Chromatic Aberration

A simple refractor lens focuses light at different distances behind it, resulting in color errors. One can choose only one focus at a time, so the remaining colors appear as defocused disks. Also, because of the differing focal points, the magnification of each color is different and white-light edges are blurred into spectra.

To get an idea of the difficulty involved, hold up a simple one-element lens and look at the transmitted image with a low power eyepiece. Such a lens often appears in toy telescopes or binoculars. Obviously, chromatic aberration is profound. Every bright object appears to be surrounded by a rainbow-hued glory, and reasonable imaging only occurs for fields comprised, not surprisingly, of only one pure color.

Early astronomers reduced the importance of simple-lens color error by increasing the focal ratio to enormous values. This stratagem increased the depth of focus until it encompassed the spread of colors, and they found that performance improved. The battle was hard to win, however, because modest increases in aperture had to be accompanied by huge increases in the focal length, and operational difficulties worsened considerably (King 1955; Bell 1922).1

Even today, residual color error is visible in nominally color-corrected instruments. Focus binoculars on a bright edge against a darker background (a good target is a window from the inside of a large room), and then decenter the image. Because the color correction is only approximate for these instruments, you will see the smearing associated with color error. In some binoculars, the color fringes are violet and green, depending on whether the bright edge is to the inside or outside of the field.

1 Many 5 x 24 finders on department-store telescopes are actually stopped-down simple lenses. If a roughly 8-mm stop is visible immediately behind the objective, it is not corrected for color. The finder is useless. Discard it.

Color errors are not really aberrations in the narrow sense that aberrations are departures from the ideal spherical shape of the wavefront. One could easily describe an aperture that focuses each color precisely on a different axial point. In any given color, the wavefront converges spherically. Such an aperture could justifiably be called perfect, yet it would not work as a telescope objective.

Another important distinction between wavefront (or "geometrical") aberrations and chromatic aberrations is the absence of interference effects in chromatic aberrations. Different colors do not mutually interfere. For most of the discussions appearing in this chapter, wave optics and ray optics are identical.

Whatever the origin of the optical degradation, color error is objectionable, and one goal of the star test is to make certain that it is as small as possible.

12.1 Dispersion

All of the image patterns presented in this book have been calculated for a single wavelength of light, so-called monochromatic radiation. An ideal lens would focus all colors at the same lens distance, and lenses would be indistinguishable from mirrors.

Unfortunately, all simple lenses are dispersive. The word dispersion represents the inability of all colors of light to move at the same speed in glass. Like the word propagation, its source is botany or agriculture. "Dispersion" originally referred to randomly spreading seed. Its optical meaning refers to the spreading of colors, but such dispersion is anything but random. In fact, if it had not been applied already to color error, dispersion might have been a better word for what is known today as "light scattering."

Red light typically has a higher speed in glass than blue light. Thus, redlight waves outrace blue-light waves while they are traversing the material. The net effect is that after passing through a prism blue light is diverted to a larger angle than red light.

Say we have two prism materials, as in Fig. 12-la, that divert or refract light to the indicated angles. The average behavior of the two prisms is the same. Each kicks yellow light to 10°, but the behavior at either end of the spectrum is somewhat different. One glass (let's call it "crown" glass) diverts red light to 9° and blue light to 11°. The other glass (call it "flint") spreads the spectrum twice as far, to 8° and 12° respectively. This second sample of glass is said to have higher dispersion because of this spreading property.

We can combine these prisms to produce interesting effects. If we invert the flint prism and place it snugly against the crown, as in Fig. 12-lb, we have made an approximately plane-parallel window. The combination passes yellow light undeflected, but red is pushed up to 1° and blue is bent down to —1°. What we have envisioned here is a straight-through dispersive element, one that spreads light into its color components, but does not refract it on the average. Such a device was once made and marketed as an objective prism by the firm of Merz and Mahler, but its expense prevented wide adoption (King 1955, p. 294). The advantage of such a prism was that the telescope could be immediately directed at the region of interest.

