In praise of the modern refractor

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Refractors traditionally were long focal length affairs with quite long tubes, which makes you wonder how astronomers like Cassini and Hevelius coped with the huge ungainly equipment they regularly used.

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We really have to take our hats off to them, considering the achievements and discoveries they made with the refractors of old.

The reason for this is because short focal lengths were difficult to figure accurately, producing colour fringing around bright objects known as chromatic aberration.

This effect could be lessened by increasing the focal length of the optics.

Today, chromatic aberration can be dealt with in short tube refractors using a range of techniques, from extra low dispersion glass to a combination of doublet or triplet lenses to control or significantly lessen the effects.

However, the very best triplets often also cost the most, so optical designers are always looking for ways of bringing the price down.

Istar Optical’s approach is to use anastigmatic optics and in the March issue of BBC Sky at Night Magazine I have the pleasure of testing their Asteria WFX 150-5 R50 anastigmat refractor.

Also in the March issue, Martin Lewis takes a look at Altair Astro’s venture into planetary webcams with their GPCAM, while Pete Lawrence covers this month’s Tried & Tested product; the Revelation 12-inch f/4 M-LRN OTA.

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All our respective observing suffered somewhat from the recent poor weather, but we pulled through in the end!


See how we got on in the March issue of BBC Sky at Night Magazine, out 18 February.