More 100-Degree-Eyepiece Choices
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Tele Vue recently announced the addition of a 21mm model to its lineup of 100-degree Ethos eyepieces and sent one to SkyNews for review.
As with all other focal lengths in the Ethos line, the 21mm has a comfortable 15mm of eye relief. And, not surprisingly, this new eyepiece performs as exceptionally as do its Ethos siblings.
In my 12.5-inch f/4.8 PortaBall reflector, the 21mm Ethos provides a magnification of 72x and a stunning 1.4-degree field of view. I enjoyed the best view of M31 that I've ever seen though this telescope, with tight stars at the edge of the field. And the wispy tentacles of the Orion Nebula extend almost to the edge of the field of view, affording a unique and detailed view of the nebula. Dobsonian owners such as myself will instantly note that this is a hefty eyepiece, weighing in at 2¼ pounds. It may be necessary to add counterweights to balance some scopes.
It was when I used the 21mm Ethos with an 11-inch f/10 Schmidt-Cassegrain telescope (SCT) that the eyepiece truly showed its advantage. The SCT, with its longer focal length, has a subsequently narrower field of view. One way to get around this is to use a focal reducer, which I have always found cumbersome: remove diagonal, screw in focal reducer, replace diagonal, insert eyepiece — repeat procedure again and again, as required.
But the 21mm Ethos in the 11-inch SCT provides a 0.74-degree field of view at 133x, a magnification I prefer for many deep-sky objects. By comparison, a Plössl eyepiece of the same focal length has slightly less than half the field of view of the Ethos. My observing partner and I enjoyed beautiful views of the Perseus Double Cluster, an open star cluster that, in my opinion, loses its beauty when observed through the SCT's typical narrow field of view.
When the 21mm Ethos was used in a Tele Vue 76 refractor, the resulting magnification of 25x and the humongous four-degree field of view provided an outstanding view of the Pleiades star cluster floating in a star-rich field.
While the 21mm Ethos excelled in the short-focal-length refractor, it was most impressive when used with the SCT — so much so that I'm going to christen this focal length the "SCT-Ethos." SCT owners are no longer restricted to relatively low magnification to enjoy a comfortable field of view.
The suggested retail price for this eyepiece is US$850, making it the most expensive production eyepiece ever offered to amateur astronomers. But excellence has its cost and this latest member of the respected Ethos family fits the tradition set by the previous five.
— Carlson, T. "More 100-Degree-Eyepiece Choices". SkyNews, (Jan/Feb 2010).
Reprinted with permission. © 2010 SkyNews Inc. All rights reserved.