20mm XW extra wide angle eyepiece

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This medium focal length Pentax provides sharp and very wide field images for both terrestrial and astronomical observing.

Terrestrially, it is an excellent medium power wide field eyepiece for detailed close-in or wide field nature studies with the 80mm and 100mm Pentax spotting scopes in overcast and sub-par light conditions, dawn to dusk.

Astronomically, its six lens/four group design makes it an excellent high-contrast choice for medium power deep space observing with long focal length refractors, reflectors, or catadioptric scopes. It is a superb low power eyepiece with short focal length refractors and fast focal ratio reflectors. An f/4.5 Dobsonian reflector, for example, will yield a 4.4mm exit pupil, excellent for detailed images of emission nebulas and galaxies. Its sharpness across the field makes globular clusters and nebulas vivid and nearly three-dimensional. The 650mm focal length TMB-105 triplet apo refractor provides 32.5x, a 4mm exit pupil, and a 2.15-degree field (over four times as wide as the Moon). These figures are all very usable for observing open clusters such as the Pleiades; large nebulas such as the Lagoon, Veil, and Orion; and large globulars such as M-13 and Omega Centauri. All will fit nicely into the field of view, with a generous framework of black velvet sky around them to set off their subtleties.

The long 20mm eye relief is ideal for those who must wear eyeglasses while observing due to astigmatism. Even those who do not wear glasses will find the long eye relief a comfortable change from the cramped eye relief that is typical of the usual short focal length eyepiece.

For terrestrial observing . . .
With an 80mm straight-through Pentax spotting scope this eyepiece provides 25x magnification and a huge 147’ field of view at 1000 yards, which are the values given in the specifications to the left.
With the 80mm angled Pentax spotting scope this eyepiece provides 26x magnification and a 141’ field of view at 1000 yards.
With the 100mm aperture Pentax spotting scope this eyepiece provides 32x magnification and a big 115’ field of view.

For astronomical observing . . .
The field of view of 70° degrees given in the specifications to the left is the apparent field of view. The actual field of view will be equal to that apparent field divided by the magnification of the eyepiece/telescope combination.

25x terrestrially
Field of view 1000 yards:
Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
70 °
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.
12.6 oz
Limited Lifetime
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1. patrick on 5/15/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
This is my favourite eyepiece. I have a Celestron 6se, and I do wear glasses, so the eye relief is really nice. I've had no complaints from the non-glass wearing folks that have looked through it. It is super sharp and a joy to look through. I love this EP!
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2. David on 3/6/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
The 20mm XW often is criticized for suffering from "field curvature". However, I have used the 20mm XW with a 120mm f/8.3 achromat and a Vixen 140 Neo-achro Petzval refractor and found that the field curvature is not signficant with these scopes. The 20mm XW is extremely comfortable and the AFOV feels bigger than the 70 degs. When I compared it with an 82 deg AFOV eyepiece with similar FL the 20mm XW didn't feel like it had a smaller AFOV. The 20mm XW has excellent coatings and provides nice bright views of nebula and galaxies and precisely rendered pinpoint stars in clusters. It also handles a barlow extremely well. I've tried every XW FL except the 3.5mm and 5mm. The 20mm XW is easily my favorite XW. Personally, I do not like XW's for lunar observations. I think there are other eyepieces that give sharper lunar views - among both simpler and widefield designs. But as a deep sky eyepiece the 20mm XW is about as good as it gets. I also had some of the best views I've ever had of Jupiter with the 20mm XW.
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Telescope Eyepiece Formula
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Pentax - 20mm XW extra wide angle eyepiece

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Pentax - 20mm XW extra wide angle eyepiece
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 AstronomicsAstronomicsAstronomicsAstronomicsAstronomics (Average: 5.00 | Users: 2)  Only registered users can submit ratings - Register Here
Our Product #: PXW20
Manufacturer Product #: 70516
Price: $329.00  FREE ground shipping - Click for more info
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MSRP: $429.00

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This medium focal length Pentax provides sharp and very wide field images for both terrestrial and astronomical observing . . .

. . . our 38th year