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Celestron computerised GoTo telescopes are very easy to set up and use, but for accurate pointing they do need aligning on some 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.
CrossAim eyepiece and illuminator. The threaded opening on the eyepiece can be seen where the illuminator is attached.
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.
Illuminator on/off and brightness control knob.
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.
Image above shows the illuminator fully attached to the eyepiece and the focus ring in a (raised) position that gives a sharp view of the double crosshair reticle.
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).
Image shows view through the CrossAim clearly showing the double cross hair reticle. Due to hand-holding the mobile phone camera some parts of the image are slightly out of focus - but the reticle appears all in sharp focus when viewing.
With the red LED illuminator switched on the reticle lines appear to "glow" red.
With the illuminator switched on the reticle lines can be seen.
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.
CrossAim eyepiece in use with an EdgeHD telescope helping to centre alignment stars.
This useful eyepiece is available from your local/favourite astronomy retailer (https://celestron.uk.com/dealers.php).