Fig. 12-1. Simplified achromatism: a) "crown" glass (dark prism) .showing dispersion of light and "flint" glass exhibiting the same average refractive power but twice the dispersion, b) the two glasses combined to generate a "straight-through" rainbow (dispersion without diversion), c) a combination that results in an achromatic prism (diversion but not dispersion).

Fig. 12-1. Simplified achromatism: a) "crown" glass (dark prism) .showing dispersion of light and "flint" glass exhibiting the same average refractive power but twice the dispersion, b) the two glasses combined to generate a "straight-through" rainbow (dispersion without diversion), c) a combination that results in an achromatic prism (diversion but not dispersion).

Figure 12-lc depicts another clever trick we can perform with these two materials. This time, we divide the flint prism in half, making another prism that only deflects yellow light to 5°. Blue light would exit this half-prism at an angle of 6° and red light at 4°. If we invert this prism as before and place it close to the crown prism, the exit angles of the combination become: yellow, 10° - 5° = 5°; red, 9° - 4° = 5°; and blue, 11° - 6° = 5°.

If one were designing a crystal chandelier, this prism combination would be a disaster. Instead of dividing the light into a profusion of sparkling rain bows, the compound prism keeps the light packed tightly in a white beam. It diverts all colors equally. On the other hand, if one were designing telescopes, it is just what is required. This device has two important characteristics. It bends light and does so colorlessly. What has been demonstrated here are all of the essential elements of an achromatic refracting telescope.

12.2 The Achromatic Lens

Figure 12-2 shows the progression that must take place to get from the achromatic prism to the achromatic lens. If we envision the lens as composed of little prism pieces, and allow the divisions to get finer and finer, we eventually get to a cylindrical lens. It takes very little imagination to rotate the other way and extend the situation to a spherical lens.

Almost as soon as the concept of dispersion was developed, this trick of achromatism was envisioned. All that was left was to find suitable materials. Isaac Newton conducted limited experiments where he compared the dispersive power of different media with their refractive power. His hasty conclusion was that dispersion and refraction were inextricably linked. Thus, dispersion could not be counterbalanced without also eliminating bending of the light beam. He reached this erroneous result by perhaps relying too much on a visceral feeling that dispersion was a property of light itself. By this argument, materials didn't matter; dispersion was proportional to refraction and thus always existed until lenses were weakened to be no different than windows. Therefore, achromatic refractors were impossible.

Fig. 12-2. How a prism achromat leads directly to an achromatic lens.

Various conflicting statements about Newton's mistake have been made. Bell (1922) said that Newton had never published this result, and King (1955) referenced a section of Newton's Opticks in which the great physicist despaired of ever curing chromatic aberration but didn't give his reasons. The confusion can perhaps be reduced by referencing yet another portion of Opticks, where Newton said (Book One, Part II, Prop. Ill, Prob. I, Exper. 8):

I found moreover, that when Light goes out of Air through several contiguous refracting Mediums as through Water and Glass, and thence goes out again into Air, whether the refracting Superficies be parallel or inclin'd to one another, that Light as often as by contrary Refractions 'tis so corrected, that it emergeth in Lines parallel to those in which it was incident, continues ever after to be white. But if the emergent Rays be inclined to the incident, the Whiteness of the emerging light will by degrees in passing on from the Place of Emergence, become tinged in its Edges with Colours. This I try'd by refracting Light with Prisms of Glass placed within a Prismatick Vessel of Water.

In other words, conditions leading to the straight-through spectrum of Fig. 12-lb do not occur, implying Fig. 12-lc is also impossible. He followed this experiment with some speculative theorems elaborating his ideas. In Newton's defense, I don't believe that discrediting the idea of achromatism was the main thrust in this section of Opticks, but the experiment was among those interpreted by later readers to be a stronger condemnation than it actually was.

Partly because of Newton's powerful reputation, opticians gave up on achromatic lenses for 50 years. Then, an English gentleman named Chester Moor Hall made the first reduced-color lens from two different materials. He kept the lens design hidden, although the trick was reverse-engineered by a nosy lensmaker who happened to be subcontracted to work on both lenses at the same time. The lensmaker didn't realize the importance of the invention, and news of it languished until it was redeveloped a generation later by John Dollond. When it appeared to be a profitable development, lawsuits were filed by London opticians saying that Dollond had stolen the idea.

