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.
