Looking for an eyepiece with the widest field of view possible on a 1.25-inch focuser? Here are three contenders for your consideration.
Plus: The Explore Scientific 68° Series provides much of the optical performance of the TeleVue Panoptic at much less cost.
Minus: The Celestron Ultima Edge ranked third in several key areas of performance, yet costs more than the Explore Scientific 68-degree Series.
Summary: If it’s a low-power, widest-field eyepiece you are after, consider the 24mm Explore Scientific from their 68° Series.
Who Are They For? Anyone willing to spend good money on a premium eyepiece that will serve you well for a lifetime of viewing.
If you have a telescope that accepts only 1.25-inch eyepieces, a 24mm eyepiece with a wide 65-degree apparent field of view is a good choice, and worth spending premium dollars on. Such an eyepiece yields the widest actual field of view possible with a 1.25-inch focuser.
Yes, you can buy 35mm to 40mm eyepieces that provide lower power. However, their apparent fields of view are much less, typically only 45 to 50 degrees, a limit imposed by the physical diameter of the 1.25-inch barrel. So how much sky they show is no more, and may well be less, than what a 24mm 65-degree field eyepiece provides. While such an eyepiece commands a higher price than the usual economy Plössls, it’s a model you’ll use for a lifetime, as it is ideal for low-power sweeps of Milky Way starfields and for panoramic views of big star clusters and nebulae.
Achieving even wider fields of view requires using a telescope equipped with 2-inch focuser and buying a 65-degree to 84-degree eyepiece with a 2-inch barrel and a focal length 27mm or longer. Now we are talking serious money!
The 24mm Trio
For this comparison I tested three 24mm eyepieces with 1.25-inch barrels: the trend-setting 68-degree Panoptic from TeleVue, an Explore Scientific from their 68° Series, and the new 8-element Ultima Edge from Celestron. The latter has an apparent field of 65 degrees, a little less than its competitors. Meade’s new 24mm Series 5000 UHD has specs similar to the Ultima Edge, but is not tested here.
I compared the trio of 24s on a 94mm f/5.4 SharpStar refractor, on an 8-inch f/6 Sky-Watcher Dobsonian reflector, and on a 6-inch f/10 Celestron Schmidt-Cassegrain, by looking at the Moon, bright stars, and starfields such as the Coma Berenices star cluster.
In all telescopes, the TeleVue 24mm Panoptic provided the sharpest images right across the entire field, a trait the “Pans” are famous for. Even at the very edge of the field stars remained pinpoint in all three test scopes.
The Explore Scientific 24mm came close to matching the Panoptic, with stars beginning to soften and enlarge in the outer 10 percent of the field, with off-axis aberrations remaining well contained at the edge.
The Ultima Edge was No. 3, with stars bloating in the outer 20 percent of the field and becoming noticeably flared by astigmatism at the edge, though much less so in the slower f/10 Schmidt-Cassegrain than in the faster refractor and Newtonian reflector. However, lateral color was well controlled; stars did not flare into colorful streaks.
All the eyepieces were also well controlled for flare and ghost images, even when aimed just away from the Moon or bright stars.
The Celestron Ultima is promoted as having an “ultra flat field,” a term which can be ambiguous. While stars were not as sharp across the field as in the competitors, what the Ultima did exhibit was a very low level of distortion, a specific optical trait.
When panning the Moon across the field, for example, its disk remained circular even at the edges of the field. Likewise, star fields did not warp as the telescope was scanned back and forth. Magnification was constant across the Ultima’s field.
By comparison, both the Explore Scientific and TeleVue showed a similar and sizeable degree of pincushion distortion. This made the Moon and star fields appear to bow out at the field edges, as if they were projected onto the inside of bowl. The Moon’s disk turned egg-shaped at the field edge. How much this might bother observers is debatable. It’s likely a design tradeoff required to achieve their sharper edge performance, which I feel is a more critical factor.
So yes, the Ultima Edge is “flatter” than the competition, in having low distortion, but not in having sharper images edge to edge.
Field of View
The Explore Scientific and TeleVue each have advertised apparent fields of 68 degrees and did prove identical for actual field. The Celestron Ultima, with its 65-degree apparent field, showed a little less sky, in a field that looked more constricted than the other two. The three-degree difference was noticeable.
More critically I found that in all the telescopes the Ultima’s field edge looked a little dark and vignetted, making the field stop appear somewhat soft and indistinct.
By comparison, in the other two eyepieces, their field was evenly illuminated right to the edge, with a sharply defined field stop, despite their wider fields and smaller physical size.
The Panoptic had the shortest eye relief, at 10mm from the top of the eyecup. However, viewing the entire field with the eyecup up was still comfortable enough. Rolling the eyecup down did allow viewing the entire field with eyeglasses on, but only just. The Panoptic is an older design; TeleVue’s newer 72 degree Delos and 62 degree DeLite eyepieces provide more generous eye relief, but lack a “lowest-power” 24mm model in their series.
The Explore Scientific’s longer 13mm of eye relief made it more comfortable to look through than the Pan, both with and without eyeglasses.
The Ultima Edge had by far the longest eye relief, which I measured to be 15mm from the top of the rolled-down eyecup, or 25mm from the deeply recessed eye lens. Even with the soft eyecup rolled up, the entire field was still visible with eyeglasses on. With eyeglasses off, the eye relief is almost too long; you have to hover your eye above the eyecup to get the best view.
However, the Ultima Edge would be an excellent eyepiece for public outreach, as it is easy to look through with and without eyeglasses, and its recessed eye lens is unlikely to get smeared.
With a main body 57mm across and 70mm tall, the Celestron Ultima Edge is physically the largest of the trio, but not so large or heavy that it won’t work well on any telescope.
The exception is on binocular viewers, where the wide bodies of paired Ultimas might collide when setting the interocular distance for people with closely set eyes.
However, even in single-eyepiece use, I found the wide and flat bottom surface of the Ultima 24’s body can hit the lock screws on star diagonals. It was difficult to adjust the screws, let alone just reach them, as they were covered by the eyepiece.
A rule of reviewing I usually follow is to not be concerned with price. It is what it is, and prospective buyers can choose to pay it or not based on the observed level of performance vs what was promised by the manufacturer.
With this eyepiece trio, though, I think it is fair to point out that, at $190 from most dealers, the 24mm Explore Scientific provides the best value — the performance vs price ratio.
If you want the best optics, buy TeleVue, though you will pay the most. That should come as no surprise.
With Celestron’s Ultima Edge costing $220, notably more than the Explore Scientific 68°, yet providing lesser performance in several key areas, it ranks the lowest of the trio in my recommendations.
Typical retail prices:
Celestron 24mm Ultima Edge: $220
Explore Scientific 24mm 68° Series: $190
TeleVue 24mm Panoptic: $320