Dollond likely had heard rumors about Hall's lens, but certainly he did enough experimentation himself to be justly credited with reinventing it. Perhaps it would be more accurate to say that Dollond was the first to reduce the achromatic lens to common practice. He was certainly the first to announce it publicly (King 1955, pp. 144-150).

12.3 Residual Chromatic Aberration

Unfortunately, the ideal materials of Fig. 12-1 do not exist. Most dispersion in the range of visible wavelengths originates with resonances in the ultraviolet. In the frequency bands of these resonances, the wonderfully transparent materials go opaque. The transparent nature of these materials (called dielectrics) in the visible spectrum does not persist for every wavelength. Over much of the spectrum, the materials are content to accept energy at the entrance side and emit it at the exit side with very little loss. The energy brushes over the molecules, disturbing them little. However, at certain frequencies these materials are unusually excited by the incident energy. For wavelengths near resonance (usually about 100 nm), the glass molecules suddenly absorb energy and convert it to heat rather than pass it along.

The material behaves like a child's swing. If the swing is pushed once every quarter hour, the pendulum motion does not build. If the swing is pushed three times per second, most pushes are poorly timed and once again, pendulum motion does not build. If the impulses are carefully timed, the energy lost to the swing (or glass) steadily increases. The swing is driven at the resonance frequency. The energy contained in the light wave doesn't float through glass anymore because it must drive the oscillation in the material. Transparency is destroyed (Hecht 1987, p. 63).

The presence of resonances in the ultraviolet causes the refractive index of optical materials over the visible spectrum to increase sharply toward the blue end of the spectrum. Most glasses also have a distant resonance in the infrared caused by molecular vibration, but this resonance affects the slopes less profoundly in the visible band.

One aim of the lens designer is to choose powers of the lenses in such a way as to cause the dispersion of the "crownlike" lens elements to cancel the opposite dispersion of the "flintlike" lens elements. Because the dispersion tilts sharply at the violet end of the spectrum and the number of acceptable materials is limited, the color correction cannot be perfect. Lens designers are unable to choose lens powers for every color so that the dispersions nest like spoons. If they are combining two lens materials, the differences of refraction toward the blue means that they can probably choose only two colors with the same focal points. The remainder of the spectrum must go where it will. Each color, naturally, will be paired with another color from the opposite side of the spectrum, but the designer can deliberately choose only two.

For most visual telescopes, the two colors that the designer attempts to bring to a common focus are red (the Fraunhofer C line at 656 nm) and blue-green (the Fraunhofer F line at 486 nm). The focal point of yellow is slightly closer to the objective, and the far ends of the spectrum (deep red and violet) are beyond the C-F focus. Violet is farthest away, but that doesn't matter. The human eye is not sensitive to violet except at high brightness, so the defocused halo of violet light is mostly invisible. The residual color spread of achromatic objectives between the two chosen colors is known as the secondary spectrum.

Color corrections do not scale with size. As the diameter of the objective increases, it must be made at higher focal ratios to squeeze the light between C and F inside the Airy disk. While small 80-mm lenses can still perform admirably at f/10, a conventional achromat six inches in diameter must be made at f/18.5 to focus the different colors as well (Sidgwick 1955, p. 67; Rutten and van Venrooij 1988, p. 55). A.E. Conrady is even more conservative, stating that the focal ratio for an 80 mm lens must be f/15 and a 6-inch should be f/29 (1957, I p. 201). Since secondary spectrum is 1/2000f for ordinary achromats, we can use Appendix E to show:

8.8 An where An is the number of wavelengths defocusing error we are willing to tolerate in the secondary spectrum, D is the aperture diameter, and F is the focal ratio. Following the depth-of-focus discussion in Chapter 5, we place An at :/2 wavelength. The formula becomes 0.23.D[mm] = F, or f/18 for the 80mm aperture and f/35 for the 6-inch. This result is as conservative as Conrady's. Nevertheless, I have observed through a 6-inch f/15 refractor and was only moderately bothered by the excess color. Perhaps the restriction can be eased to 1 wavelength without too much loss. In this case, it drops to 0.12D[mm] = F, which is the same as Sidgwick.

