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All eyepieces contain the following parameters:
Eyepiece Barrel: 0.965”, 1.25” and 2” (or 24.5mm, 31.75mm and 50.8mm)
The focal length of the eyepiece determines the magnification, commonly around 3-32mm
Apparent Fields of view (AFoV): 40°, 50°, 68°, 82°, 100°
Eye relief: Often between 5mm to 20mm?
Exit pupil: 0.5mm to 7mm?
See below for an explanation of the above two terms!
1 – Real image 2 - Field stop 3 - Eye relief 4 - Exit pupil
An eyepiece consists of several elements and has a set focal point called the ‘exit pupil’ or ‘Ramsden disc’
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This diagram shows a Plössl eyepiece that consists of two identical doublets, which are reversed, giving this eyepiece its other name of ‘symmetrical’
The Plössl eyepiece traditionally has around 50º AFoV.
The Plössl is one of the most common contenders for upgrading from the supplied eyepieces of a telescope, it gives relatively sharp views and it's cost is very reasonable.
Plössls retail at around £20-£30
The higher mag eyepieces are no good for spectacle wearers as the eye relief is much too short. In this case you should consider the BST StarGuiders listed below
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A 19th century innovation, many modern wide field eyepiece designs involve a negative lens group, followed by a positive lens group which forms the final image. Collectively these negative groups are called Smyth lenses and invariably their purpose is to correct for aberrations, esp. field curvature.
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The BST Explorer/StarGuider
60° AFoV at a reasonable cost
Good eye-relief – around 15 to 18mm
This eyepiece uses the negative Smyth lens and as a result is very well corrected, the wider AFoV is also desirable and at a fairly reasonable cost (around £55)
A 5-element design consisting of two achromatic elements with extra elements in between, invented by Heinrich Erfle (working for Carl Zeiss) during WWI for military purposes
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This illustration shows how much the eyepieces from Galileo's time have progressed.
The Baader Hyperion eyepiece
The Baader Hyperion 68º ‘modular’ eyepiece
The focal length can be changed by the addition of fine tuning rings (FTR’s) 14mm & 28mm
Baader Hyperion’s do not perform well in fast scopes
The only problem with the idea of FTR's is that you try changing them when you have freezing cold fingers, also in this vein detaching and reattatching the nose-piece will be problematic
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The original take on this configuration is known as the Zeiss (Kohler) Erfle, while Panoptic is the final Al Nagler take on it in 1985 when he released his "wide angle Panoptic".
Erfles prefer longer focal length telescopes for superior performance on and off-axis
Al Nagler
Al Nagler changed the concept of visual astronomy with an 82° eyepiece, he coined the term ‘spacewalk’
Every Tele Vue eyepiece is tested at f/4 to ensure sharpest full field possible. It is the nature of optics that being the best in fast scopes assures the best performance in 'slow' scopes.
Tele Vue eyepieces, by thier nature, are very expensive starting at over £300
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Tele Vue Optics is a Chester, New York-based astronomical optics company known primarily for its premium brand of speciality eyepieces and apochromatic refractor telescopes. Founded in 1977 by Al Nagler, an optical engineer from The Bronx who designed simulators used in the Apollo program, the company originally made projection lenses for large projection-screen televisions, but is well known in the astronomy community for its products.
Some of the Tele Vue Naglers along with a 2.5x Powermate
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Samples of Nirvana eyepieces
82º eyepieces at a reasonable price
Focal lengths: 4mm 7mm 10mm 13mm 16mm
With a slim 47mm waistline, the Nirvana is less bulky than most similar eyepieces
These eyepieces are retailed under a number of different names, one of the most significant being the 'TS-Optics 1.25" Ultra Wide Angle Eyepiece UWAN - 82° field of view'
At around £90 these eyepieces are highly recommended!
A more complex design of the Erfle - the ES 100º
Nine elements in five groups
The Smyth converter is slightly over-compensating in order to reduce the width of the field group in the Erfle II section (where barrel distortion shrinks the field)
Choices can be based on what you want to view
1) Deep Sky and Clusters - 32mm (28x); 18mm (50x); 12mm (75x)
2) Planetary, Double Stars & Moon - 12mm (75x); 8mm (112x); 5mm (180x)
Magnifications based on an f/9…900/100mm refractor telescope
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