This Meade zoom eyepiece is fully multicoated, with an 8mm to 24mm focal length range and a soft rolldown rubber eyecup. Focal lengths are marked in 1mm increments for repeatable power settings (for example when two eyepieces are used in a binocular viewer). It uses a lanthanum rare earth glass field lens for a long eye relief that’s suitable for virtually unvignetted use with eyeglasses.
As with almost all zooms, the field of view of this eyepiece decreases as the focal length increases. This results in a narrower apparent field of view at low powers and a wider apparent field at high powers than you can achieve with similar focal length single magnification eyepieces. This is just the reverse of what most people expect from eyepieces – which is a wider field at lower powers, where you’re usually observing larger objects that don’t need as much magnification to fit in the eyepiece field.
Of course, when you factor in the increasing magnification as the focal length decreases, the actual field of view still gets narrower as the power increases. However, a single magnification eyepiece in the 24mm focal length range will typically have an actual field of view that’s 25% wider than the zoom set at the same focal length. At an 8mm focal length, the zoom will typically have a field 10% wider than a single magnification eyepiece of the same focal length.
Likewise, the usable eye relief changes as the focal length is changed, in this case decreasing somewhat in the middle focal lengths and increasing to a maximum at the focal length extremes. Contrast is also generally a little lower than you’ll find in single magnification eyepieces of similar focal lengths. If these minor anomalies are not a problem for you, a zoom offers the convenient ability to precisely match the observing magnification to the seeing conditions, as well as offering a virtually infinite number of magnifications in a single reasonably-priced eyepiece.
The specifications given to the left are for the zoom set at its longest focal length of 24mm. With a 2000mm focal length telescope, such as an 8” f/10 Schmidt-Cassegrain, the 40° apparent field at 24mm will result in an actual field of view that’s 0.48° across.
At the 16mm focal length, this zoom has a 42° apparent field (giving a 0.34° actual field with the same 8” f/10 Schmidt-Cassegrain) and 12mm of eye relief.
At a 12mm focal length, the apparent field is 45° (a 0.27° actual field with the same 8” f/10 Schmidt-Cassegrain) and the eye relief is again 12mm.
At an 8mm focal length, the zoom’s apparent field increases to 55° (a 0.22° actual field with the same 8” f/10 Schmidt-Cassegrain), with 15mm of eye relief.
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