The range of Celestron’s computerised GoTo telescopes are so very easy to set up and use, but for accurate pointing they do require aligning on some celestial “alignment targets” at the beginning of your astronomy session. A common question that is asked is what can be used to help centre alignment stars in the telescope? Celestron offer their CrossAim eyepiece to help with this.
This fully multi-coated optic plossl eyepiece has a 40-degree apparent field of view and has a 12.5mm focal length. You can easily determine what magnification it will have with your telescope by dividing the focal length of your telescope (in mm) by 12.5.
The eyepiece has an external red LED illuminator (to help keep your dark adapted vision), which allows the double cross hair reticle (see later) to be clearly seen through the eyepiece when it is used in darkness, under the night sky. The illuminator simply attaches to the eyepiece by screwing the illuminator unit into the threaded side opening on the eyepiece.
The red LED can be switched on and off, and its brightness varied (varying the brightness of the reticle crosshairs to a suitable level) using the rotary knob located on the top of the illuminator unit.
The eyepiece has an adjustable focus ring located at the top (i.e. nearest where your eye will be placed). This allows you to adjust the internal optics so that the cross-hair reticle (see below) is in focus for your eye. The focus ring has a knurled pattern on the edge for improved grip.
When looking through the eyepiece what do you see?
You will see the reticle which, for the CrossAim, is a double crosshair type. A double crosshair reticle is a pair of closely spaced horizontal and vertical “lines”. Where these lines intersect “creates” a small square “box” which allows you to centre a target (e.g. star) or keep a guide star centred. The image below shows an image taken through a CrossAim eyepiece using a hand-held mobile phone up against the eyepiece lens (see notes below image).
With the red LED illuminator switched on the reticle lines appear to “glow” red.
In use, the CrossAim eyepiece simply goes into where you would use a “normal” eyepiece for observing and is swapped out once you have finished using it. In addition to help alignment process, the eyepiece can also be used for other purposes too such as polar alignment or for centring guide stars.
This useful eyepiece is available from your local/favourite astronomy retailer our ourselves (>>here<<)