Long Eye Relief 3mm 1.25" eyepiece

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This very short focal length Astro-Tech 3mm Long Eye Relief eyepiece is superb for comfortable high magnification lunar and planetary observing with short focal length refractors and fast focal ratio/short focal length reflectors. It has a very flat field that keeps small details sharply in focus from one edge of its 55° field of view to the other. The Astro-Tech’s very high contrast makes it an exceptional choice for splitting close binary stars and globular clusters and for picking out faint low contrast deep space objects, such as the smaller planetary nebulas.

The 3mm focal length of this Astro-Tech Long Eye Relief eyepiece is very probably too short for use with long focal length refractors, reflectors, or catadioptric scopes under anything less than absolutely perfect (and accordingly very, very rare) seeing conditions. For example, using it with a 2000mm 8" f/10 Schmidt-Cassegrain will result in 667x (83x per inch of aperture) and a 0.3mm exit pupil (about 1/75th of an inch across). Both of these far exceed the rule of thumb maximum values of 50-60x per inch of aperture and a 0.5mm exit pupil for usable observing under good to very good seeing conditions, even 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.
55°
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.

3mm
Number of optical elements:
7
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.
8.6 oz.
Warranty:
2 years
Reviews from Cloudy Nights (www.cloudynights.com)
These reviews have been written by astronomers just like you and posted on the Cloudy Nights astronomy forums . . .
Review of Astro-Tech LER Eyepieces

User Ratings/Reviews from our Customers (www.astronomics.com)
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Helpful Formulas
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.

Enter your telescope's focal length: mm   



Astro-Tech - Long Eye Relief 3mm 1.25" eyepiece

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Astro-Tech - Long Eye Relief 3mm 1.25" eyepiece
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The comfortable 20mm of eye relief and outstanding flat field optics of this Astro-Tech 3mm Long Eye Relief eyepiece makes it a super choice for all high power observers, not just eyeglass wearers . . .





. . . our 34th year