................................................................. . = T H E M A R T I A N C H R O N I C L E = . . . . The electronic newsletter by the International Mars Patrol, . . an observing program of the . . Association of Lunar and Planetary Observers . . Edited by: J.D. Beish . . June 1991 No. 4 . ................................................................. COLOR FILTERS FOR PLANETARY OBSERVING A set of photo-visual color filters is the most important ob- serving aid the planetary astronomer can have in his/her arse- nal of telescopic accessories. Color filters help overcome image deterioration caused by atmospheric scattering of light, permit separation of light from different levels in a planetary atmosphere, increase hue contrast between areas of differing color, and reduce irradiation within the observer's eye. All of these factors increase the sharpness of surface and cloud details that are seen on the planet Mars. Standard color Wratten Filters for scientific research are made by Eastman Kodak Co., and they may be ordered in 3x3-inch square thin gelatin sheets from Kodak photo dealers or may be purchased from major telescope dealers in glass with threaded mounts for attachment to the eyepiece. A basic set of tricolor filters useful for color image comparison or "Blinking" at the eyepiece is a must for the planetary observer, e.g., yellow (Wratten-12), green (W-58), and blue (W-80A) or W-38A). An advanced set would also contain as orange (W-21 or W23A), red (W-25), violet (W-47), and magenta (W-30, minus green) An efficient method to use gelatin filters is to mount 1/4-inch strips into glass or plastic slide mounts. Do not use slide mounts with "Newton Rings suppression" glass -- for some reason, rings will appear. Observers can then hold the filter mount between the eye and the ocular with three or four different color filters on hand. This method has been used by the ALPO Mars Recorders for years with satisfactory results. Using color filters at the telescope can be a most challenging and rewarding experience, although several weeks of practice may be necessary to condition the observer to detect those subtle wisps of clouds and hazes often observed in the Martian atmos- phere. Without the aid of these filters, many interesting phe- nomena may go undetected. Bright Martian atmospheric clouds and hazes are classified by the color of light in which they are best observed. Standard color Wratten Filters produced by Eastman Kodak Co. for scien- tific research are systematically used by observers either at the eyepiece or in combination with a camera at the telescope. Observers using filters other than Kodak Wratten Filters supplied their respective filter characteristic curves to insure compatibility. During all observations with permissible seeing conditions, Kodak Wratten color filters are systematically used to detect suspected meteorological activities. Blue-white clouds appear bright in blue and violet light. Red and green filters are used to search for low fog's and hazes with close attention to discrete bright areas accompanied by surrounding yellow- ish haze. Checks are made from time to time with a red filter to either confirm or dispel any suspected Martian dust clouds. While shorter wavelength light reveals low altitude meteorology or surface white frost areas, green and blue-green filters also enhance seasonal white clouds. Due to the scattering of blue light by Mars' atmosphere, blue filter observations with frequent cross checking with red fil- ters aid in the detection of higher cloud formations and hazes. Alternating from blue to red light (blinking) is a tried and proven way to detect cloud activity. The use of light blue and violet filters will surely bring out those subtle and low intensity blue and blue-white clouds often observed near the Martian equator. Without the aid of color filters, planetary system cloud banding and other low contrast meteorology is almost impossible to detect. Table I. Eastman Kodak Wratten Color Filters used for visual and photographic observations of Mars. Transmission Characteristics are given. RED (W25,29) -- Gives maximum contrast of surface features, polar cap edges, enhances fine surface details, and dust cloud bound- aries. ORANGE (W21,23A) -- Further increases contrast between light and dark fea tures, penetrates hazes and most clouds, and brightens dust clouds. YELLOW (W8,12,15) -- Brightens desert regions, darkens blue and brown fea tures. NOTE: Do not use yellow filter to deter- mine Martian dust clouds. GREEN (W56,57) -- Darkens red and blue features, enhances frost patches, surface fogs, polar projections,and polar cap boundaries. NOTE: There is no true green features on Mars. BLUE-GREEN (W64) -- Enhances ground fogs, limb and polar hazes. BLUE (W80A,38A)-- Shows atmospheric clouds, discrete white clouds, and limb hazes, and darkens reddish features, which probably contributes to the illu sion of blue or violet haze. VIOLET (W47) -- Shows blue and blue-white clouds, limb hazes,equatorial cloud bands, polar cloud hoods, and dark dust clouds,and aids in the detection of the Violet Clearing phenome- non. MAGENTA (W30,32) -- Enhances red and blue features, darkens green features. NOTE: There is no true green features on Mars. THE COLOR OF MARTIAN DUST CLOUDS In the past, astronomers have identified Martian dust clouds and/or obscurations as "yellow clouds." Confusion over the colors of Mars is nothing new. Observers often report several of Mars' darker albedo features, or "maria," to appear quite green or even blue at times. This has led scientists of the past to speculate that vegetation grew on Mars during its springs and early summers. When we observe bright Mars against the dark nighttime sky, the planet's color hues are often per- ceived as complementary to the dark back ground sky. This effect is known as "simultaneous contrast" [See Sky & Telescope, Vol. 77, No. 5, May 1989, p.474, "What's New on Mars?" by William K. Hartmann]. While on photographic patrol of Mars using the University of Hawaii's 24-inch telescope atop Mauna Kea, J. Beish observed this "simultaneous contrast" phenomenon first hand. On several occasions the gray volcanic ash strewn over the surrounding fiery red volcanoes began to appear greenish to him as the Sunlight grew dimmer. If we consider that Mars only varies from a dark gray-or ange/brown to a slightly brighter grayish-orange or grayish brown, it becomes interesting when attempting to describe the color of Martian dust clouds as "yellow." While dust clouds may appear yellowish when observed without the aid of color filters, they are in fact brighter in red and orange light than they are in yellow light. Thus it may be necessary to reclassify these clouds as simply "dust clouds." EFFECTS OF TELESCOPIC IMAGES IN EARTH's ATMOSPHERE Several atmospheric conditions and physical effects are modified by the use of color filters at the telescope: SCATTERING interposes a luminous veil between the observer and his/her subject. Scientists have shown that for particles in a planet's atmosphere of a given size, the scattering is inversely proportional to the fourth power of the wave length of the light. Hence, violet light of 400 nm is scattered about 16 times more than deep red light of 800 nm; Earth's daytime sky is blue as a result of this property. The Martian atmosphere scat- ters light in the same manner and thus allows us to observe Martian aerosols at the different relative atmospheric depths. PRISMATIC DISPERSION by our atmosphere is most evident when a star or planet is seen near the horizon. It results from refraction being less for the longer wave lengths where the red appears nearer the horizon and violet toward the zenith. ATMOSPHERIC PENETRATION. To explore an atmosphere similar to our's to various depths, molecular scattering can be exploit ed. Since the shorter wave lengths are scattered more, it follows that ultra violet light scarcely penetrates an atmos- phere at all, violet light penetrates to some depth, blue still deeper, while blue-green may reach the solid surface. Orange and red light certainly penetrated to the surface. COLOR CONTRAST is affected by sharpness of boundaries and by differences in color and shade. Light yellow and orange filters are useful in judging the colors of the low-hue cloud belts and zones of Jupiter and Saturn. To bring out a white area on a reddish background, a green filter is useful. IRRADIATION occurs between adjoining areas of unequal brightness. The amount the bright area appears to encroach upon the faint er one is approximately proportional to their intensity dif- ference. This is evidently a physiological effect, originating within the eye itself. A deep red or orange filter reduces this effect while observing a planet at the telescope.