Longitudinal Chromatic Aberration
Often called longitudinal color, or axial color, this is the primary aberration affecting refracting telescopes. It can also arise in any system which uses lenses, although it tends to be most problematic in pure refracting instruments.
How Longitudinal Color Arises
When light rays pass from one medium to another–say, from air into glass–the light is bent. How much the light is bent depends on two factors: the two media (air and glass, in this case), and the wavelength of light. Different wavelengths of light are refracted by differing amounts. Blue light is refracted more strongly than red light. This is what causes the colors of light to split after passing through a prism.
Above: A glass prism refracts different wavelengths of light by different amounts, causing the colors to separate
The same thing happens in a raindrop to create a rainbow. In this case the media involved are air and water, rather than air and glass, but refraction occurs wherever light passes into a medium of different density. This also means light is bent again upon exiting a raindrop or prism.
Above: Sunlight refracted by a raindrop to create a rainbow
The same principle applies to a telescope objective. Since blue light is more strongly bent than red light, blue light will focus closer to the objective than red light. This means it is impossible to focus all the colors simultaneously. If you focus the telescope for red light, blue light and green light will appear out of focus. If you focus for blue light, red and green light will appear out of focus.
Above: Longitudinal chromatic aberration in a simple lens
How Telescopes Reduce Longitudinal Color
A simple positive lens like the one shown above causes red light to focus farther from the objective than blue light. A simple negative lens causes the same effect except that the light diverges rather than converges. Another way to think of this is to imagine a negative lens having a virtual focus in the direction of incoming light. This means the effect of the negative lens is the same as positive lens only in the opposite direction.
Above: A negative lens, showing virtual focus points
By combining a positive lens and a negative lens with the proper optical properties, it is possible to get two colors of light to focus to the same point. Telescopes are designed to focus red and blue light to the same point. This leaves green light focused to a different point, but the overall chromatic aberration is significantly reduced. A telescope which focuses two wavelengths of light to the same point is called achromatic.
Above: A doublet telescope objective focuses red and blue light to the same point, while green light focuses short
The difference in focus between the green light and the red/blue light is called the secondary spectrum. The greater the secondary spectrum, the greater the amount of chromatic aberration. The remaining secondary spectrum can be minimized in several ways. The simplest is to just increase the telescope’s focal ratio. The longer a scope is relative to its aperture, the smaller the secondary spectrum will be. Of course, this results in a long telescope. The larger the aperture the longer the focal ratio must be. A 4″ doublet refractor needs to be 4 feet long to reduce the secondary spectrum to a very small level. A 6″ telescope would need to be 9 feet long, and an 8″ would be 16 feet long!
A better way to reduce the secondary spectrum is to use specialized types of glass, called extra-low dispersion glass, or ED glass. ED glass is more expensive than normal glass, but it allows higher levels for chromatic correction in shorter telescopes.
A further means of reducing chromatic aberration is to use three lens elements. By combining the right types of glass, three wavelengths of light may be brought to the same focus point. This type of objective is called apochromatic. In such a telescope, red, green, and blue light all focus to a common point. This leaves other wavelengths such as violet focusing to different points, but the chromatic aberration is significantly less than in a doublet.
Above: An apochromatic triplet lens focuses three wavelengths of light to the same point
Again, using special types of low dispersion glass can further reduce the remaining aberration to extremely low levels. Note that ED doublet refractors are often called apos, short for apochromatic. While they may have very little chromatic aberration and give excellent image quality, they are not true apochromats in the strict sense of the word. The term achromatic is defined as bringing two wavelengths of light together, while apochromatic means to bring three wavelengths together. There is also a term called super-achromatic where four wavelengths of light coincide, but this is often unnecessary with modern glass types being good enough that an apochromatic triplet is all that is ever required.
Telescopes with Longitudinal Chromatic Aberration
All telescopes with lenses will have some amount of longitudinal color. This color is very small in apochromatic refractors and ED doublet refractors. Color is practically negligible in catadioptric designs such as Schmidt-Cassegrains and Maksutov-Cassegrains, at least for visual use. Color aberrations can be seen in high-resolution images with SCTs, but the amount is still considerably less than in a typical refractor. Focal reducers, coma correctors, and field flatteners tend to introduce color aberrations, but these aberrations remain small or even invisible in most designs. Chromatic aberration is primarily a concern for refracting telescopes.
Lateral color, the subject of the next section, is more of a problem than longitudinal color in certain designs (such as Maksutovs).
Telescopes without Longitudinal Chromatic Aberration
The only telescopes truly free from longitudinal color are pure reflecting systems such as Newtonians, Classical Cassegrains, and Ritchey-Chrétiens. Of course, using any of these telescopes with a focal reducer, coma corrector, or field flattener will introduce chromatic aberration, although usually of a negligible amount.
High quality apochromatic refractors may be considered free from chromatic aberration for all intensive purposes, although technically they have very low amounts of aberration rather than strictly being free of it. Schmidt derivatives such as SCTs and Schmidt-Newtonians have very low amounts of chromatic aberration, since Schmidt corrector lenses have very little optical power (compared to, say, a Maksutov corrector), but some aberration may be detected in CCD images where the camera has an extended spectral range compared to the human eye.