Pub Date : 2016-02-14DOI: 10.2352/ISSN.2470-1173.2016.6.RETINEX-023
D. Marini, C. Bonanomi, A. Rizzi
Stars, galaxies and some nebulae emit light, differently from solar system planets and satellites that mainly reflect Sun’s light or nebulae that also partly reflect nearby stars light. Light emission is concentrated on specific wavelengths corresponding to transition states of atoms that compose the object. Professional photographers and astronomers use special narrow band filters to detect spectral light emission. Using monochromatic CCD cameras a multi filter photograph can be taken, producing at least long, middle and short wavelength snapshots that can be processed to give full color pictures. Amateurs can use wide-band filters or even color cameras. Colors in astrophotography do not correspond to perceivable colors by human vision system (HVS) and our visual system did not evolve to perceive these kinds of images. Any way we still have to consider our perception when creating pictures to observe cosmic objects photos, that have been rendered using the so called representative colors, selected to show the captured wavelength bands with the purpose to make visible what is scientifically relevant. The typical application field of the Retinex based algorithms is that of natural images, since their purpose is to simulate some behaviors of the human visual system. However we can use HVS properties to enhance astrophotographs and increase local contrast, thus allowing researchers to detect non-visible structures and lay people to be fascinated by richness of cosmic objects. We will present the results of applications of some Retinex based algorithms to astrophotographs. We will discuss their efficacy, compared to traditional methods and discuss possible developments. Introduction Since the launch of Hubble telescope (1990), the first out of atmosphere orbiting telescope, a large amount of new photographs of deep sky objects have been acquired for scientific research purpose and public distribution. A famous picture that contributed to diffuse discoveries of the structure of the universe is the socalled Pillars of Creation in the Eagle nebula Messier 16 (see figure 1 – downloaded from [1]). Let’s explain this image. The staircase structure is due to the structure of Hubble telescope camera system, WFPC-2 (Wide Field/Planet Camera-2 wiffpick) [2], composed of 4 cameras, the top right one having double resolution to observe details of planets, to be down scaled to compose the large field picture. The second observation is the peculiar color channels distribution. The image has been captured through three narrow band filters centered around the emission lines of specific atoms of gas molecules of nebulae: O III (Oxygen III, 501.2 nm Δλ 2.7 nm), Hα (Hydrogen alpha, 656.4 nm Δλ 3.5 nm) and SII (Sulfur II, 673.2 nm Δλ 4.72nm). If we interpret these wavelengths as color bands we see that there is no blue component: OIII is around green and SII, Hα are in the orange-red interval. To explore this color rendering we have downloaded the three band pictures fr
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