This focal-ratio discussion sounds a disturbing note about the so-called perfection of conventional refractors. Perhaps it also says something about the human tolerance for chromatic errors.

Lens designers have found other useful telescopic color corrections besides pulling C-F into common focus. In the early years of astrophotography, special-purpose achromats were corrected for the orthochromatic emulsions then in common use. Orthochromatic plates were most sensitive to blue through near-ultraviolet light, and completely insensitive to red. Photographic telescopes could only be focused by the tedious process of taking actual exposures, but they gave sharper images on the plate than a visually-corrected lens.

Since the designer had to take the whole optical system into account, the personal preferences of the observer were figured into the design. Color correction curves published by Bell (p. 91) showed that the finest makers of the 19th century favored bringing the F (blue-green) line into common focus with the deep red at 680 nm (the B line). Much of this color correction shift is presumably caused by the chromatic aberration inherent in the eye and in the eyepieces used at the time (Taylor 1983).

12.4 The Apochromat

Achromatism can be compared to tying the spectrum in a knot. The brightest parts of the visual spectrum are deliberately folded into the tightest bundle, with the deep red and the violet ends hanging out like shoelaces. Some of the earliest optical workers (most notably Peter Dollond, son of the achromatic lens developer) tackled the cause of these spectral defects. Dollond could choose from only a handful of glasses. He reasoned that if a flint element were "designed" from a composite of two glasses, then the dispersion of that element could be tuned so that it would nest more closely with the opposite dispersion of the crown element.

He made and marketed such a triple objective, but apparently the lens was designed by trial-and-error. In any case, the glasses of the time were not yet good enough to consistently allow such refinement. It was not until 1892 that H. Dennis Taylor produced an apochromatic lens using comprehensive optical theory. He not only corrected secondary spectrum more fully, but folded the violet tail of the spectrum close enough to the visual to allow the blue-sensitive photographic plates of the time to use the same focus as the human eye (King 1955).

Using three different forms of glass (or two, if exotic glasses are used) allows the lens designer to put an extra kink in the dispersion curves, which in turn allows the simultaneous focus of three chosen colors. The color spread is knotted yet again. Often, the colors selected for common focus are further separated than the Praunhofer C and F lines. One such correction brings the line C, e, and g into common focus, or red, yellow-green, and violet (Kingslake 1978, p. 86). The deep-violet and deep-red ends of the spectrum are tucked in closer to the visual focus. Typically, the residual tertiary spectrum has been reduced a factor of five to 1/10000/ (Rutten and van Venrooij 1988, p. 54).

However, the proper behavior of the bright portions of the spectrum is no guarantee that the spectral tails are close to focus. Much depends on what the designer has in mind. Some apochromatic refractors might be designed for purely visual use, minimizing the spread of focus between C and F. These apochromats might bring violet only a little closer than ordinary doublets. Others may be designed for pinpoint photographic images without using filters, focusing deep into the violet with only slightly improved visual correction.

12.5 Testing Refractors for Geometrical Aberrations

Since each color, in effect, goes through different apparent thicknesses of glass, one would anticipate that the correction for other aberrations might vary over the spectrum. In fact, some aberrations aren't corrected at all. Much depends on how many free parameters the lens designer is allowed to play with.

For example, the variables of a doublet are all four of the curves, the separation of the lenses, the position of the aperture stop, and the glass formulations (of which there are hundreds of important types). If allowed to vary these parameters at will, designers can focus two chosen colors simultaneously and adequately correct for coma and spherical aberration over much of the spectrum. However, if designers are deliberately hamstrung by cost limitations or other considerations, they may make a cemented achro-mat out of inappropriate glasses. Such conditions can be so confining that the designer cannot correct spherical aberration and coma at the same time, even though it is possible with careful choice of glasses (Kingslake 1978, p. 171). Most makers of quality astronomical refractors are not constrained in this manner, but the cheapest consumer refractors may well offer limited aberration correction.

