  Radio astronomy studies the Universe by observing the radio emissions of
celestial bodies. Different methods can be used to recover the sky brightness
distribution (SBD), which describes the distribution of celestial sources from
recorded data, with the output dependent on the method used. Image quality
assessment (IQA) indexes can be used to compare the differences between
restored SBDs produced by different image reconstruction techniques to evaluate
the effectiveness of different techniques. However, reconstructed images (for
the same SBD) can appear to be very similar, especially when observed by the
human visual system (HVS). Hence current structural similarity methods,
inspired by the HVS, are not effective. In the past, we have proposed two
methods to assess point source images, where low amounts of concentrated
information are present in larger regions of noise-like data. But for images
that include extended source(s), the increase in complexity of the structure
makes the IQA methods for point sources over-sensitive since the important
objects cannot be described by isolated point sources. Therefore, in this
article we propose augmented Low-Information Similarity Index (augLISI), an
improved version of LISI, to assess images including extended source(s).
Experiments have been carried out to illustrate how this new IQA method can
help with the development and study of astronomical imaging techniques. Note
that although we focus on radio astronomical images herein, these IQA methods
are also applicable to other astronomical images, and imaging techniques.
