5mm XW extra wide angle eyepiece

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This short focal length Pentax eyepiece has a very wide 70° field of view and very long 20mm eye relief, a uniquely useful combination not matched by other eyepieces. It uses eight lenses in five groups to provide outstanding medium to medium high magnification astronomical observing with short focal length refractors and fast focal ratio/short focal length Dobsonian reflectors. Its long eye relief is particularly helpful if you must wear eyeglasses while observing, although all observers will benefit from its wide field and sharp images. It provides about the maximum usable magnification with a typical 8” or 10” f/10 Schmidt-Cassegrain telescope. It has good contrast for seeing subtle lunar and planetary details, and for seeking out small planetary nebulas.

It will generally be useful with long focal length refractors, reflectors, or catadioptric scopes only under very good or better seeing conditions. Average seeing will limit its usefulness. Using it with a 2000mm 8” f/10 Schmidt-Cassegrain, for example, will result in 400x (50x per inch of aperture) and a 0.5mm exit pupil (about 1/50th of an inch across). Both of these are very close to the generally-accepted maximum values of magnification per inch of aperture and exit pupil size for usable observing under good to very good seeing conditions for a reasonably experienced observer.

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.

20mm
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.

5mm
Number of optical elements:
8
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"
Weight:
The weight of this product.
14 oz
Warranty:
Limited Lifetime
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1. patrick on 5/15/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I want to love this as much as I love my 20 XW. But I don't. And I think it might be due to the skies in my area and perhaps the scope. The views are not as sharp the 20. If I get a different opinion I will update this, but for now, it's just ok. The quality of the EP is great and the eye relief is nice too. If you have super clean skies, I wouldn't hesitate to try it.
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2. Chris on 4/2/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
The XW series of Pentax eyepieces are my go-to tools for most every observing session. I observe with a f/7 3.5" undriven refractor. The magnification of the 5mm XW is hitting about as high as I like to go in the city. The eye relief is generous (I observe with glasses) and the clarity/quality of the image surpassed anything else I've tried in the same FL. You may upgrade your scope, but this eyepiece will stay with you.
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Telescope Eyepiece Formula
To calculate the magnification and approximate actual field of view of this eyepiece on your telescope, enter the focal length of your telescope below and press the "Calculate" button.

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Pentax - 5mm XW extra wide angle eyepiece

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Pentax - 5mm XW extra wide angle eyepiece
 AstronomicsAstronomicsAstronomicsAstronomicsAstronomics (Average: 4.00 | Users: 2)  Only registered users can submit ratings - Register Here
Our Product #: PXW05
Manufacturer Product #: 70512
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This short focal length Pentax eyepiece has a very wide 70° field of view and very long 20mm eye relief, a uniquely useful combination not matched by other eyepieces . . .





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