Hack58.Enhance Lunar and Planetary Contrast and Detail
Hack 58. Enhance Lunar and Planetary Contrast and Detail
Choose a basic set of filters to improve the view. Just as photographers use filters to alter the images captured by their cameras, astronomers use filters to alter the images visible in their eyepieces. A filter, by definition, can add nothing to an image; it can only take away. But, like the sculptor who creates an elephant from a block of stone by cutting away everything that doesn't look like an elephant, an astronomer may use filters to create more detailed views of some objects by removing extraneous light to allow the subtle details of the object to show through. Astronomical filters are available in several sizes. The most common are those that fit standard threads on 1.25" and 2" eyepieces and the visual backs of Schmidt-Cassegrain Telescopes (SCTs). Meade goes its own way, using a non-standard thread on their 1.25" and 2" eyepieces and filters, and Questar uses still a different thread. A few filters are available for 0.96" eyepieces, which are common in Japan and with some old telescopes but rare elsewhere. Filters for 1.25" eyepieces are available in by far the widest variety. The selection for 2" eyepieces is somewhat more limited, and those for SCT visual backs are even more limited.
4.16.1. Enhancing Color ContrastColor filters, sometimes called planetary filters or Lunar/planetary filters, are simply discs of colored glass, ground optically flat on both sides and with antireflection coatings applied to both surfaces. By selectively transmitting parts of the visible spectrum and blocking other parts, color filters can enhance visual contrast and detail in bright objects like Luna and the planets. All color filters use the century-old Kodak Wratten numeric designations familiar to photographers. Any two filters that have the same Wratten designation, regardless of manufacturer, have similar or identical transmission characteristics. Transmission curves are available for all Wratten color filters. These curves graph the transmission (or its inverse, the density) of a filter across the visible spectrum. Figure 4-31 shows an example transmission curve, this one for a #58 Green filter. The same data may be presented in tabular form, as shown in Table 4-7. These data tell us that the #58 Green filter passes most of the green light, with maximum transmission centered on about 530 nanometers (nm). Blue-green wavelengths are attenuated, and everything shorter than about 465 nmblue through ultravioletis blocked completely, as are yellow through red wavelengths from about 620 nm (orange) through 700 nm (deep red). At about 700 nm, the #58 Green filter begins providing higher transmission, reaching nearly 100% at wavelengths of 800 nm (infrared) and higher. Figure 4-31. Transmission curve of a Wratten #58 Green filter
Some experienced Lunar/planetary observers swear by color filters, while others (including us) find them of very limited use. Although a filter changes the appearance of the object dramaticallya #25 Red filter, for example, turns everything bright redthe degree of contrast and detail enhancement is usually quite subtle. Color filters are not a magic bullet for enhancing Lunar/planetary observing, but they are relatively inexpensive and you may find the enhancements they provide worth their small cost.
We think the four color filters listed in Table 4-8 are most useful for general Lunar/planetary observing with mid-size to large telescopes. You can buy them individually for $10 or $15 each in 1.25" or $20 to $25 each in 2" versions. Some astronomy specialty vendors sell this or a similar selection as a set, often at a significantly lower price versus buying them individually. If you want to start with just one color filter, we think the #80A Medium Blue is the best choice for mid-size or larger scopes. Although it is not necessarily the best choice for any one purpose, it is helpful for many objects, from Luna to Venus to the superior planets. The #80A even serves as a "poor man's" light pollution filter, somewhat reducing the yellow-pink skyglow caused by low-pressure sodium vapor lights. Unfortunately, the #80A is often a bit too dark for 4.5" and smaller scopes, particularly at high power. For such scopes, we recommend the #15 Deep Yellow as the best general-purpose filter. The #82A Pale Blue is a jack of all trades, and it would be our second or third choice for any size scope.
Table 4-9 lists the four filters we consider most generally useful after the basic set. The #47 Violet, although it is too dense for use in small scopes, is probably the best overall choice for viewing the inferior planets, particularly Venus. The similarly dense #25 Red lightens warms colors and darkens cool colors dramatically, and it is the best choice when you need a deep-cutting contrast filter. The #11 Yellow-Green and the #21 Orange straddle the #15 Deep Yellow in both density and effects.
If you have the Basic and Supplemental sets and still feel the need for more color filters, consider one or more of those listed in Table 4-10. In general, filters in this group simply fine-tune the effects of the more commonly used filters. We consider all of these filters highly optional, so much so that we no longer own any of them.
4.16.2. Dimming the ImageNeutral-density (ND) filters and Polarizing filters, sometimes called moon filters, are used when you need to dim the image, for example, as if you are viewing Luna at low power. ND filters are available in various fixed densities from quite light to very dark, and are simply dyed glass filters with a neutral gray shade. ND filters may be labeled by their percentage transmission (e.g., an ND50 filter has 50% transmission and an ND25 25%) or logarithmically (e.g., an ND 0.3 filter has 50% transmission, an ND 0.6 25%, and an ND 1.0 10%).
Polarizing filters are, in effect, variable neutral-density filters. They comprise two layers of Polarizing material in a mount that allows the layers to be rotated relative to each other. As you change the relative position of the Polarizing layers, the visible neutral density varies from moderate to high. We consider neutral-density and Polarizing filters to be useless except in vary large instruments. Part of that, we suppose, is because the primary purpose of a telescope is to gather light, and most astronomers consider "wasting light" to be stupid, if not downright sinful. But the real reason is that there are better alternatives. If, for example, Luna is too bright in your eyepiece, that's nature's way of telling you to use more magnification, use a contrast filter, or both. |
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