An apochromat focuses three chosen colors to the same point (for a more precise definition, see Buchdahl 1970). Enough free parameters remain in three-element apochromats that the designers can make a superbly corrected lens, minimizing chromatic variation of spherical aberration (spherochromatism) at more than one wavelength and suppressing coma. Also, apochromats can be adequately corrected at faster focal ratios. Six-inch apochromats are routinely made at f/9. An apochromat should present as good an image as diffraction will allow, and should accordingly be tested thoroughly.

You should test for geometrical aberrations in refractors by using an eyepiece filter. In fact, using such a filter to suppress the polychromatic nature of white light is recommended even for inspection of reflector optics. A very deep yellow or green filter is recommended. After you have tested at a central color, you may wish to change filters to red or blue to verify that other aberrations are small at the edges of the visual spectrum. Except for limiting the color band, testing refractors for other aberrations is the same as testing reflectors.

12.6 The Star Test for Chromatic Aberration

Different colors do not mutually interfere. Hence, chromatic aberration does not manifest itself in modifications of the diffraction rings, either focused or defocused. Chromatic aberration appears as a different focus position for each color. The shift can be a lateral one, as for a mild prism, or it can be a longitudinal offset. For all real lenses, each color has a slightly different focal length.

To see color effects, pull off all filters and focus on a white star or an artificial source. Be sure to filter the flashlight for a nighttime artificial-source test of chromatic aberration (see Chapter 5).

12.6.1 Wedge, Assembly Errors, and Atmospheric Spectra

Look for smearing of the focused image into a short lateral spectrum, the effect of decentering or wedge in optical components. Either of these problems puts a red fringe on one side of the image and a blue fringe on the other side, although the yellowish light of the outer planets often mixes with this light to make it appear green. Decentering may also cause other aberrations, depending on the details.

Decentering is a sideways shift of the elements with respect to one another. Wedge is an extremely shallow prism that is added to the optical system. It results from having an element thicker on one side than the other. After years of trouble-free use, wedge can appear suddenly after the lens is disassembled for cleaning. Makers sometimes cleverly remove the last bit of wedge in their objectives by canceling it between lens elements. Thus, if the crown element has a wedge of 0.04 mm and the flint element has a wedge of 0.03 mm with a maximum tolerance of 0.02 mm, the total wedge can be reduced to 0.01 mm by putting the thick part of the crown next to the thin part of the flint. An unsuspecting owner can turn these elements to yield a total wedge of 0.07 mm, or more than 3 times the tolerance. Upon reassembling the lens elements, look for alignment marks, either arrows or scratches, on the edges of the disks.

Another accident can also occur when refractor lenses are taken apart for cleaning. Refractors are so long-lived that even well cared-for instruments will eventually acquire too much internal grime. Occasionally, owners improperly invert the crown element during cleaning. This error sometimes happens even to huge observatory instruments, as is documented in Leslie Peltier's Starlight Nights (1965). He had obtained a 12-inch Clark refractor that had a hideous purple glow around star images. Doubting that the Clarks would deliberately release such a poor instrument, since it would damage their reputation in professional circles, Peltier speculated that the crown element had been inadvertently flipped during washing in the past. He inverted the element once again, and recovered the fine performance of the original lens design. Any instrument that is air-spaced may suffer this indignity, regardless of size. I once saw it in a good 2-inch refractor, but in the case of this miniature doublet, the inversion also damaged the spherical correction.

Usually, the direction of spectral dispersion is vertical. In this case, the atmosphere and not the telescope is at fault. The same spectrum would appear in a similar-sized reflector, and certainly no mechanism exists in a reflector to cause the same dispersion. The presence of a vertical spectrum could either be caused by cool air puddling at the bottom of the telescope, or (more likely) a slight prismatic effect in the atmosphere itself.

Observers commonly witness a color spread in low-lying planets and bright stars. Any object below 45° elevation is likely to be smeared to a small extent. Such errors are unlikely for artificial sources, but in the sky they are all too prevalent. Rotation of the tube will serve to isolate this error to air effects. Choose a star closer to zenith.

12.6.2 Star Test for Conventional Astronomical Visual Doublets

The design of the crown-flint refractor lens froze into place in the 19th century. Individual makers chose slightly different residual dispersions curves, but all were more-or-less bound by the availability of materials. The quality and homogeneity of materials have improved, but the simplest astronomical refractors are still made using designs that would have been recognized by makers in the 1800s.

