Curing the refractor blues.
FOR MANY AMATEUR ASTRONOMERS, refractors are the only telescopes worth owning. Such enthusiasm is understandable, for refractors do have undeniable virtues. When properly made they are all but impervious to loss of collimation, maintaining their optical alignment even when subjected to rough handling. They rapidly achieve thermal equilibrium with their surroundings and generally maintain their image quality while doing so. The absence of a central obstruction due to a secondary mirror perceptibly improves performance on the planets. And last but certainly not least, refractors have a unique aesthetic appeal.
A Colorful History
The simplest and most affordable refractors have two-element objective lenses made from ordinary crown and flint glasses. These so-called achromatic doublets suffer from an optical defect known as longitudinal chromatic aberration. The typical achromatic objective designed for visual use has yellow-green light (with a wavelength around 555 nanometers, where the eye is most sensitive) come to focus slightly closer to the objective than blue (around 486 nm) and red (around 656 nm). Violet light reaches focus even farther back from the objective (see the diagram on page 56). As a result, when bright stars look sharply focused to the eye they appear surrounded by purple halos composed of defocused red, blue, and violet light. This secondary spectrum also bleeds across extended images. Lunar shadows cease to be jet black, and delicate low-contrast planetary details become washed out.
The residual spurious color of an ordinary achromat cannot be totally eliminated, but it can be reduced to almost imperceptible levels if the focal length of the lens is sufficiently long. The secondary spectrum is usually deemed unobjectionable when the blur circles of defocused red and blue light do not exceed three times the diameter of a star's Airy disk formed by the telescope in yellow-green light. This criterion correlates quite closely with the old rule of thumb stating that if the focal length of an achromat is equal to three times the square of its aperture in inches, then its residual color will not perceptibly impair its visual performance. This formula yields the following focal lengths and focal ratios for a given aperture:
Aperture Focal length Focal ratio (inches) (inches) 3 27 f/9 4 48 f/12 5 75 f/15 6 108 f/18 8 192 f/24
Many small doublet achromats satisfy this criterion, but in apertures greater than 5 inches or so, such instruments become prohibitively cumbersome and unwieldy. (The 40-inch Yerkes refractor would require a tube 400 feet long!) The traditional compromise has been to employ focal ratios of f/12 to f/16 and simply tolerate the color error. Nevertheless, the secondary spectrum of large doublet refractors is so pronounced that it almost invariably comes as a rude shock to observers accustomed to the views provided by reflectors.
During the 1980s, apochromatic refractors by Takahashi, Astro-Physics, and Tele Vue appeared on the market. The objectives of these instruments have elements made from exotic (and expensive) glass or fluorite, which all but eliminate chromatic aberration, even at comparatively fast focal ratios of f/6 to f/9. This dramatic reduction in focal length means that even a 7-inch apochromat can be transported to a star party or remote dark-sky observing site. The optical performance of these apos quickly became the stuff of legend, and they have emerged as the standard by which other instruments are judged today. However, in apertures of 4 to 6 inches this level of performance commands premium prices ranging from $400 to more than $2,000 per inch of aperture.
In recent years, Celestron, Meade, Orion, and Konus, as well as other firms, have introduced achromatic refractors in apertures of 4 to 6 inches with relatively fast focal ratios of f/8 to f/10. Refractors of this aperture have never been as affordable as these Chinese-made instruments, but their color correction is mediocre at best, particularly in the popular 6-inch f/8 models.
Filters to the Rescue
For decades observers working with achromats have used color filters (usually yellows like Wratten 4, 8, 12, and 15 or greens like Wratten 11, 56, and 58) to block the secondary spectrum's defocused red, blue, and violet light. Wratten filters use dissolved organic pigments tailored to transmit certain wavelengths and absorb others. The transitions from passing to blocking light are not very abrupt, however, so eliminating the secondary spectrum requires rather dense filters that appreciably dim the image in addition to imparting a pronounced yellow or green hue. Some observers tolerate these color shifts, but many find them very objectionable.
Sophisticated filters for suppressing the secondary spectrum of achromats have recently appeared on the market. Often referred to generically as minus-violet filters, they employ a multitude of thin, vacuum-deposited dielectric coatings to exclude selected wavelengths in much the same fashion as the light-pollution and nebula filters introduced 25 years ago. Constructive and destructive interference at the interfaces of these layers permits much sharper cut-on and cut-off of the transmitted wavelengths, resulting in significantly higher overall light throughput than conventional dyed-glass filters as well as less pronounced changes in the color balance of the image.
