
NGC 422 WF2 F450W 100 s 25/01/94 (103:358,494:749)
NGC 330 PC1 F450W 80 s 27/01/94 (499:754,99:354)
3. Image Restoration
The restoration of the motion blurred images is carried out in three stages: (1)
standard restoration using a model PSF, (2) determination of the motion­blur
function, and (3) final restoration. The initial model PSFs were computed using
Tiny TIM Version 4.0. For each observation, the PSF position coincides with
the peak position of a bright star.
Figure 1. WFPC 2 image of NGC 422 (WF2, exposure = 100 s).
Top left: original observation. Top right: standard Tiny TIM decon­
volution. Bottom left: MEM restoration (final). Bottom right: Lucy
restoration (final).

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In order to generate the motion­blur function, a standard Tiny TIM PSF
restoration is made for each observation. The blurring function is determined
by averaging the images of several bright stars in the initial restored frame.
The final PSF is constructed by convolving the motion blur function with the
Tiny TIM PSFs.
The restorations were done using the maximum entropy method (MEM)
(Wu 1992, 1994) as implemented in STSDAS. The restored image adopted is the
ME solution after approximately 100 iterations. The Richardson­Lucy method
has also been tested in this experiment using the task lucy implemented in STS­
DAS (White 1993; Stobie et al. 1994). The motion­blurred images of NGC 422
and NGC 330, and their restorations are illustrated in Figs. 1 and 2, respectively.
Figure 2. NGC 330 WFPC 2 (PC1, exposure = 80 s). Top left:
original observation. Top right: standard Tiny TIM deconvolution.
Bottom left: MEM restoration (final). Bottom right: Lucy restoration
(final).
4. Discussion and Conclusions
The point spread function for data taken in gyro­hold mode is the convolution
of the normal PSF with the motion blurring function. Because our PSF models
are now quite accurate, deconvolution of a motion­blurred image with a model
PSF yields an image in which the remaining shapes, and features associated with
point sources, define the motion­blur component. This residual also encompasses

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Figure 3. Motion­blurred PSF. Left: PSF of NGC 422 WFPC 2
(WF2, exposure = 100 s). Right: NGC 330 WFPC 2 (PC1, exposure
= 80 s).
any additional mismatches between the model PSF and the observed PSF in the
absence of motion blur.
WFPC 2 is undersampled, especially in the UV in WF mode. In order to
obtain a better representation of the motion blurring function, several (partially)
restored star images are averaged together. The success of the method relies
upon having several bright but unsaturated star images in the field of view.
Our experiments indicate that gyro drift is not uniform linear motion, but
rather must be described by a two­dimensional function. Indeed, the final PSF
obtained for the NGC 422 image shows a multiply­peaked structure, indicating
that within the exposure time the telescope dwelled longer at certain locations
than in others. A surface plot of this PSF and the final PSF of the NGC 330
image are shown in Figure 3. Both the MEM and Richardson­Lucy algorithms
implemented in STSDAS were successful in restoring the image with a multiply­
peaked PSF, and gave virtually indistinguishable results.
The standard image restoration techniques developed for use on aberrated
HST images can be adapted for removing the motion blur from snapshot images
taken under gyro­hold pointing mode. This should allow observers to obtain
optimal spatial resolution and dynamic range on many of these data sets.
Acknowledgments. Support from ST ScI's Image Restoration Project,
funded by NASA as a contract augmentation to NAS5--26555, is gratefully
acknowledged. The authors thank M. Shara for providing his HST WFPC2
observations prior to publication.
References
Bond, H. E., ed. 1994, Hubble Space Telescope Cycle 5 Call for Proposals
(Baltimore, Space Telescope Science Institute)
Stobie, E. B., Hanisch, R. J., & White, R. L. 1994, in Astronomical Data Anal­
ysis Software and Systems III, ASP Conf. Ser., Vol. 61, eds. D. R.
Crabtree, R. J. Hanisch, & J. Barnes (San Francisco, ASP), p. 296sto­
biee
White, R. L. 1993, in Newsletter of ST ScI Image Restoration Project, ed. R. J.
Hanisch (Baltimore, Space Telescope Science Institute), p. 11
Wu, N. 1992, in Astronomical Data Analysis Software and Systems II, ASP
Conf. Ser., Vol. 52, eds. R. J. Hanisch, R. J. V. Brissenden, & J. Barnes
(San Francisco, ASP), p. 520

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Wu, N. 1994, this volume, p. ??