Apertura’s SWA eyepiece series is a fairly economical wide-angle eyepiece line that is ideal for use in telescopes with focal ratios of f/5-f/6 and slower. The Apertura SWAs are identical to the SWA eyepieces offered by Agena Astro, Orion, Omegon, and a few others, though not all of the other brand variations of these eyepieces have the Aperturas’ twist-up eyecups.
The Apertura SWAs are available in 2” and 1.25” formats. The 2” eyepieces come at focal lengths of 38mm, 32mm, and 26mm, while the 1.25” SWAs are available in 10mm, 15mm, and 20mm focal lengths.
The Apertura SWAs are, more or less, Erfle eyepieces. This is the same design that the GSO SuperView eyepieces and the “goldline” or “redline” eyepieces use – though the 9mm and 6mm goldline/redline use an internal Barlow lens to increase eye relief and improve edge-of-field correction. Most other “wide angle” and “SWA” type eyepieces are Erfles or some variation on it.
The Erfle design dates back to the 1910s, being patented in 1921, and can be seen as sort of an incremental improvement on the Plossl and Kellner. It uses a cemented doublet field lens with a concave surface on the outside and a convex interior surface, a middle lens which is convex on both sides and an eye lens with a convex side towards the interior and a flat surface facing the observer’s eye. The Erfle design was intended to improve the apparent field of view of binoculars and spotting scopes during World War I and many binoculars made during or after World War II use Erfle eyepieces to achieve a wide apparent field of view. The Erfle is optimized for a 60-degree apparent field but some manufacturers have stretched it as far as 85 degrees; however it generally performs extremely poorly towards the edges of the field if it is not limited to 70 degrees or less.
The Erfle design works best at focal ratios of f/8 or above. The Apertura SWAs stretch the eyepiece design’s apparent field to 70 degrees, which of course means the outer edges are not as well-corrected. The main issue as you approach the edges of the field of view is astigmatism, which manifests as cross or line-shaped stars and is often confused with coma. It is caused by the way the Erfle design bends light entering at an angle. The more severe the angle (i.e. the wider the field lens or faster f/ratio) the worse this is. Thus, the longer the focal length of the eyepiece, the worse the astigmatism gets.
The Apertura SWAs show noticeable astigmatism to some degree in telescopes faster than about f/8 and it becomes seriously detrimental to the view at f/6, while the shorter focal lengths don’t start to have issues at all unless you use a telescope with a focal ratio of f/6 or faster and they still work acceptably at f/4.5 or so. Based on a poll of 10 observers, we can conclude the SWAs perform best with regards to edge-of-field astigmatism as follows:
|“Tack sharp”||Above f/10||Above f/10||f/9 or above||f/8 or above||f/7 or above||f/6 or above|
In short, these eyepieces are inevitably compromised in any fast Dobsonian. However, it should be noted that coma is going to be worse than the SWAs’ edge-of-field astigmatism at f/5 or below anyway, which means that without a coma corrector a nicer eyepiece isn’t going to be of much benefit. The 38mm SWA also produces too big of an exit pupil to be used in a scope faster than f/6 given its focal length.
If you have a refractor, SCT, Maksutov, or unusually slow Newtonian, you will get pretty much pinpoint sharp views with any of the Apertura SWA focal lengths available. If you have a fast scope, the Apertura SWAs’ astigmatism can usually be at least somewhat tolerated.
The other performance aspects of the Apertura SWAs are very good. There is some lateral chromatic aberration but you are unlikely to notice it except with the 10mm and maybe 15mm when viewing the Moon and planets. The blackened lens edges, baffles, and multi-coatings do a good job minimizing glare and internal reflections. There is some field curvature in all of these eyepieces owing to the Erfle design but you may or may not notice it unless you are using a refractor.
With the exception of the 10mm, the Apertura SWAs all have enough eye relief to be pretty comfortable to use, and the twist-up eyecups can be set to whatever height you find comfortable. However, the 10mm has a mere 10mm of eye relief, much like a Plossl or Kellner of this focal length, and is rather uncomfortable to use as a result.
Which To Buy?
The 38mm Apertura SWA maxes out the achievable true field of view in a 2” barrel format – though the focal length and edge-of-field astigmatism ruin its viability with a fast scope below f/6. If you have a typical 6” f/8 or 8” f/6 Dobsonian, a longer f/ratio refractor that takes 2” eyepieces, or a catadioptric, this eyepiece is perfect for low-power viewing. The 32mm provides a little more power and is okay if you have a 10” or 12” Dob, for instance, though its edge-of-field correction is going to of course suffer in a fast scope. The 26mm SWA is kind of redundant, given that many scopes are supplied with a 25mm or 26mm Plossl, and the 32mm or 38mm gives you a wider field. The 20mm and 15mm Apertura SWAs are good and perform similarly to a redline or goldline eyepiece but with a comfier eye guard and slightly better contrast owing to better quality control and blackened lens edges. We would not particularly recommend the 10mm unit owing to its short eye relief and lateral chromatic aberration.
In addition to the various clones from other brands such as Agena Astro, SVBONY, Orion, and Omegon, there are a few other eyepiece lines we recommend which are similar to or better than the Apertura SWAs in performance.
SWA-Type (60-72 degree AFOV)
- The GSO SuperView eyepieces perform similarly to the SWAs but with slightly different focal length options.
- The Baader Hyperion eyepieces provide more eye relief at shorter focal lengths than the SWAs and are slightly better in fast f/ratio scopes, with a 68-degree apparent field of view.
- The APM Ultra-Flat Field (also sold as Celestron Ultima Edge and Orion UFF) eyepieces provide a sharper field than the SWAs – albeit only 65 degrees across.
- The Explore Scientific 68-degree and Tele-Vue Panoptic eyepieces (which share the same optical design) are tack-sharp even in fast f/ratio telescopes and feature a 68-degree apparent field of view.
UWA-Type (82-degree AFOV)
- The Meade PWA and Astro-Tech UWA eyepieces are well-corrected even in fairly fast telescopes though they do have minor field curvature and lateral chromatic aberration.
- The Tele-Vue Nagler and Explore Scientific 82-degree eyepieces are extremely sharp eyepieces, typically with good eye relief and little in the way of edge-of-field aberrations.