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CHARACTERISTICS, ABERRATIONS AND TECHNICAL TERMS

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CHARACTERISTICS, ABERRATIONS AND TECHNICAL TERMS

In today¡¯s eyepiece market, there is a lot of confusion about eyepiece characteristics and problems. What follows will be a basic guide:

This summary does not review eyepiece types, nor go into every possible design parameter for an eyepiece. It will merely define some terms in CHARACTERISTICS, and describe the usual problems seen at the telescope in ABERRATIONS.

CHARACTERISTICS

1. Resolution. This can be measured by the use of a grid or line test, but, if those tests are performed, the results are not published, so we do not have tests to compare. Our own eyes tell us which eyepiece is sharper and which is less sharp. There is more to sharpness than merely the center of the field, however, and it is the resolution of the outer perimeter of the fields of view that really set apart the superb eyepieces from the good. If you¡¯d like to make your own test, use small details on the planets and Moon to compare one eyepiece with another. Subtle though they may be, differences will soon display themselves.

2. Apparent Field of View. This is the size of the field our eye sees when looking into the eyepiece. The manufacturers can quote larger fields of view if allowing distortion at the edge of the field, but that is for later discussion. Suffice it to say, 30-45 degrees is considered a narrow field, 50-60 degrees considered a normal field, 65-70 degrees considered widefield, and 78-90 degrees considered an ultra-wide field of view. Many people have trouble seeing the entire field at once if the field is ultra-wide.

3. True Field of View. This is determined by the telescope. Essentially, the field stop of the eyepiece encompasses a linear field of view from the telescope. The true field of view depends on the linear scale of that field stop, as determined by the magnification of the eyepiece in that telescope. Telescope focal length divided by eyepiece focal length equals magnification, and (approximately), the eyepiece apparent field divided by the magnification equals the true field. I say approximately because distortion at the edge of the field in an eyepiece changes this relationship somewhat.

4. Focal length. Eyepieces have intrinsic focal lengths and work as simple magnifiers of the focal image of the telescope. Shorter focal lengths produce higher powers.

5. Eye relief. This is the distance above the eye lens (the lens closest to the eye in the eyepiece) that the eye can be held and still allow the observer to see the entire field. There is no true standard for measurement of this parameter, but it is usually described as the distance the cornea is from the center of the eye lens. Note that glasses wearers will need 18-20mm or more of this parameter, whereas otherwise eye reliefs can be as short as 8mm without being short enough to cause the eyelashes to continually brush the eyepiece. Eyepieces can be found with shorter eye reliefs than that, but they are not comfortable to use for long periods.

6. Contrast. This is the most misunderstood of all parameters of performance. It will be very difficult to measure or come up with a system of measurement for this characteristic, because it describes many different characteristics all lumped together into one word. It encompasses light scatter (the reflected/scattered light in the eyepiece FOV), quality of coatings, light transmission, light absorption, surface accuracy of the lenses, and all the aberrations eyepieces are prone to. Ultimately, even the resolution (sharpness) of the eyepiece comes into play when attempting to see the faintest stars. This term is vague as it is applied to eyepieces, because so many external factors come into play (the mirrors, seeing, transparency, fatigue, eye quality, etc.) that have nothing to do with the eyepiece. At best, a direct comparison of two different eyepieces with the exactly same focal lengths, used in the same scope, on the same night, on the same target, at nearly the same time, and seen by more than one observer, can show you some valid differences. But change any of the factors mentioned, and the outcome is suspicious. Apparent field even enters into the picture, as eyepieces of small apparent fields seem to have their fields of view surrounded by blackness. This elimination of peripheral light may allow the maximal opening of the eye¡¯s pupil, reduce field brightness, and give a perceived edge to the narrower field of view. Dark adaptation is critical for evaluating contrast, as the better dark adaptation will see fainter details. And finally, eye problems such as cataracts will have profound effects on the contrast evaluation. Contrast is an elusive parameter, yet we all use the word, so it is put in here as a descriptor of eyepieces.

