| Color filters thread into your eyepieces to increase the visibility of faint planetary details by selectively changing the light transmission of individual colors reflected from features on the planet’s surface or in its atmosphere. This makes low-contrast features stand out more clearly. For example, an #80A blue filter gives you a better view of the huge planet-sized cyclone on Jupiter known as the Great Red Spot (often closer in appearance to a Faint Pink Spot at the present time) by absorbing most of the red light reflected from it. This makes the Great Red Spot darker than the surrounding cream-colored cloud mass (which is relatively unaffected by the blue filter), making it stand out with more contrast so it’s easier to see. True, the filter will turn the entire planet blue, but most visual observers feel that the increased contrast of the Great Red Spot and the fainter brown Jovian storm belts against the bright cloud background more than compensates for any loss of true color. Because of this color change, color filters are used only for black and white planetary photos, since the color change is not apparent in a black and white print, but the contrast change most certainly is. Filters increase photographic exposure times, so you’ll have to experiment to determine the correct exposures. By using the appropriate filter, you can also enhance cloud details in the Martian atmospheres. A #47 violet filter, for example, sharpens clouds high in the atmosphere, while a #58 green filter penetrates more of the atmosphere, sharpening clouds closer to the surface. If a cloud on Mars appears sharper with a green filter, it is probably near the surface. If a violet filter gives superior definition, however, the cloud is at a higher altitude. Saturn’s white storm bands, always subtle against the cream-colored body of the planet, will invariably be more visible with proper filtration. A #58 green filter, or a blue filter (#38A, #80A, or #82A), are the most useful for enhancing these low-contrast white features. Irradiation, a physiological property of your eye itself, can also be a problem in planetary and lunar observing. Irradiation causes bright areas to bleed into adjoining dark areas, blurring their boundaries and making the apparent size of the dark areas shrink. Irradiation shows itself, for example, by the apparent bloating of the white polar caps on Mars to unrealistic proportions, almost to where they seem to float above the planet’s dark surface. It also shows itself by the seeming disappearance of thin, dark lunar rilles and Martian ‘canals’ against the lighter lunar or planetary background. Irradiation is less apparent when images are dimmer, so using a neutral density, polarizing, or color filter greatly reduces the problem. Dual threads on some filters let you stack two filters together – for example, to combine a neutral density filter with a color filter to simultaneously reduce glare and enhance contrast. The light transmission of stacked filters is equal to the product of the individual transmission values. For example, stacking a 50% transmission #96ND3 filter and an 83% transmission #8 yellow for lunar observing results in 42% transmission (.50 x .83 = .42). The description for each individual color filter describes the effect the filter has on various solar system objects. It also gives the light transmission value for the filter, should you want to stack filters. |
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