A telescope has two roles optically, the first is to gather light and the second is to focus that light, a refractor excells at gathering light being the only unobstructed optical design. The traditional achromat refractor has some issues focusing all wavelengths (colours) at exactly the same point resulting in colour fringing or chromatic abberation (CA).
To combat this an achromat refractor is often made in longer focal lengths such as f9 as this reduces the CA to some extent if the aperture is small to medium sized, up to around 5" (127mm) provides good but not perfect results.The problem
CA robs precious contrast from the view you see, especially important when observing fine planetary detail. You really need all the visible wavelengths of light to come together at focus for the best performance and view. The answer used to be impractically long focal ratios of f12 and f15, making a 5" refractor over 6ft long in some cases.The solution
Modern special dispersion (ED) glass types and exotic triplet designs have all but eliminated CA but at a much higher cost than the basic achromat, both in terms of material costs and lens production. Now you can have a refractor where essentially all colours focus together and at short focal ratios which means practical, manageable length instruments.ED Doublets
The most cost effective solution is the Extra low dispresion glass doublet or (ED) refractor using one special dispersion element to focus virtually all wavelengths extremely closely, resulting in a near perfect in focus star image. At focal ratios of f6 or greater this solution is ideal for visual observers and casual imagers, however it is not perfect for serious imaging as there are still traces of CA and at f6 / f7 the field is not completely flat. Field flatteners can be added to help with corrections for imagers who want better field curvature control with today's large chips in commercial cameras.Triplet Apo's
Triplet apochromats are the ultimate answer for advanced visual and photographic applications, three lenses are used either air or oil spaced using at least one ED lens to correct for colour and fleld curvature is also better in the triplet. With this level of complex lens arrangement there are some practical drawbacks, air spacing insulates and slows cooldown and oil spacing restricts the design more than air spacing. Of course these are the most expensive to produce and assemble correctly but for many they are the ultimate astronomical telescope.Final thoughts
Essentially unless you are very serious about imaging the ED doublet is likely to be the best choice. Virtually free of CA and offering extremely high light transmission and contrast with only four air to glass surfaces combined with lighter weight and a robust cell design these are very desirable options. The ED doublet is more expensive than an achromat but modern manufacturing and competitive retail pricing has made these more affordable than ever.
Popular ED doublets are produced by SkyWatcher
with the Equinox and DS Pro ranges up to 150mm aperture, aswell as models from APM
in 140mm and 152mm apertures.