13mm 70° field AF Series eyepiece, with dual 1.25" and 2" barrels

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The 13mm focal length of this Astro-Tech 70° Field AF Series dual-barrel eyepiece is designed for unsurpassed wide field observing with any telescope type, under virtually any seeing conditions – from fair to superb. It is truly excellent for medium magnification lunar, planetary, and globular observing with long focal length refractors, reflectors, or catadioptric scopes. Contrast is very good, rivaling that of orthoscopic and Brandon eyepiece designs, but with a substantially wider field of view than either. Its high contrast also makes it a good choice for resolving detail in planetary and emission nebulas, while its sharpness across the field makes globular clusters vivid and nearly three-dimensional.

An f/4.5 Dobsonian will yield a 2.9mm exit pupil, for bright images of nebulas and galaxies. With short focal length refractors, such as the Astro-Techs or TMBs, it’s a low to medium power eyepiece. With the 603mm focal length Astro-Tech AT90EDT triplet apo, for example, it provides 46x, a 1.94mm exit pupil, and a 1.5° field. With the 506mm focal length TMB-92 Signature Series triplet apo, it provides 39x, a 2.36mm exit pupil, and a 1.8° field. On both scopes, the fields are three times as wide as the full Moon or more. These figures are all very usable for observing open clusters such as the Pleiades, large nebulas such as the Lagoon, and large globulars such as M-13. All will fit nicely into the field of view, with a framework of black velvet around them to set off their subtleties.

With its dual barrel design, the 13mm Astro-Tech 70° AF eyepiece can be used in both 2” and 1.25” focusers and star diagonals with no adapters needed.

The 17mm eye relief of this 13mm Astro-Tech is ideal for those who must wear eyeglasses when observing due to astigmatism. Vignetting will be minimal for eyeglass wearers, although the very wide field may require some moving of the head from side to side to see the entire field, even for those who don’t wear glasses.

This Astro-Tech comes with both 1.25” and 2” barrels, dust covers for both ends of the eyepiece, and a plastic bolt-type eyepiece container.

The optical performance of this 13mm Astro-Tech would be exceptional at any price. At its actual and very sensible price, it is truly outstanding. Well corrected for all aberrations, high in contrast, flat and sharp across its entire 70° field, it is an extraordinary value. We feel that this Astro-Tech 70° Field AF Series eyepiece equals or surpasses the performance of exotic designs costing twice its very reasonable price. It would be a welcome addition to any astronomer’s observing kit.

Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
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.

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.

Number of optical elements:
Barrel Size:
The industry standard diameter, as of 2011, is 1.25" and is the most common.  Other sizes available are .965" and 2".
The weight of this product.
16.8 oz.
2 years
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1. Robert on 3/3/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
Not nearly as good as the 17mm or 22mm versions. Similar field curvature, but it simply doesn't sharpen up at the extreme edge of field quite as nicely. The outer 15% suffers from quite a bit of astigmatism making pinpoint focus impossible. It actually shows bright stars as rainbow comets with the tails pointing to the center at sharpest focus. Quite beautiful, but not desired. The center is extremely sharp out to about a 50 degree AFOV. It is quite sharp from there to a 60 degree AFOV. From there to the edge, it quickly turns into a blurry mess at sharpest focus. Had the field stop been set for a 60 degree field, it would be nearly perfect. Compared to a 14mm Pentax XL, it is nearly as sharp in the center but nowhere near as sharp at the edge, even accounting for the wider field of view. It snaps into focus almost as easily as the Pentax. The eyecup screws off the same as the 17mm and 22mm versions. With it removed, it is just as easy to take in the view as with a Pentax XL or XW, with just a few less mm of eye relief, but still easy with glasses. Nearly parfocal with the 17mm and 22mm versions as well as the Pentax XL and XW lines. It is good if you keep your object centered. Don't let it drift to the edge. Jupiter looked horrible near the edge but quite good in the center. Barlowing helped very little and made the exit pupil a bit difficult to hold.
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Astro-Tech - 13mm 70° field AF Series eyepiece, with dual 1.25" and 2" barrels

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Astro-Tech - 13mm 70° field AF Series eyepiece, with dual 1.25" and 2" barrelsImage showing the fixed 1.25" barrel and the removable thread-on 2" barrel of the Astro-Tech 13mm AF Series 70° field eyepiece.
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This 13mm Astro-Tech AF Series very wide 70° field dual-barrel eyepiece is usable for medium power observing with any scope type, under virtually any seeing conditions – from fair to superb . . .

. . . our 34th year