The following star test does not apply to modern advanced refractors made from uncommon materials. It is based on a star test done by the author on a 4-inch f/15 Alvan Clark refractor built in 1881. This test used Polaris as a target. It is essentially the same description found in Taylor (in 1891), who probably used similar instruments, and would apply to ordinary doublets even today.

Inside focus, a very pale yellow-green disk with a trace of a magenta fringe is visible. Just beyond focus to the outside, a somewhat surprising red spot appears in the center. This wonderfully tiny crimson point of light is astonishing to someone who has never noticed it before. Taylor says that its origin is in the deep red beyond the C line.

An additional factor contributes to the appearance of this red dot. It approximately coincides with the location for which yellow-green is 1 wavelength out of focus. The diffraction pattern of yellow-green light for this situation looks like Fig. 12-3a, which shows an annulus with a hole neatly punched in it. Eq. 5.1 gives the focus shift to +1 wavelength defocus as 8F2X. The fraction of the focal length is just 8F2X/f. For a 4-inch f/15 refractor in yellow-green light this ratio amounts to 8(225)(2.2 x 10-5)/60 = 0.00066.

a) yellow-green b) red, same position a) yellow-green b) red, same position

defocus = 1

defocus = 0

Fig. 12-3. The "red dot" effect just beyond /ocus in conventional doublet refractors.

defocus = 1

defocus = 0

Fig. 12-3. The "red dot" effect just beyond /ocus in conventional doublet refractors.

For conventional doublet refractors, the difference between yellow-green and C (red) focus is 0.0005 times the focal length. For the same location as the yellow-green focus of Fig. 12-3a, red light slightly beyond the C Fraunhofer line is being focused in the pattern of Fig. 12-3b. Thus, a convenient little hole is present in the bright yellow-green diffraction pattern through which the red focus peeks.

Beyond the red spot, the pale greenish disk expands once more. Taylor says that sometimes a green fringe appears on this disk, but it didn't appear or was very weak in the Clark. Farther out, an indefinite blue focus is supposed to form at the center. Focus is much too strong a word, however. The 4-inch showed a blue-violet fuzzy ball that never condensed well enough to form what could be termed a focus.

Taylor's observations were done with a Huygenian eyepiece while the modern views were supplied with a modified Orthoscopic eyepiece. This difference probably accounts for the few changes, together with variations in optical design. Also, the Clark was not coated, and it had the residual coloration of glasses at the time it was made, thus slightly affecting comparisons with modern doublets.

A dim in-focus image is mostly colorless. Its most objectionable feature is a watery purple or violet glow that forms around bright objects such as the planet Venus. Taylor said it best when he described a planetary image as a sketch in black and white, where the artist made a last pass dabbing on the far red and violet colors with a sponge. However, the bad effects of this halo should not be exaggerated. It attracts notice on only the most dazzling objects, and even then it seems to interfere only slightly with the ability to discern detail. On conventional refractors smaller than 80 mm, this purple glow is almost unnoticeable. It begins to become intrusive at 4

and 5 inches (100 to 130 millimeters), but only for large instruments does it become objectionable. I saw it in a 6-inch f/15 refractor used as a guide telescope for the Schottland 16-inch Schmidt camera (King 1955, p. 370). Saturn had a crisp white-yellow disk surrounded by a bright purple blur.

Poorly made modern doublets show behavior that does not match the usual C-F color correction. A strong halo of red or greenish-blue surrounding the focused image is certainly a reason for concern. Such behavior is not normal.

Lesser amounts of chromatic aberration can be also be discerned by examining the fringes of the image just inside focus. The wavelength that focuses closest to the lens in a normal doublet is about 550 or 560 nm (yellow-green). If the place where the achromat folds the spectrum is too close to the red end, the inside-focus fringe is blue with perhaps a small green component. If the fold is too close to the blue end, the fringe is a scarlet red instead of magenta (Sidgwick 1955).

