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The Celestron X-Cel 5mm is a high powered eyepiece for planetary and Lunar viewing. The 5mm focal length makes for a high magnification whether your using a fast, slow, long or short telescope. Unlike the majority of eyepieces out there Celestron have used 6 lenses (or elements as they're known) in the X-Cel's manufacture.
Most eyepieces use 4 or a maximum of 5 lenses as generally the fewer elements there are in an optical system the clearer and sharper the image will be. The extra elements have not been introduced to increase field of view (as may be expected), the field is only 55 degrees. The reason for the added glass is to increase the objective aperture, to put another way the lens you look through.
At shorter focal lengths (such as 5mm) some eyepieces can feel like you're looking through a pin prick, this can cause headaches and eye strain and is clearly undesirable. The X-Cel doesn't suffer from this problem and is very comfortable to look through. It also features a very generous 20mm of eye relief. Eye relief being the distance one must position ones head away from the lens to see the whole image. Due to the eye relief this eyepiece is suitable for people who wear glasses.
Optically the X-Cel performs well for it's price range (£60). Special low dispersion glass has been used on the most curved lenses to minimise light scatter and other abberations. Each lens element has been fully multi coated to improve light transmission.
If you buy more than one eyepiece from the X-Cel line you will be pleased to hear they are all par focal. That is to say that once you have achieved focus with one you won't need to adjust focus when swapping for another X-Cel eyepiece.
The body of the eyepiece feels strong and durable and features a rugged rubber grip ring to help avoid any accidents (especially common when observing in the freezing cold with un-gloved hands). Another handy feature is that the silver metal barrel has a safety groove cut into it, this stops the eyepiece falling out of the telescope if not properly secured.
The X-Cel is heavier than an equivalently focal lengthed plossl (due to it's extra lens elements) but this shouldn't be enough for any significant balancing issues. Cosmetically this is quite a stylish eyepiece, long and slender with a white, orange and black colour scheme.
Recommended especially for the older observer whos eyesight may not be once what it was. Having said that younger observers can still benefit from the comfort this eyepiece offers. Optically the X-cel is above average, good contrast and a sharpness across 90% of the field means undriven telescopes will be able to let the planet drift through almost the entire field of view before having to reposition the scope to keep the object crisp.
To conclude as optically proficient as the X-cel is, there's no doubt its major selling point is it's oversized objective and generous eye relief. I would give it a 3 out of 5 for it's flat field and decent colour rendition (especially important on a planetary eyepiece) as well as it's mininmal light scatter, but I would give it a 5 out of 5 for comfort, a large and soft rubber eyecup compliments the long eye relief and the benefit of the large objective element should not be under estimated. Earning the X-cel a very respectable 4 out of 5 overall.