Tele Vue 21mm Ethos eyepiece
When Tele Vue introduced its Ethos eyepieces in 2007, the design defined a new benchmark for ultra wide-angle eyepieces.
They sport an apparent field of view of 100°, the widest apparent field of any commercial eyepiece – ever.
The 13mm focal length was the first out of the gate, closely followed by the 8mm, 6mm, 17mm and 10mm versions.
Noticeably lacking was a long focal length, but now Tele Vue has delivered the 21mm Ethos, which may just be the new crown jewel for ultra wide eyepiece lovers.
It also marks the longest focal length in the Ethos series – in effect, the capstone.
The main benefit of wide-field eyepieces is the sense of immersion from their wide fields of view – especially useful in undriven telescopes since objects stay in the eyepiece for longer.
But wide fields also let you see more true field (the apparent field divided by the magnification) for any given magnification.
And maintaining true field size with higher magnifications increases the apparent contrast of the view, revealing more detail in deep-sky objects.
The 21mm Ethos is an imposing eyepiece, one of the largest to come out of Tele Vue, but it does have a bevelled safety undercut built into the barrel, so it’s less likely to slip out.
Optically, we evaluated the Ethos in several telescopes with fast focal ratios (large Dobsonians with f/4.2 and f/4.5 focal ratios) as well as Tele Vue’s own f/5.4 NP-101 apo refractor.
Using the essentially perfect NP-101 meant that any defects spotted would originate elsewhere in the optical system, such as the eyepiece.
Visually, the performance is stunning.
The field is flat and the whole view comes to focus at the same point.
The eyepiece also lacks any detectable astigmatism – stars focus to points, not crosses.
Pincushion distortion – the curvature of what should be straight lines – is basically nonexistent and interior light baffling is excellent, so there are no problems with internal reflections or stray light.
Colour rendition is perhaps on the cool side for Tele Vue eyepieces, but this seems to be the norm for the Ethos line.
There are only two possible drawbacks: price and size.
At a few grams over a kilo, it’s the heaviest eyepiece in Tele Vue’s current line up, surpassing both the 31mm Nagler Type 5 (0.99kg) and the 41mm Panoptic (0.95kg).
In fact, the only heavier eyepiece we’re aware of is the venerable Tele Vue 20mm Nagler Type 2, which weighed in at an astounding 1.05kg.
You’ll want to make sure that your balance system and your telescope focuser can handle the weight.
However, with the price of the eyepiece, that probably won’t be a concern for the people who buy it: this is an eyepiece for astronomers who won’t stand for anything less than the best.
There’s been a lot of discussion as to whether this is going to be the longest focal length in the Ethos line up.
Ultimately, the true field size of any eyepiece is restricted by the maximum size of the field stop.
For a 2-inch focuser, that works out to around 46mm for the eyepiece, and the 21mm Ethos field stop is 36.2mm.
So by field stop size alone there is room for another model.
But due to weight and size constraints, Tele Vue has announced this is the longest focal length Ethos it plans to make.
The only eyepieces that offer a wider true field of view are the 31mm Nagler type 5 and the 35mm and 41mm Panoptics – which don’t offer the magnification, contrast and immersive viewing of the Ethos.
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This review appeared in the March 2010 issue of Sky at Night Magazine