Some of the new minus-violet filters block a carefully selected portion of the yellow-green spectrum to further reduce the yellow or green hue imparted to the image, thus retaining a more pleasing natural appearance. Sirius Optics achieves this localized "dip" in its filter's transmission curve by using a complicated stack of dielectric layers. Baader Planetarium offers a yellow-absorbing filter made from a custom melt of glass doped with the rare-earth element neodymium. Called the Moon & Skyglow filter, it is intended as a general-purpose light-pollution filter for deep-sky observing. It can be stacked with the Baader Fringe-Killer minus-violet filter to achieve a similar effect to the Sirius filter.
My 14-inch Schmidt-Cassegrain telescope shares its German equatorial mount with several Chinese-made refractors, including 4-inch f/9.8 and 6-inch f/8 achromats from Celestron. The larger refractor served as the principal test bed for evaluating the filters described here, which were borrowed from their respective manufacturers for review. The objective is well corrected for spherical aberration and is free of astigmatism and significant zonal errors. Despite this high optical quality, it lacks a "sweet spot" of unambiguous precise focus due to its pronounced color error, which impairs its performance on the Moon and planets.
I took advantage of nights of good seeing late last year and early this year to compare views of the Moon, Mars, Jupiter, and Saturn. The filters I tested included Orion's V-Block; Sirius Optics' MV1 and MV20; and Baader Planetarium's Contrast-Booster, Fringe-Killer, and Neodymium filters. All were installed in a filter wheel so they could be rapidly compared.
All the filters exhibit common performance traits. The most dramatic effect is that images snap into an unambiguous focus once the blue-violet fringing and hazes are attenuated. The following notes record my impressions at the eyepiece.
Despite the presence of violet halos along the limb and surrounding detached peaks and crater rims near the terminator, I find unfiltered views of a cool, bluish gray waxing gibbous Moon at low and medium power very aesthetically pleasing. However, the filters are truly indispensable for observing minute features near the limit of resolution at magnifications exceeding 150x. Without a filter, the craterlets on the floor of Plato, the delicate network of rilles near the crater Ramsden, and the tiny summit pit atop the Kies dome can be glimpsed only intermittently, but they stand out boldly through all of the filters.
Predictably, the lunar surface is a vivid lemon yellow through the Wratten 12 filter that I have traditionally used to sharpen lunar images with this scope. The other filters achieve comparable gains in sharpness and clarity while giving perceptibly brighter images and imparting a far less garish array of hues: straw yellow through the Baader Contrast-Booster; an almost imperceptibly pale yellow through the Fringe-Killer; yellow-green through the Sirius MV1 and the Orion V-Block. (Green is more predominant than yellow with the Orion filter). The Sirius MV20 and the stacked combination of the Baader Fringe-Killer and Neodymium filters give the most neutral lunar images, which have a very dilute and surprisingly unobtrusive salmon tint.
Viewing the red planet through the 6-inch refractor with these filters is nothing less than stunning. The high surface brightness of Mars makes the violet haze of the secondary spectrum particularly bothersome, and once a filter is interposed the radial streaks around Solis Lacus and dappled mottling in Mare Erythraeum pop into view. The edge of the retreating polar cap is diffuse and poorly defined without a filter; the filters sharpen this boundary and reveal hints of intricate scalloping. Although the color shift is rather pronounced with the Baader Contrast-Booster, its very aggressive blocking of short wavelengths gives the best views.
Without a filter, Jovian detail begins to wash out above 180x, even in steady seeing. All of the filters extend the range of useful magnifications to at least 250x on a steady night. Jupiter's bright zones take on a yellow cast through both the Baader Contrast-Booster and Fringe-Killer filters, although this color shift is so subtle through the Fringe-Killer that I tend to quickly forget its presence. The Sirius MV1 and Orion V-Block lean more toward chartreuse, while the MV20 gives a very pale peach or yellowish pink tint to the view.
