Tele Vue 3.7mm Ethos-SX 110° eyepiece

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Magazine Verdict: 
Tele Vue 3.7mm Ethos-SX 110° eyepiece

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Tele Vue

When hearing about the latest – and supposedly final – addition to the Ethos line, it would be easy for one to be a bit dismissive. How many times will Tele Vue bring out yet another eyepiece with a wider apparent field size?

With its short focal length of 3.7mm – and fairly hefty price – at first glance the Ethos-SX would seem to appeal mainly to advanced lunar and planetary observers, many of whom prefer simple eyepieces with fewer lens elements than the Ethos-SX has.

However, there are several advantages to this eyepiece, not least that its apparent field size of 110° really does live up to its SX moniker – which stands for Space eXperience. With an increase of 21 per cent on the rest of the Ethos 100° line, the extra field of view is immediately apparent. Although the SX may only provide an extra 10° of apparent field, that makes for a truly unique spacewalk experience – agoraphobics need not apply.

Tele Vue has always pushed its designs, but this latest is historically rather significant for company founder Al Nagler. The Ethos-SX’s 110° apparent field is the same the same field as the optics he designed 40 years ago for the lunar module simulator used to train NASA astronauts for the lunar landings.

To put the eyepiece through its paces, we used it in a Tele Vue NP-101, a 4-inch apo refractor, and an 8-inch f/4.5 Newtonian for observations of bright planetary nebulae and double stars. So what were the views like? Well, the field is flat and sharp, and the colour rendition – like the rest of the Ethos line – is fairly neutral. Contrast is good and light transmission is extremely high, despite the number of lens, elements. There’s also no sign of internal reflections.

As a wide-field, deep-sky eyepiece, it’s unsurpassed in focal length and it gives up little to nothing when compared to dedicated planetary eyepieces. In the 4-inch apo, the eyepiece is nothing short of stunning – its only failing being some slight colour visible at the extreme edge of the eyepiece when highlighted against the Moon.

Somewhat surprisingly – given its short focal length – we found the eyepiece to be extremely useful in our 8-inch, f/4.5 Dobsonian. When observing planetary nebulae at high powers, traditional eyepieces tend not to provide much of a frame of reference and tracking becomes critical. But with its large field and high contrast, the Ethos-SX doesn’t have these problems, making it an exceptional eyepiece for this type of observing. For comparison, the true field provided by the Ethos-SX is larger than an 8mm Plössl’s or even a 5mm Nagler Type 6’s.

Adaptable eyepiece

The SX is designed to function with or without its 2-inch adaptor – designed specifically for the eyepiece. So much so, that with the adaptor fitted, it appears to be part of the eyepiece itself. As a whole, the unit is well baffled and will take 1.25- or 2-inch filters. It depends on your particular telescope, but we’re betting that you’ll opt to keep the adaptor on all the time. One final note here – the lens caps that come with the eyepiece are designed for use with the 2-inch adaptor. If you elect to leave it in 1.25-inch mode you’ll need to dig up a 1.25-inch end cap to protect your investment.

So, does this herald an entire new line of 110° eyepieces? Well, according to Tele Vue, the short answer is ‘No’. The Ethos-SX is not a scaled design and this particular configuration does not lend itself to being reproduced in longer focal lengths. It might be more fitting to look at the SX as the Ethos capstone. 

The Ethos-SX is suitable if you’re a beginner or advanced astronomer who can afford the price tag; if you’re interested in planetary observing with small, short focal length telescopes; or even if you’re hankering after a high-power option for a larger scope. With its latest eyepiece, Tele Vue has delivered a true space simulator experience with ‘spacewalk’ views.

Wide-field, high-power views

The hallmark of the entire Ethos line is the massive apparent field of view that they provide, and the SX goes one better by extending the viewable area by over 20 per cent. Wide fields of view allow for easier observations at higher powers and make finding and centring objects far easier, especially if you’re using an un-guided Dobsonian. However, another benefit of such a wide field is that it allows you to maximise image contrast while minimising sky background glow.

This has been Tele Vue’s intent since the company was first formed in the 1980s and the theory behind the introduction of the eyepieces that set the original standards for wide fields of view: the Tele Vue Naglers.

This review first appeared in the January 2011 issue of BBC Sky at Night Magazine.

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