A zoom eyepiece is an eyepiece that combines a wide range of different focal lengths into one single eyepiece unit. These “zooms” (as they are often referred to) are sometimes overlooked in favour of having a batch of single individual fixed focal length eyepieces, as fixed focal length eyepieces offer higher optical quality. However zoom eyepieces can still be very good optically, and can be very useful and convenient under many circumstances.
A benefit of a zoom is that you do not have to carry multiple (between 3-5 or so) fixed focal length eyepieces around with you. Next, imagine that the sky has just cleared but there’s another cloud bank fast approaching. This may mean you have just about enough time to get your telescope out for a very quick peek at the sky but don’t want to waste precious time by bringing out many different eyepieces. A zoom can be very convenient and a time saver. Another example that comes to mind is if you have friends and family over (or you attend e.g. a star party) and you want to be able to show many people the surface of the Moon (usually a very popular target!). Being able to zoom in and out, with a simple twist of the eyepiece, is much easier and convenient than keep swapping individual eyepieces in and out of the telescope. Its also actually quite impressive and dramatic for those looking through the telescope to be able to see themselves “falling into” and “zooming away” from the Moon’s surface too! Finally, if you are at a star party with many people you may not want to take along your more expensive eyepieces, so a low cost quality zoom eyepiece covering a wide range of focal lengths could be ideal!
Celestron offer a high quality 1.25″ zoom eyepiece with multi-coated optics. The zoom eyepiece “starts off” with a (low magnification) focal length of 24mm and an apparent field of view of 60° with 18mm of eye relief. The eyepiece has a (high magnification) focal length of 8mm, an apparent field of view of 40° with an eye relief of 15mm. The eyepiece is ideal for both Lunar and planetary observing as well as terrestrial use too. Markers are labelled on the eyepiece barrel at 24mm, 18mm, 12mm and 8mm to allow you to see what focal length your eyepiece is set to. To find out what magnification you will get with such an eyepiece, divide the focal length of your telescope (in mm) by the focal length you are at. For example with a Celestron 8″ SCT with a 2032mm focal length, the zoom eyepiece will give ~85x at 24mm and 254x at 8mm. The zoom fits into a telescope focuser or a diagonal just like a “normal” fixed focal length eyepiece.
The eyepiece has a wide textured rubber twist-barrel (for “zooming”) for improved grip (especially if you are using gloves), It has a fold back rubber eyeguard for viewing with or without glasses. The 1.25″ barrel has internal threads to allow the use of filters (e.g. a Moon filter).