Image Restoration in Astronomy

The field of digital image restoration has a quite long history that began in the 1950s with the space program. The first images of the Earth, Moon (mainly of the opposite side) and planet Mars were, at that time, of unimaginable resolution. However, the images were obtained under big technical difficulties such as vibrations, bad pointing, motion due to spinning, etc. These difficulties resulted, in most cases, in medium to large degradations that could be scientifically and economically devastating. The need to retrieve as much information as possible from such degraded images was the aim of the early efforts to adapt the one-dimensional signal processing algorithms to images, creating a new field that is today known as “Digital Image Restoration and Reconstruction”.

The application of early image restoration techniques to these images was very successful. If we compare the raw data obtained by the spacecraft with the “final” products (obtained also by adding several images) one could think that they were obtained using different equipment and in different epochs.

Since their introduction, the techniques of image reconstruction and restoration have been a “must” in all scientific disciplines involving projections or interferometric data as medical tomography, seismology, magnetic resonance imaging, and even some astronomical applications such as mapping in radio astronomy. If we look at the “dirty map” of a radiointerferometric observation (see Fig. 1) obtained with the Very Large Array (VLA), it is almost impossible to distinguish anything but the image of the beam. However, after the restoration process the images of tiny extended sources (see Fig. 2) can be easily recognized.

Figure 1

Figure 2

However, for a long time image restoration was considered as a luxury in other fields such as optical astronomy. Nonetheless image restoration was applied to images coming from space such as the case of the images taken in 1986 of comet Halley by the spacecraft Vega and Giotto. Again, the raw images of the comet obtained, for example, by the Vega mission have almost nothing to do with the elegant “peanut shape” images of the comet Halley nucleus.

In 1990 something happened which changed the situation of image restoration in the field of optical astronomy. After the launch of the $2000 million Hubble Space Telescope (HST) an “impossible” mistake was discovered in the main mirror. The mirror had a severe problem of spherical aberration because it was polished with the help of a faulty device and checked with the same faulty device. Thus, the checking was perfectly coherent with the polishing but the curvature of the mirror was wrong. Since a single minute of observing telescope time cost about $100.000, any effort to improve the images was cheap. Since then, a substantial amount of work has been done in image restoration directed towards optical astronomy. As result of such efforts, it was possible to correct the aberrant HST images.

Raw image of planet Saturn obtained with the WF/PC camera of the Hubble Space Telescope. 

Restored image of Saturn using Richardson-Lucy algorithm (Don’t bother about the algorithm now: D).

In the time elapsed since the beginnings of the digital image restoration era the restoration techniques used have improved enormously. Nowadays, restoration is routinely carried out on many astronomical observations. As another example, the final product of the HST Deep Space Survey (HST-DSP) recently delivered, is an elegant and colorful series of very deep images of a selected area. However, the raw data (needed for some astronomical measurements) contain a great number of cosmic-ray hits, readout noise, low signal to noise ratio, etc. 

Original (a) and restored (b) images of the spirall Galaxy NGC 450/UGC 807

Courtesy: The contents and the images of this article is taken from 

R. Molina, J. Nunez, F. J. Cortijo and J. Mateos, "Image restoration in astronomy: a Bayesian perspective," in IEEE Signal Processing Magazine, vol. 18, no. 2, pp. 11-29, Mar 2001.