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30mm Wide Scan Type III Eyepiece
The popularity of wide-field eyepieces has surged in recent years, and, to a certain extent, so has their cost and complexity. Systems containing more than the usual handful of elements are required to achieve good correction of aberrations over wider fields especially in the shorter f/ratio telescopes we see today. It was with some trepidation that I heard of the new Wide Scan Type III eyepiece. I tried one and decided that, for its cost, it might have a place in my eyepiece box.
The 30mm Wide Scan Type III is a 5-element 2 inch barrel diameter eyepiece similar in size to a larger Erfle. The unit is 11 cm (4.33 inches) long, about 5.7 cm (2.24) inches in diameter, and weighs in at around 455 grams (1 lb). The barrel is a chrome surface which is about 4 cm in length and the standard 2 inches (5.08 cm) in diameter, with a safety groove for set screws in its upper third. The 2 inch barrel is threaded for standard 2 inch filter threads and the interior is grooved to reduced scattered light.
The main section of the eyepiece is a hard black surface with a rubberized knurled base for better gripping in the hand with an eye lens end that has a standoff or fixed “eyecup” to help position the eye.
Optically, the lenses are said to be multi-coated, and from the looks of things, this is true. The coatings had no visible flaws and yielded an overall greenish cast to light reflecting off the lenses. The eye lens is about 3.7 cm across and was slightly concave on its outer surface. The field stop on this eyepiece measures out to 44 mm, and is slightly smaller than the field lens itself, which appeared to be slightly convex. The nature of the outer surfaces of this eyepiece might indicate that it could be a variation on the Erfle design, which also has 5 elements. The apparent field of view is stated to be 84 degrees, and an optical bench measurement showed this to be accurate. The eye relief is difficult to measure (probably in the 11 to 13mm range), but the fixed eyecup above the eye lens prevented me from seeing quite the entire field of view with my glasses on.
I put the eyepiece to the test using my 10 inch f/5.6 Newtonian, my 100mm f/6 refractor, an 8 inch Celestron Schmidt-Cassegrain and a 14 inch Celestron Schmidt-Cass. In larger focal ratios such as the f/10 SCTs, the eyepiece performed fairly well, with fairly sharp star image over much of the field of view. For a more stringent test, in the C14, I slewed Antares from one edge of the field to the other and the eyepiece did start to show a little astigmatism as the star got into the outer portions of the field.
However, it was nice to be able to fit the entire moon in the field of view with that scope, even if the edges weren’t perfectly sharp! The view was fairly high in contrast, so the coatings looked to be doing their job.
However, in shorter f/ratios, the performance did decline in the outer third of the field. The eyepiece yielded a whopping 4.1 degree true field in my 100mm f/6 refractor, but at the cost of some noticable astigmatism. This became most apparent from about 70% of the field radius out to the edge, where star images became short arcs rather than points. It wasn’t quite as bad as the astigmatism I had seen in the 24mm Speers-Waler, but it was present. However, the views of the star fields of the Milky Way were quite stunning even with the aberration present.
There is some pin-cushion distortion with this eyepiece which is noticable if looking at straight lines near the edge of the field. However, it isn’t excessive, and in fact, is comparable to that found in some of my Plossl eyepieces with their much smaller apparent fields of view. There was little chromatic aberration present in this eyepiece (except perhaps towards the edges when the eye is moved out of its normal centered observing position), so it is fairly well corrected in that regard. Still, it looks like this design could be a somewhat “pushed” Erfle, and that the design was stretched perhaps a little too much really shows when used at focal ratios below f/10. Use of a Barlow improved things somewhat, but did not eliminate the astigmatism. I noted no severe kidney beaning or blackout, although you do want to keep your eye well-centered on the eyepiece and right up to the hard eyecup “guide” if you want to see the entire field. Compared to things like the Meade 14mm Ultrawide or the TeleVue 31mm Nagler, this eyepiece’s performance wasn’t exactly stellar, but it didn’t cost nearly as much as those two eyepieces either. If users want tack-sharp star images across an 80+ degree apparent field, they should be ready to pay the big bucks for a more complex eyepiece design that delivers that kind of quality.
In summary, the 30mm WIDE SCAN III eyepiece is not in the same league as the more expensive wide-field designs, but it does offer some interesting wide-field performance which some people might find useful, especially in an eyepiece that costs less than $250.