CrossAim 12.5mm dual crosshair Plössl

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This Celestron “CrossAim” 12.5mm illuminated reticle eyepiece has dual crosshairs for use with any make or model telescope and/or off-axis guider. In addition to manually guiding long exposure photographs, it can be used for aligning go-to telescopes on the sky each night when you go out to observe, 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.

The 1.25" Celestron 4-element Plössl guiding eyepiece has a 40° apparent field of view and a double line crosshair reticle. For guiding, this reticle pattern allows you to position the guide star in the middle square of the double crosshairs or at the intersection of two lines, whichever is easier for you. The crosshairs can be focused to match your individual eyesight.

The 12.5mm focal length of this eyepiece provides reasonable guiding magnifications with most telescopes. The optics are fully coated for increased contrast and good light transmission on faint stars. The 10mm eye relief is substantially longer than competitive Plössl guiding eyepieces. The 10mm eye relief will vignette a portion of the field for those who must wear glasses while observing. It is not a debilitating drawback, however, as the central crosshair box remains visible even with glasses and the crosshair focusing mechanism has enough range to allow most observers to remove their glasses and still focus sharply on the crosshairs.

The supplied cordless illuminator has a rotary on/off switch with an integral brightness level control. It uses three 1.5 volt L1131C button-type alkaline watch batteries or their equivalent (189, LR54, AG10, etc.)

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.

For guiding tolerance purposes with your particular scope, you can calculate the area of sky subtended by the small central box formed by the dual crosshairs by aiming at a star near the celestial equator. Turn off the scope drive and count how many seconds it takes for the star to drift from one side of the box to the other. One second of time equals 15 arc seconds of sky.

Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
40°
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.

10mm
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.

12.5mm
Weight:
The weight of this product.
4.5 oz.
Warranty:
2 years
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Celestron - CrossAim 12.5mm dual crosshair Plössl

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Celestron - CrossAim 12.5mm dual crosshair Plössl
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Our Product #: 4310
Manufacturer Product #: 93235
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This 12.5mm Celestron “CrossAim” 1.25" Plössl design double crosshair reticle guiding/centering eyepiece comes with a variable brightness cordless illuminator for use with any make or model telescope and/or off-axis guider . . .





. . . our 34th year