Celestron 8-24mm Zoom Eyepiece
By Chuck Hawks and Robert Fleck
This eyepiece is Celestron's entry in the modern zoom eyepiece sweepstakes. Celestron is a major telescope manufacturer and a major player in eyepieces, with their highly successful Omni, X-Cel, Ultima LX and Axiom LX lines of fixed focal length accessory oculars. However, the Celestron zoom ocular is classed as a specialty eyepiece and is not officially a part of any of those established lines.
Variable focal length (zoom) lenses are standard equipment on most cameras these days, zoom riflescopes have practically taken over that branch of optics and zoom binoculars are proliferating, so it is not surprising that zoom oculars for astronomical telescopes are becoming increasingly popular. Most major ocular brands now offer one or more mid-range zoom eyepieces in the 7-21mm or 8-24mm range. These are 3:1 zoom ranges, considered the most practical range for high quality zoom optics at the current state of the art.
The Celestron zoom ocular is fairly large, but not excessively so. In physical size, it is similar to competitive offerings from Meade, Vixen and Tele Vue. (Those oculars have previously been reviewed by Astronomy and Photography Online.) This zoom is manufactured in Red China by Synta Technology, Celestron's partner and corporate owner.
The external finish is matte black with a wide, checkered, rubberized griping area around the zoom ring. There is a rather stiff rubber eyecup approximately ¼" deep at the top of the ocular, similar to those provided on X-Cel oculars. Beneath this eyecup are T-threads, which makes eyepiece projection photography with (D)SLR's simple. This is particularly handy for use with Newtonian reflectors, as they are typically backfocus challenged. In external shape, diameter and general styling, the Celestron zoom strongly resembles the X-Cel oculars, but lacks their ED glass element and trademark orange band above the grip ring.
"Celestron" and "8-24mm Zoom" are screen printed around the lower part of the zoom ocular. 8mm, 12mm, 18mm and 24mm are printed along the lower edge of the zoom ring to indicate the focal length settings; these numbers are printed upside down, presumably for viewing from the top, which is practically impossible at night. A triangular index mark on the non-rotating part of the lens barrel beneath this row of numbers indicates the focal length selected.
The 1.25" diameter mounting barrel is chrome plated and incorporates a wide safety grove. All air to glass lens surfaces are fully multi-coated. The inside of the eyepiece is matte black to minimize internal reflections and the mounting barrel is threaded to accept filters.
Celestron's optical design uses four elements in three groups. The Celestron zoom's physical weight, at 7.9 ounces, is slightly heavier than the Meade and Tele Vue zooms.
The Celestron zoom is in the somewhat unusual position, for a Celestron product, of being the least expensive of the popular zoom oculars. No doubt this is due to its simpler optical design. The typical online discount price, as these words are written, of the premium Tele Vue 8-24mm Click Stop Zoom is $210-$240 and the Vixen 8-24mm Click Stop Zoom is about $220. The Orion Zoom! 7.2-21.5mm is $160, the Meade Series 4000 8-24mm Zoom runs $125-$130, the Orion 7-21mm Explorer II sells for $68 and the Celestron 8-24mm Zoom is only $60-$65. If price is any guide, it tends to indicate that the Celestron zoom is either grossly inferior or a heck of a good deal. We were understandably curious to see for ourselves which description applies. First, here are some specifications for the Celestron 8-24mm zoom ocular:
We found the Celestron zoom to be virtually parfocal throughout the zoom range with only minor focus adjustments necessary, providing that you focus at 8mm and zoom out. The eye guard, folded in the photo at the top of this page, is deep enough to block out most stray ambient light. We found that it wanted to be erect and refused to stay folded down for long, which can be mildly irritating.
You can easily hear the zoom helicoids moving the inner tube when you turn the large zoom ring, which occupies the entire middle area of the eyepiece body. This Celestron is not as smooth or quiet when zooming as the previously reviewed Tele Vue Click Stop Zoom, but to its credit, it turns with less resistance, so you are less likely to twist the ocular in your star diagonal when zooming.
There is a noticeable loss of sharpness as you look toward the edge of the field of view at all focal length settings. This seems typical of zoom oculars, as we noticed the same thing with the Meade and Tele Vue zooms. Like all zoom oculars, the apparent field of view (AFOV) decreases as the focal length increases. A generous 60-degree AFOV diminishes to a stingy 40-degrees at 24mm.
To evaluate its relative performance we viewed a number of objects, both astronomical and terrestrial, using a Stellarvue SV115T telescope. We compared the Celestron zoom to a premium Tele Vue Click Stop Zoom and also to a 12mm Tele Vue Radian fixed focal length eyepiece. The 12mm Radian seemed to have a slight advantage in contrast and resolution over the zoom oculars (set at 12mm) and it is sharper at the edge of the field of view, but these differences are subtle, not dramatic. Most observers would probably be hard pressed to see the difference in center sharpness between the Radian and the zoom oculars.
Comparing the two zooms, it was difficult for us to see any appreciable visual difference. In every case, each detail we could see with one zoom, we could see with the other. This was true at all focal length settings. The Celestron's simpler design should result in more lateral color error than the TeleVue zoom, but the terrestrial and deep sky objects that we were able to observe were not capable of revealing this and the moon was in its "new" (absent) phase. The Tele Vue zoom operates more smoothly, as mentioned previously, and its click stops at the 8mm, 12mm, 16mm and 24mm settings are a considerable convenience, particularly at night when it is very difficult to read the focal length settings printed on the Celestron zoom. The Tele Vue also seemed to have a larger useable area of center sharpness. When viewing terrestrial subjects, the color rendition of the two oculars is subtly different, but we were unable to decide which is objectively more accurate. Both are entirely satisfactory. At night, of course, it simply does not matter.
Our subjective impression is that the Tele Vue Click Stop Zoom is superior mechanically and is probably a hair better optically, but objectively it is difficult to quantify that impression without recourse to an optical bench. Certainly, at its modest retail price, the Celestron 8-24mm zoom eyepiece is a bargain. If you don't already own one of these modern 8-24mm zoom oculars, you should order yours today!
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