Dispersion

So far we have mentioned little about the color aspects of planetary image processing, but a vital understanding of color and the eye/brain perception of what appears on your monitor screen is essential.

From latitudes well away from the equator, the planets are never going to be directly overhead. This instantly causes a problem with respect to atmospheric dispersion, i.e., the splitting up of colors into a spectrum. We have all seen the way in which a prism splits light up into its constituent colors. Well, the Earth's atmosphere causes the same effect: the lower the object's altitude, the worse the dispersion. The effects are especially notable on the Moon, where bright crater edges will be fringed with red and blue. Unfortunately, atmospheric dispersion is significant enough to severely limit a telescope's resolution on any planet lower than 35 degrees altitude. A planet at 90 degrees altitude, that is, directly overhead, will have no color disper sion. At 60 degrees altitude (a typical best case scenario for observers in the U.K.), the visual spectrum from red to blue will be smeared across 0.35 arc-seconds, i.e., the theoretical resolution of a 30-cm telescope. Move down to 45 degrees altitude and the visual spectrum will be smeared over 0.6 arc-seconds, i.e., roughly the resolution of a 20-cm telescope. Things then get dramatically worse! At 30 degrees above the horizon dispersion will be 1 arc-second and at 18 degrees, 2 arc-seconds. Of course, at these low altitudes there will be other undesirable effects, too, as the light is coming through a lot of air, seeing will suffer and the image will look dimmer.

Fortunately, for the webcam imager, there is a partial solution to atmospheric dispersion. All digital images are constructed from red, green, and blue values, which can, at the user's discretion, be separated into their respective channels. For example, Registax has a feature called RGB shift (Figure 8.1) in which the user can

Figure 8.1. Registax' RGB Shift tool enables the red, green, and blue layers to be moved to compensate for atmospheric dispersion.

choose to move the red, green, and blue components of each image with respect to each other until no color fringes are seen at planetary limbs or around bright craters. Of course, this is not a perfect solution, but, aesthetically, a planet without blue and red fringes on opposite edges, looks much better. Needless to say, when a planet transits the local meridian (due south from the northern hemisphere and due north from the southern), it is at its highest point and this is the point of least dispersion. Another solution to dispersion is to use an optical arrangement by which prisms reverse the damage inflicted by the atmosphere. This might seem like a horrendous optical problem, but, in fact, AVA (Adirondack Video Astronomy) has recently marketed an affordable wedge prism corrector that can be set to correct atmospheric dispersion at a variety of altitudes. I remember seeing such a device in 1984, when I visited the legendary optician Horace Dall in his home, but now such devices are available commercially.

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