Filters

Colored filters

Colored filters are used to bring out features on the planets and to dim the glare of the Moon. Table 6.2 gives specific examples.

Regardless of who makes them, these filters are identified by the equivalent Kodak Wratten filter numbers, and the threads are standardized so that filters of all brands fit nearly all eyepieces. Two-inch eyepieces take 48-mm camera lens filters.

If there is room, it is often more convenient to hold the filter by hand between your eye and the eyepiece.

Any filter darkens objects of the complementary color. For instance, the bluish-gray markings on Mars are brought out by a red filter. The belts of Jupiter, which are grayish-blue or purplish, stand out when viewed through a yellow filter.

A strongly colored filter also sharpens the image in unsteady air or when a planet is near the horizon. Like a prism, the air bends light of different

Table 6.2 Filters for lunar and planetary observing

Color

Wratten numbers

Used for

Red

25 (dark), 23A (lighter)

Mars, some features on Jupiter and Saturn

Yellow

15,12

Increased contrast on all planets,

especially Jupiter and Saturn

Green

58 (dark), 56 (medium)

Jupiter's Great Red Spot; polar caps

on Mars; lunar surface detail

Pale blue

80A, 82A

Bright comets; lunar surface detail

Blue

38A (dark), 47 (very dark)

Mars during "blue clearings", Venus

wavelengths differently, and by rejecting everything but a narrow band of wavelengths, the filter reduces the resultant blurring.

You can also get interesting results by using a nebula filter (next section) on the planets. Narrow-band nebula filters transmit a narrow segment of the spectrum in the blue-green range; broadband nebula filters block yellow while transmitting wavelengths above and below it. Both have been recommended particularly for viewing Jupiter.

Light-pollution (nebula) filters

Nebula filters, also called light-pollution filters, darken the glare of city lights without filtering out stars and nebulae (Figure 6.10). This is possible because mercury-vapor and sodium-vapor streetlights emit light only at specific wavelengths. Thus, filters can be designed to block them out.

While blocking streetlights almost completely, such filters transmit much of the light of stars and galaxies, because starlight covers the whole spectrum. Hydrogen nebulae such as the Orion Nebula and Ring Nebula come out even better: they, too, emit only at narrow bands wavelengths, different from those of the streetlights, and their light can go through the filter almost unattenuated.

Filters of this type are expensive because they cannot be made of dyed glass. Instead, the glass is coated with several partly reflective layers whose thickness and spacing are tuned to specific wavelengths. The result is called an interference filter and looks almost like a mirror when viewed in daylight.

There are several kinds of light-pollution filters. The broadband type is best for viewing deep-sky objects other than hydrogen nebulae, for photography, and for use at mildly light-polluted sites; it cuts streetlight glare while transmitting as much of the spectrum as possible. Narrow-band (high-contrast) filters give the best view of hydrogen nebulae at heavily light-polluted sites; they transmit only the most visible wavelengths of ionized hydrogen, including hydrogen-beta and oxygen-III but not hydrogen-alpha.

Light-pollution filters do not help as much today as they did when first invented 20 years ago because a greater variety of outdoor lights is in use, and their emissions cover more of the spectrum.

Figure 6.10. Nebula filter darkens the sky background, making nebulae more visible.

Sun filters

Filters for viewing the Sun must be placed in front of the telescope, and only filters specially made for the purpose, and known to be safe, should be used. Loosely mounted aluminized Mylar filters give good results; high-quality glass solar filters are available at somewhat higher cost.

If not properly filtered, a telescope aimed at the Sun gathers enough light to blind the observer, melt its plastic components, and even set itself on fire. That is why the concentrated light of the Sun must never be allowed to enter the telescope. You cannot judge the safety of a solar filter by looking at it; most filter dyes transmit far too much infrared light - enough to cause serious eye injury -even if they look comfortably dark. For more information see Celestial Objects for Modern Telescopes and Astrophotography for the Amateur.

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