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 and providing high contrast views 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 acceptable levels if the aperture is small to medium sized, up to around 5" (127mm) provides very good but not perfect results.
The problem CA robs precious contrast from the view you see and makes critical focusing vague, 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 viewing experience. 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. The combination of excellent colour correction and unobstructed aperture provides unbeatable light transmission and contrast over and above other optical designs.
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 design 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.
Why are most refractors white? Most premiun refractor telescopes are finished in white for very good reason, the obvious one is of course visibility, in low light you can see the tube far easier than a dark colour. Thermally white is ideal, it absorbs virtually no heat from the Sun in daytime making Solar astronomy more practical, hot optics are not going to show good images. Another benefit is cosmetic, finger marks from handling are near invisible on white but horribly prominent on dark or gloss black tubes.
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. ED triplet designs are becoming much more affordable for those seeking the very best performance, especially for imaging but the ED doublet remains a really superb choice.
Thoughts on optical test charges? We do not charge customers for optical testing on a brand new instrument, we feel the brands we sell are of excellent optical quality and we certainly could not justify charging customers a hefty fee to prove this. Some of our brands include a test report provided by the manufacturer at no extra cost. As a customer you are paying for an instrument that should already be assured to reach the level of performance the manufacturer has stated. If in the extremely unlikely event any optic was not performing correctly this is of course covered under warranty at no cost to the customer!
Some ED refractors to consider Popular ED refractors are produced by SkyWatcher with the Evolux, DS Pro and Esprit ranges up to 150mm aperture, as well as models from Altair Astro in 60mm to 152mm apertures and StellaMira with models from 66mm to 125mm apertures. These options are more affordable than ever before and can offer incredible performance previuosly impossible at these price points.