Flat Field 16mm 1.25" eyepiece

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The 16mm Astro-Tech Flat Field is a very good medium power eyepiece for lunar observing, resolving globular clusters (and open clusters with short focal length refractors), picking up detail in emission and reflection nebulas, etc. It has a very flat field that keeps your image sharply in focus from one edge of its wide 60° field to the other. This flat field makes it a great match to the new breed of flat field/coma-free catadioptric telescopes.

The flat field of the 16mm Astro-Tech lets you scan the lunar surface with a long focal length scope, for example, and see sharply defined craters and rilles filling the entire field, rather than having the image soften and blur disconcertingly towards the edges as you often find is the case with the curved field of conventional eyepiece designs.

When used with short focal length refractors, the Astro-Tech’s flat field shows you views of open clusters in which all the stars in the cluster are in focus, not just those in the center of the field. Dobsonian owners will also appreciate not having to constantly tweak the focus as objects drift across their field of view. Its 16mm focal length is a good choice for filling in the gap between the 25mm or 26mm eyepiece that comes standard with most scopes and a high power planetary/binary star splitting eyepiece.

This 16mm Astro-Tech Flat Field has a good 15mm eye relief that lets even eyeglass wearers use the eyepiece with virtually no vignetting.

Best of all the 16mm Astro-Tech Flat field eyepiece is priced well below similar design eyepieces from other suppliers, and with double the warranty.

Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
60°
Eye Relief:
Eye relief is the distance from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the circular image is formed, sometimes called the “Ramsden Disk.” This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you’ll usually need at least 15mm of eye relief or longer if you want to see the full field of view with your glasses on.

A note on our eye relief figures: Quite often, our eye relief figures will differ from those of the manufacturer. This is because we measure the “usable” eye relief, while the manufacturers specify their usually-longer (but technically correct) “designed” eye relief.

The eye lens of the eyepiece is normally recessed below the rubber eyeguard or rubber rim of the eyepiece to keep the lens from being scratched during use. An eyepiece might have a “designed” eye relief of 15mm (and the eye relief will truly measure 15mm from the eye lens to where the image forms). However, if the eye lens is recessed 3mm below the eye guard, the Ramsden Disk forms only 12mm above the eyepiece body (the 15mm “designed” eye relief, less the 3mm of eye relief made unusable by having the eye lens recessed into the body of the eyepiece). This “usable” eye relief of 12mm (measured from the rolled-down eyeguard – the closest point you can get your eye to the eye lens – to where the image forms) is the eye relief figure we would measure and list in this website.

Why is it important to list the “usable” eye relief? For those people who don’t wear eyeglasses while observing, a few mm difference between the eye relief they expect from the manufacturer’s literature and the shorter eye relief they actually get in real life doesn’t mean a lot. They can simply move a little closer to the eyepiece to see the full field, and never realize that the eye relief is a little shorter than they expected. However, some people must wear eyeglasses while observing, because of severe astigmatism. These observers can’t move closer to the eyepiece if the eye relief is shorter than expected because their glasses get in the way. For these people, the real life “usable” eye relief is more important than the technically correct but sometimes not fully usable “designed” eye relief. We measure and list the actual usable eye relief so that people in the real world can pick the eyepieces that will work best for them.

15mm
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

16mm
Number of optical elements:
5
Barrel Size:
The industry standard diameter, as of 2011, is 1.25" and is the most common.  Other sizes available are .965" and 2".
1.25"
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Astro-Tech - Flat Field 16mm 1.25" eyepiece

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Astro-Tech - Flat Field 16mm 1.25" eyepieceImage showing a typical Astro-Tech Flat Field eyepiece with the eyecup retracted for eyeglass use and with it fully extended for use without glasses.
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Our Product #: ATFF16
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This Astro-Tech 16mm Flat Field medium power eyepiece is designed to keep objects sharply in focus across its entire 60° field of view, making it a good choice for filling in the gap between the 26mm eyepiece that comes standard with most scopes and a high power planetary/binary star splitting eyepiece . . .





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