Imaging with a Refractor
The refractor is the oldest telescope design. It is by far the most familiar design (it's what Galileo used in the 17th Century, and it's what Marvin the Martian used in the cartoons). Refractors have been used to produce some of the most stunning wide-field amateur astrophotos ever taken.
Above: The optical layout of a refractor telescope
The refractor has become a very popular design for wide-field imaging. It is especially popular among astroimagers who already own long focal length telescopes such as Schmidt-Cassegrains. Part of the reason for its increased popularity lies in advances in optical manufacturing technology that are allowing very good refractors to be built less expensively than in the past. However, the best refractors remain quite expensive instruments. Why are refractors so expensive?
Apochromatic vs. Achromatic Refractors
There are two classes of refractors: achromatic and apochromatic. (Apochromatic is very often shortened to "apo".)
Achromatic refractors are generally inexpensive and suffer from chromatic aberration. When light passes through a refractor lens, it is bent to reach a focus point at the back of the telescope. Each wavelength of light is bent by a slightly different amount. Thus, red, green, and blue wavelengths do not necessarily focus at the exact same position. Telescope designers create the lens such that it focuses red and blue light to the same point. But since the human eye is most sensitive to green light (especially at night), the telescope produces its best image in the green portion of the spectrum. The green point of focus does not coincide with the red and blue point of focus. This is chromatic aberration. Specifically, the difference between the green focus and the red/blue focus is called secondary color. Minimizing secondary color is the purpose of fancier refractor designs.
Above: An achromatic lens usually consists of two elements. The different wavelengths of light do not meet at the same focus point, causing chromatic aberration.
Above: An apochromatic lens normally has 3 or more elements. The different wavelengths meet at the same point of focus, effectively eliminating chromatic aberration.
Apochromatic refractor lenses are designed so that at least three colors of lights (rather than just two) reach the same focus. And they are designed such that the difference between the primary focus point and the focus point of the remaining colors (such as violet) is extremely small. In this way, chromatic aberration and secondary color is essentially eliminated.
Understanding Refractor Terminology
Refractors seem to have a lot of terminology associated with them. Here are some definitions of common terms.
Achromatic -- An achromatic objective is one in which red and blue light is focused to the same point, but there is residual secondary color from green light focusing to a slightly different point. Apochromatic -- An apochromatic (apo) objective uses 3 elements or special glass (or both) to focus red, green, and blue light to one point and minimize secondary color. Chromatic Aberration -- The aberration that results from different wavelengths of light not focusing to the same point. Doublet -- A refractor objective (lens) consisting of two glass elements. ED -- Extra-low dispersion glass, which has special properties that reduce secondary color. Similar to fluorite but less expensive. Fluorite -- Fluorite is a special type of glass (actually a synthetic crystal) which has unusual properties yielding low secondary color and exceptional image quality. Very expensive. Secondary Color -- The difference in focus between the primary wavelengths (red/blue in an achromat, red/green/blue in an apo) and the remaining wavelengths. Triplet -- A refractor objective (lens) consisting of three glass elements.
A new third class of refractors has started to pop up, called semi-apochromatic. Semi-apo is a very subjective term. One manufacturer's semi-apo may be quite better than another's (and there are even variations among a single company's models). A semi-apo is really an achromat that uses special glasses to minimize (but not eliminate) secondary color. Technically, "apo" is a strictly defined term, meaning three colors come to the exact same focus. However, companies are beginning to use the term simply to mean "very low chromatic aberration". While any scope termed "apo" is going to be good, there is a wide variation in quality between manufacturers. Semi-apos and inexpensive apos are often still doublet designs (two elements) using low-dispersion glass, rather than triplet designs. This means the secondary color is much less than in an achromat, but is not truly eliminated as is the case with a true apochromat.
Only apochromatic or good semi-apochromatic refractors are really suitable to CCD imaging. Some people would argue that only the best apochromatic refractors should be used, but if your goal is simply pretty pictures, a high quality semi-apo is a very good choice as well. The reason refractors can run into trouble for CCD imaging purposes lies in the spectral sensitivity of the CCD chip. Most refractors are designed to eliminate (or minimize) chromatic aberration over the wavelengths to which the human eye is sensitive. However, CCDs can see well beyond the limits of human vision, into the near ultraviolet and infrared regions of the spectrum. Refractors are not necessarily corrected for these wavelengths, and so aberrations can appear. In most cases these aberrations are very slight. In the case of achromatic refractors these aberrations, especially in the UV end of the spectrum, will be extreme and can significantly degrade an image.
Also, triplet refractors, since they have an extra lens element, allow designers to minimize other aberrations such as. That said, here is an image taken through a semi-apochromatic doublet refractor costing $500 (although the mount and CCD cost much more).
Above: CCD image of the Orion Nebula through a small apochromatic doublet refractor.
The primary reason refractors are less common for CCD imaging is simply cost. Apochromatic refractors are by far the most expensive of the three most common types of telescopes. High-quality apo refractors cost nearly $1000 per inch of aperture (and some are even more pricey). Refractors over 7" in aperture are also rarely manufactured and are large and unwieldy instruments. Since most amateur astronomers own only one telescope, and use that telescope for both imaging and visual observation, aperture becomes more important than if the telescope were being used for imaging alone. For taking pictures, aperture is of little importance, while for visual observing it is everything! For the price of a small apo refractor, a quite large Schmidt-Cassegrain can be had. However, for the astroimager who desires the absolute best quality, an apochromatic refractor is hard to beat. Many imagers use an SCT for imaging smaller targets, and piggyback a small refractor on top of the SCT to image wide-field targets.