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Classicals and Modern Eyepieces

Technical Insights Posted on Fri, May 23, 2014 17:35:49

When speaking about eyepiece designs most astronomers classify the eyepieces in “old but good” designs like the Plössl eyepiece (field 45°) and the “new and large” designs with large apparent field of view, from 60° (a “small” large) to… 120° (definitively a “large” large).

I was surprised, and I bet most of the readers will also be, that a very large eyepiece design existed in WWII for submarine periscopes with an apparent field of view of 120°! The design of this eyepiece (called Koehler eyepiece) was available after WWII and look like this:
Eleven lenses are used inside and the apparent field of view is 120°. The happy customers who found one in WWII surplus and tested it on their telescope are all enthusiasts.

But the Koehler design is still a classical design. Before computers existed and was available to a large public, opticians did calculations by hand. It was a consuming task so the opticians calculated one “formula” for an eyepiece (let’s say 20mm) and other eyepieces were enlargement (40mm) or reduction (10mm) of the “master” 20mm eyepiece.

A classical eyepiece design is a design were all eyepiece share exactly the same design, scaled to the intended focal. By example a 20mm Plössl is exactly the double than a 10mm Plössl, except for the apparent field of view that stays constant. If the 20mm Plössl has an eye relief of 16mm and internal lenses of 18mm; the 10mm will have an eye relief of 8mm and internal lenses of 9mm.
Classical designs have one important drawback: for short focal eyepieces the eye relief is so short that observations are difficult: you can not keep eyeglasses and staying in position so close to the view lens is a tremendous and rapidity tiring task impacting quality of observations.

A modern eyepiece design
involves computer assisted design. Each eyepiece focal length uses an adjusted design, different from all other eyepieces in the same group. Calculating and adjusting the design for each focal length will be a time-life task without a computer.

Eyepieces based on a modern design share the same field of view -but unlike the classical design- keep the eye relief constant. A group of eyepieces based on a modern design will be about the same diameter, with the shorter focal eyepieces being the longer.

As eyepieces based on modern design share the same eye relief short focal eyepieces are very comfortable to use (user can keep its eyeglasses) at the expense of a large and quite heavy construction.

Photokina Köln 16-21.09.2014

Events Posted on Thu, May 22, 2014 20:41:51

Every two years there is one focal point in the sector: photokina, the
world’s leading imaging fair. photokina brings together the industry,
trade, professional users and end consumers with a passion for
photography — in a way no other event can match.

From September 16 to 21,
2014 in Cologne, Sky-Watcher will present its latest products and hope to meet you for discussing the future in amateur astronomy.

Sky-Watcher at ATT Essen 2014

Events Posted on Thu, May 22, 2014 19:38:33

The ATT is Europe’s biggest exhibition especially for amateur astronomers, presenting state-of-the-art telescopes and accessories. It is the place where manufacturers and experienced amateur astronomers from all over Europe come to share their thoughts.

Sky-Watcher was presenting new 2014 products on its stand: the Dob18, the ED mount and the Star Adventurer mount.

The Dob18 is unlike previous dobsonians created by Sky-Watcher. The entire structure is using a skeletal
like design to save on weight. The primary mirror design provides fast
cooling and light weight! The primary is 2″ in
thickness and has a focal ratio of f/4.12 producing a low eyepiece
height! Please
remember this is still a prototype.

The ED mount is a revolutionary equatorial mount designed for newtonian astrographs. The ED mount can track past the meridian without the need to flip the RA axis (like on German equatorial mounts) and is far more compact and sturdy than any fork mount.

Last but not least the cute Star Adventurer mount was the little star of the show. This equatorial mount designed for deep-sky large field photography and time-lapse animations supports up to 5kg. payload and can be configured for a multiple of roles.

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