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MEADE TELESCOPE EYEPIECES

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Meade Series 5000 Ultra Wide Angle 30mm 2 inch Eyepiece, 82 Degree Field of View
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Meade UWA Eyepiece 07744, Series 5000 Ultra Wide Angle 30mm 2" Eyepiece, 82 Degree Field of View
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Meade Series 5000 Ultra Wide Angle 20mm 2 inch Eyepiece, 82 Degree Field of View
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Meade UWA Eyepiece 07743, Series 5000 Ultra Wide Angle 20mm 2 inch Eyepiece, 82 Degree Field of View
MSRP: $187.00
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Meade 14mm Series 5000 1.25 inch Ultra Wide Angle UWA waterproof
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Meade UWA Eyepiece 07742, 14mm Series 5000 1.25" Ultra Wide Angle waterproof
MSRP: $162.00
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Meade 8.8mm Series 5000 1.25 inch Ultra Wide Angle waterproof
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Meade UWA Eyepiece 07741, 8.8mm Series 5000 1.25" Ultra Wide Angle waterproof
MSRP: $162.00
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Meade Series 5000 Ultra Wide Angle 5.5mm 1.25 inch Eyepiece, 82 Degree Field of View
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Meade 07740 Eyepiece, Series 5000 Ultra Wide Angle 5.5mm 1.25" Eyepiece, 82 Degree Field of View
MSRP: $162.00
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Meade Eyepiecs Kit Series 5000 HD-60, 07736 w/ aluminum carrying case
Meade Eyepiecs Kit Series 5000 HD-60, 07736 w/ aluminum carrying case
Item No. 712-18020
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Meade 07736 Eyepiecs, Series 5000 HD-60, six of the HD-60 eyepieces in an aluminum carrying case
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Meade Series 5000 25mm HD-60 6-Element Eyepiece, #07735 (1.25 inch)
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Meade 07735 Eyepiece, Series 5000 25mm HD-60 6-Element Eyepiece, 1.25"
 
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Meade Series 5000 18mm HD-60 6-Element Eyepiece, #07734 (1.25 inch)
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Meade Series 5000 18mm HD-60 6-Element Eyepiece, #07734 (1.25")
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Meade 07734 Eyepiece, Series 5000 18mm HD-60 6-Element Eyepiece, 1.25"
 
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Meade Series 5000 12mm HD-60 6-Element Eyepiece, #07733 (1.25 inch)
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Meade Series 5000 12mm HD-60 6-Element Eyepiece, #07733 (1.25")
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Meade 07733 Eyepiece, Series 5000 12mm HD-60 6-Element Eyepiece, 1.25"
 
Our Price: $79.99
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Meade Series 5000 9mm HD-60 6-Element Eyepiece, #07732 (1.25 inch)
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Meade Series 5000 9mm HD-60 6-Element Eyepiece, #07732 (1.25")
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Meade 07732 Eyepiece, Series 5000 9mm HD-60 6-Element Eyepiece, 1.25"
 
Our Price: $79.99
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Meade Series 5000 6.5mm HD-60 6-Element Eyepiece, #07731 (1.25 inch)
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Meade Series 5000 6.5mm HD-60 6-Element Eyepiece, #07731 (1.25")
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Meade 07731 Eyepiece, Series 5000 6.5mm HD-60 6-Element, 1.25"
 
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Meade Series 5000 4.5mm HD-60 6-Element Eyepiece, #07730 (1.25 inch)
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Meade Series 5000 4.5mm HD-60 6-Element Eyepiece, #07730 (1.25")
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Meade 07730 Eyepiece, Series 5000 4.5mm HD-60 6-Element Eyepiece, 1.25"
 
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Meade MA 12 mm Illuminated Reticle Eyepiece - Corded
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Meade MA 12 mm Illuminated Reticle Eyepiece - Corded
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Manuf. No. Meade 07065

Meade MA 12 mm Eyepiece Illuminated Reticle - Corded
 
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Meade MA 12 mm Illuminated Reticle Eyepiece - Wireless
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Manuf. No. Meade 07066

Meade MA 12 mm Eyepiece Illuminated Reticle - Wireless
 
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Meade Plossl 9 mm Illuminated Reticle Eyepiece - Corded
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Meade Plossl 9 mm Illuminated Reticle Eyepiece - Corded
Item No. 712-42020
Manuf. No. Meade 07067

Meade Plossl Eyepiece 9 mm Illuminated Reticle - Corded
 
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