Image Processing Reference
In-Depth Information
Chapter 9
Case Study: Noise Removal from
Astronomical Images
The previous chapters have included examples of filtered images and a discussion
of the implementation of morphological and logic-based filters. This chapter pres-
ents a case study showing how a morphological filter may be designed for a specific
type of noise in images, namely astronomical images.
Imaging instrumentation is widely used in space-based astronomy and solar
physics where it has the potential to produce excellent pictures. However, these are
frequently degraded by bursts of cosmic ray ions that saturate the charge-coupled
devices (CCDs) and produce an overlaid speckle. This is a source of frustration to
observers and can obscure vital detail. In this chapter it will be shown that the
speckle may be removed from the image using a type of nonlinear filter known as a
soft morphological filter.
Soft morphological filters comprise a branch of nonlinear image processing
that is particularly effective for noise removal. They originate from the field of
mathematical morphology but their operations are less harsh since the structuring
elements used are designed to have “soft” boundaries. The implementation of such
filters makes extensive use of rank-ordering operations.
The chapter will describe how a training set may be created for the images and
how the optimal filters may be derived using genetic algorithms. The results of pro-
cessing the images with the optimal filters will be presented. Finally, experiences
of implementing the filters in programmable hardware will be given.
9.1 CCD Noise in Astronomical and Solar Images
CCDs are used in many space-based astronomy and solar physics imaging instru-
ments, such as the Hubble Space Telescope's wide-field and planetary cameras, the
Solar and Heliospheric Observatory (SOHO), the Extreme Ultraviolet Imaging
Telescope (EIT), and the Large Angle Solar Coronagraph (LASCO
1
).
121
