Eclipse viewers reach astronomical prices

It seems the sky's the limit when it comes to Friday's solar eclipse, with eBay vendors asking huge sums for viewers that cost just a couple of quid to make.

While sunglasses aren't powerful enough for looking at the sun, special eclipse viewers are available that protect the eyes. But while they usually cost just a pound or two - or even come free with astronomy magazines - some people are trying their luck and asking for a fortune.

One listing asks for £51.91 for a viewer consisting of a three-by-five-inch piece of cardboard with filter inserted - although the vendor may not get much interest, as the predicted delivery date is in two weeks' time. Others are asking for similar amounts.

As excitement rises, it's become difficult to get hold of eclipse viewers, and even the cheapest ones available online are costing £14 or more.

So where can you get hold of a pair without being ripped off?

The Sky at Night magazine is offering a free pair with every issue, although copies have been selling out fast. Some Amazon vendors still have them available, although again delivery times could be a problem. Some photographic shops may have them for sale.

But it's vital to make sure that what you're buying really is safe.

As NASA notes: "The Sun can only be viewed directly when filters specially designed to protect the eyes are used. Most such filters have a thin layer of chromium alloy or aluminium deposited on their surfaces that attenuates both visible and near-infrared radiation," notes NASA.

"A safe solar filter should transmit less than 0.003% (density~4.5) of visible light (380 to 780 nm) and no more than 0.5% (density~2.3) of the near-infrared radiation (780 to 1400 nm)."

This is very little light indeed - if in doubt, just look for CE certification.

If you're friendly with a welder, you could be in luck, as welders' goggles rated 14 or above are sufficiently powerful to be safe for watching an eclipse.

Alternatively, you can make a pinhole viewer that will display an upside-down image of the eclipse on a piece of paper. Just make a small hole in a piece of card and hold it up to the light; as the sun's rays pass through, they will create a perfect image, projected onto another piece of paper or the ground.

According to NASA, practically any small hole will do, from a colander to loosely interlaced fingers. Just make sure you never look directly at the sun itself.

The timing of the eclipse varies slightly across the UK, but starts at roughly 8.30 on Friday morning, reaching its peak around an hour later. There's detailed timings for different British cities here.

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