24mm illuminated centering eyepiece

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This Astro-Tech #AT24IL illuminated 1.25” centering eyepiece is the perfect eyepiece for aligning go-to telescopes on the sky each night when you go out to observe. It is also ideal for checking your scope’s polar alignment for long exposure astrophotography, for centering guide stars in an off-axis photoguide scope for astrophotography, or for testing the accuracy of your computer pointing or the periodic error of your tracking system. Priced as much as 40% less than competitive centering eyepieces, the #AT24IL is yet another example of the Astro-Tech philosophy of “more performance for less money.”

Most conventional illuminated reticle eyepieces have a short focal length and limited eye relief, which can make for uncomfortable viewing, particularly if you wear eyeglasses. In comparison, the Astro-Tech #AT24IL has a long 24mm focal length for a wider field that makes it easier to locate guide stars. This virtually eliminates the need to swap out a high-power illuminated reticle with a lower power eyepiece to locate or center objects.

Its very long 20mm eye relief and soft roll-down rubber eyecup make it very comfortable to use, whether you wear glasses to observe or not. You can use this Astro-Tech centering eyepiece with any make or model of telescope or off-axis guidescope.

The eyepiece reticle has a single etched crosshair precisely centered in eyepiece field of view for accurate results. A ribbed ring at the top of the eyepiece rotates to focus the crosshairs sharply to match your eyesight. The five-element optical system is fully multi-coated on all air-to-glass surfaces with high transmission anti-reflection coatings for bright, crisp views. The edges of the lenses are blackened for high contrast. The barrel is threaded for standard 1.25” filters.

The metal-body cordless red LED (light emitting diode) crosshair illuminator has a rotary on/off switch with an integral brightness level control. Two supplied 1.5V watch/camera/calculator button batteries will power the illuminator for many hours of use. The supplied batteries are model GPA76, but you can also use the Duracell, Maxell, National, Panasonic, Sony, or Toshiba LR44; the Eveready or Philips A76; etc. When replacing batteries, be sure the battery – signs face the LED portion of the illuminator and the + signs face the on/off switch.

Please note: While the manufacturer normally supplies batteries at no cost with this product as a convenience, the battery charge or useful life is not guaranteed. The batteries may arrive with their charge depleted or very low. In such a case, you will have to purchase replacement batteries locally, as battery life is not covered by the manufacturer’s warranty.

Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
50°
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.

24mm
Weight:
The weight of this product.
4 oz.
Warranty:
2 years
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Astro-Tech - 24mm illuminated centering eyepiece

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Astro-Tech - 24mm illuminated centering eyepieceClose-up of the illuminator of the Astro-Tech #AT24IL.The Astro-Tech #AT24IL illuminator open, showing the batteries.
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Our Product #: AT24IL
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The economical Astro-Tech #AT24IL illuminated 1.25” centering eyepiece helps you align your go-to telescope on the sky, check your polar alignment for astrophotography, center guide stars in an photoguide scope, and more . . .





. . . our 34th year