The achromatic refractor is the traditional image of the astronomical telescope, portrayed in text books, fiction, the media and the movies. The long white tube with a lens at one end and the eyepiece and focuser at the other. Why does this telescope have such a long tube compared to its aperture? For small aperture telescopes (80mm to 140mm), refractors are in many was the ideal instrument. Refractors have closed tubes, suffer less than Newtonians and Cassegrains with internal tube air currents, give very sharp high contrast images, are excellent for deep-sky photography, excellent lunar and planetary performance, and also ideal as a solar telescope with the appropriate white light filter over the front aperture. They are however more expensive aperture for aperture than other telescope types, and the achromatic refractor suffers from one aberration that can affect on-axis images.
Chromatic aberration is an image error that arises because of the interaction between glass and the wavelengths of light. White light (polychromatic light) is made up of visible wavelengths that we see as individual colours. This is the visible spectrum from violet to red. Remember the old school experiment of passing white light at a particular angle through a prism? The light spreads out after exiting the prism and splits into the colours of the spectrum. The different colours represent different wavelengths of light, and demonstrates that glass has a subtle effect on different wavelengths of light. Lenses refract different wavelengths of light so that the focus positions of the wavelengths are separate from each other, and as white light is made up of all visible wavelengths, we can see that there may be a problem. A lens has a different focal length for each colour (wavelength). This means the focal point for each colour is different. This is longitudinal colour orlongitudinal chromatic aberration.
If we could somehow bring two colours from both ends of the spectrum to the same focal position, we could, in the process, bring the focus positions of other colours closer to each other. We are doing exactly that when we achromatize an objective lens. We are reducing the effects of this wavelength dispersion, and we do this by adding a second lens to the objective, typically making a lens pair out of a crown glass and a flint glass. A properly designed crown and flint objective will greatly reduce the dispersion over that of a single lens, by a "folding" of the focus positions and in so doing, create a re-combination of the focus positions of the red and blue wavelengths. This still means that there is some dispersed colour that is not corrected. Residual unfocused wavelengths in the on-axis image that are not corrected, are visible as diffuse colour fringing, contribute to the image as a Chromatic Aberration and has the term Secondary Spectrum. This is often visible as a violet haze surrounding bright objects such as stars, planets or the limb of the moon. If we increase the focal ratio (focal ratio is focal length divided by aperture), to at least f/9, we can see a reduction in secondary spectrum over an f/5 or f/6 achromatic refractor of the same aperture. This is essentially why astronomical refractors have been traditionally long tube telescopes. Increasing the focal ratio of a lens decreases secondary spectrum. Most achromatic refractors designed for astronomy up to about 120mm in aperture, have focal ratios of >f/8. Anything less than that, and the higher magnification performance is affected by secondary spectrum. Larger achromatic refractors require even greater focal ratios to realise a similar degree of colour correction. At lower magnifications, chromatic aberration does not create much of a problem in the image, and as refractors provide good image sharpness, they are ideal as rich field telescopes, (a marketing name for telescopes that can be used at low magnification for spectacular crisp wide-field images). Some modern GOTO telescopes are small achromatic refractors with f/5 or f/6 objectives, as they provide the most spectacular lower power images for those new to astronomy. For higher power, it is always best to choose a longer focal ratio achromatic refractor.