Contrast is markedly improved, revealing delicate features like white ovals, hints of structure in the Great Red Spot, and the ragged edges and rifting of the equatorial belts. In unfiltered views of a transit of Europa, the satellite's tiny disk disappears soon after it passes in front of Jupiter's globe, but through all of the filters I can follow it more than halfway to the planet's central meridian.
The Baader Fringe-Killer and Sirius MV20 filters preserve a surprising amount of color information, although more secondary spectrum remains. Through both of these filters Jupiter's equatorial belts retain their ruddy pastel hues, and I can still see the robin's-egg blue of the festoons protruding into the Equatorial Zone from the northern edge of the South Equatorial Belt. These features are just amber-gray halftones through the Wratten 12 and Baader Contrast-Booster filters.
The improvement on Saturn is just as striking as it was for Jupiter. In unfiltered views, the planet's globe is devoid of detail except for a diffuse equatorial belt and a dusky patch surrounding the pole. The filters afford glimpses of subtle, low-contrast banding and give the polar hood a well-defined border. The ring system also sharpens dramatically. The Cassini Division in the rings stands out much more boldly, and, in the steadiest moments, I can glimpse concentric tonal gradations in ring B and a central dusky shading in ring A. Without a filter, the gap between the interior of ring B and the globe is awash in a "noise" of purple haze that masks the faint "signal" of the crepe ring. Despite a modest reduction in image brightness, the crepe ring is visible at a glance in the ansae of the rings through all of the dielectric filters; the denser Wratten 12 filter doesn't fare as well in this respect. On Saturn I don't find the color shift objectionable through any of the filters (no doubt due in large measure to the overall intrinsic yellowish color of Saturn's globe), but only the Sirius MV20 and the stacked combination of the Baader Fringe-Killer and Neodymium filters preserve the difference between the colors of the globe and the rings. The planet is tack sharp through the other filters, but the views are reminiscent of old sepia-tone photographs.
A Few Cautionary Remarks
My 6-inch f/8 achromat gathers more than twice as much light as its smaller 4-inch f/9.8 cousin, and it also exhibits far more secondary spectrum. Light really is a precious commodity with the smaller aperture, so the filters that offer the best performance with the 6-inch instrument don't fare as well with the smaller scope. The Baader Fringe-Killer is the filter of choice with the 4-inch because it provides the highest overall light transmission and the brightest images, permitting me to use magnifications of 160x to 200x on good nights.
Baader touts the fact that its filters are made from homogenous glass free from striae and plane-parallel to 14-wave rather than raw, flame-polished glass. When the various filters were installed in a filter wheel during the lion's share of my testing, they were located so close to the focal plane that I wasn't able to detect any differences in image quality that I could attribute to the filter substrates. However, when the filters were installed at the entrance of a binocular viewer that placed them more than five inches inside the focal plane, image degradation was evident at 210x through several filters, but not through those made by Baader. Bear this in mind if you plan to use a binocular viewer or install a filter at the entrance of a star diagonal.
According to the lyrics of a recent Sheryl Crow recording, "It's not having what you want, but wanting what you've got." A $60 filter won't turn your $600 achromat into a $6,000 apochromat. Chromatic aberration will be reduced, but it certainly won't be eliminated.
With the filters that block the secondary spectrum more aggressively, you'll have to accept the warm hue imparted to the image. Some observers will quickly grow accustomed to the color shift, but others never will. This tint is much less pronounced through the Sirius MV20 filter and the stacked combination of the Baader Fringe-Killer and Neodymium filters, which were my strong favorites on most subjects with the 6-inch refractor. Impressions of color are notoriously subjective, and there is admittedly an element of personal taste involved here.
If you're prepared to sacrifice some degree of color fidelity in exchange for crisper images, you will be able to use higher magnifications and resolve a wealth of otherwise inaccessible lunar and planetary detail with all these filters. The fast 4- to 6-inch Chinese achromats certainly aren't the only instruments that will benefit from them. The performance of large, long-focus doublet refractors, particularly the gems by legendary 19th-century makers like Clark and Brashear that grace scores of club and college observatories, will also be enhanced.
A research chemist by day, Ohio native and contributing editor THOMAS DOBBINS divides his astronomical pursuits between observing the Moon and planets and writing about the history of telescopes and solar-system observations. His latest book, Epic Moon, was coauthored with William Sheehan and published by Willmann-Bell in 2001.