12.6.3 Star Test of Apochromats or Advanced Refractors

Coloration is far less noticeable in apochromats. Taylor claimed that the out-of-focus star disks in his photovisual lenses were virtually colorless. I have never inspected a Cooke photovisual lens, so I cannot provide confirmation. According to Sidgwick (1955, p. 201), a focused photovisual lens has a dazzling yellow-green disk with a purplish-red fringe. It should be pointed out that a weak red fringe appears on point sources even in reflectors, although it is difficult to observe. The diameter of the diffraction disk increases with long wavelengths, and the telescope is unable to pack red light into the small bundle.

My apochromatic refractor has a slight magenta fringe at a short distance inside focus and a green fringe at the same position outside focus. In focus, no color is obvious. This description is consistent with yellow or green focus nearest the objective and red or blue focus slightly farther away. A blurred blue-violet focus forms nowhere. Presumably, violet is folded back near the focus of other colors. Because violet is not bright, it can no longer be seen in the "noise" of the other strong colors. The location of the red focus is not far enough behind that of yellow-green to be distinctly noticeable.

The white appearance of a focused star image or the muting of color in views of the planet Venus (in comparison with a conventional doublet) should be a powerful indicator that an apochromat or advanced refractor is properly color-corrected. You should tolerate little spurious color in any modern triplet or fluorite-doublet design. Planetary images should not show easily discerned blurry colors.

Make certain you are not blaming the objective lens for color errors in your own eye. Most people do not realize that their eyes are not achromatic. In fact, the erroneous assumption that the human eye is achromatic led later scientists to question Newton's research on dispersion. In normal daylight vision, the eye-brain system is able to process out much of the color error encountered. Astronomy is no common activity, however, and the processing is subverted during observation. Taylor mentions how the apparent color correction of a telescope was upset with change of magnification and exit pupil.

The answer to this problem is to use the eye at a reduced pupil size. As the magnification of the telescope increases, it illuminates less of the eye's pupil. The eye's color correction improves with smaller pupil size for the same reason that refractors perform better at high focal ratio. Most perceived chromatic aberration is then produced in the telescope instead of the eye. Star test with a short focal length eyepiece.

12.6.5 The Eyepiece

An objective can present a perfectly acceptable image that is destroyed by imperfect achromatism in the eyepiece. One may be unjustly blaming the telescope's objective lens for an error that happens later in the optical train.

Fortunately, the modern general-purpose eyepiece is designed to work passably well even with a steep f/4 light cone. Most refractors that we would want to star test operate at f/9 and higher. At these mild focal ratios, compound eyepieces such as Orthoscopies or Plossls work superbly. Not unless they have been improperly made or assembled do they add significant chromatic aberration to the image. Still, they perform best when the star is in the center of the field. Do not make chromatic aberration judgments on decentered stars.

The easiest way of checking your eyepiece is to change to a different eyepiece and see if the color error goes away. Also, put the suspect eyepiece in a reflecting telescope and see if the color difficulty is still present.

12.7 Conclusions and Remedies

About the only optical problems discussed in this chapter that are changeable are those that deal with atmospheric effects or improper assembly of the lens cell. Most other color errors must be handled at the factory. You must be very cautious in blaming a perceived chromatic aberration on the instrument. Personally, I have never seen a quality refractor

12.7. Conclusions and Remedies

with grossly improper correction of chromatic aberration. I have heard of a few cases secondhand. Even department-store models seem to get color correction right although they botch nearly every mechanical feature on the instrument. I have seen fast richest-field refractors or large binoculars that had only marginal color correction, but specialty refractors must be judged to a different standard than the lunar-planetary models. Owners of these telescopes should realize that they have traded increased color error for a wider field.

Testing with a bad eyepiece presents the greatest opportunity for error. Be certain that the suspected chromatic aberration appears in many eyepieces, preferably not of the same type. Do not test with the Huygenian or Ramsden eyepieces that are often included with the telescope. If a Barlow is used, make certain that it is achromatic. Because of the high focal ratios of most refractors, some companies have been known to include simple-lens Barlows. Such a lens is usable, but it generates an incorrect color correction in the star test. Perform the test at high magnification to avoid color problems in your eye, and be absolutely sure that an unusual coloration in your light source is not shifting the results. Finally, test on multiple occasions before the final assessment is made.

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