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William Optics Swan Eyepiece- 20 mm

Posted by Chad Moore   03/05/2005 12:00AM

William Optics Swan Eyepiece- 20 mm
[ARTICLEIMGR="1"]I suppose if money were no object, eyepiece reviews would be relegated to a minor forum topic. We’d all go out and buy the fancy eyepieces with the green lettering and that would be that. Or perhaps we could split hairs with our eyepiece of choice, Tak LE, Nagler, Leica and the like over breakfast after a great night of observing. But for many of us, settling in on an eyepiece collection is a decade long affair. Mine actually started two decades ago in high school as I tucked away money from an after school bike mechanic job to purchase a Meade UltraWide eyepiece.

I’ve been impressed with my William Optics diagonal so I was eager to try the William Optics Swan eyepieces when they surfaced on the Anacortes web site. I ordered the 20mm for 78 dollars which gives me a 2.5 exit pupil on my 5” refractor; a wide enough field for sweeping and starhopping, but enough magnification to discern small galaxies.

The Swan in Daylight
The eyepiece is surprisingly lightweight for a wide field eyepiece. The specs say 72 degrees. The barrel is aluminum, not brass, and has the typical WO undercut. The anodized barrel has no scratch marks after three nights of hard use. Barrel fit is a bit loser than a Televue or Ultima eyepiece, but better than most. Overall, very nice. I partly disassembled the eyepiece, revealing that it is constructed on par with Japanese eyepieces such as the Celestron Ultimas, but better than an Orion Sirius eyepiece or other Chinese makes. The lens edges do not appear to be blackened. It has a reasonably comfortable fold down eyeguard and a nice rubber grip for cold weather glove use. The surface reflection seems well controlled with the typical green multicoatings.

Performance in a 5” F8 Refractor
The first clear night after the box came in, my girlfriend and I set up the Burgess achromat refractor in the driveway and tried out the new eyepiece. I was pleasantly surprised when I brought the beehive cluster into focus. It was not as breathtaking as a Panoptic, but at one third the cost it looked just fine. It is a nice focal length in combination with this telescope as I rarely had to switch to a lower power eyepiece (a 30 mm BW Optik) or a higher one (a 12.5 mm Ultima). The eye relief was comfortable, and I can roughly confirm the manufacturer published 17 mm measure. It was easy to center the eye and get a crisp field stop, and there was no kidney bean effect.

We looked at a variety of objects ending with Saturn, comparing the 20 mm Swan to a 30 mm and 18 mm Celestron Ultima, a 25 mm Orion Sirius, and a 18 mm Televue Radian I borrowed. It doesn’t take long to notice that the star images soften toward the edges. I estimated the onset of softening at 50% field diameter. It gradually increases toward the edge, but despite this shortcoming it did not detract from the overall view. So while the overall field of view is 1.4 degrees, the sharply focused area was only 0.7 degrees. Using the same subjective criteria on the 25 mm Sirius yielded 75% sharp field or about 0.9 degrees of sharply focused stars. Examining the high quality Celestron 18 mm Ultima yields about a 90% sharp field, or 0.85 degrees. The 18 mm Televue Radian was sharp to the very edge, yielding 1.1 degrees of sweet spot. So this eyepiece design does sacrifice the angular size of the sharpest center for the inclusion of a larger overall field. In actual use, though, this was not a handicap. The eyepiece has a nice spacewalk impression and the deficiencies are sub-clinical. In other words, you really don’t notice unless you are looking for them.

On stellar objects, on-axis sharpness was almost identical to the Ultimas and after careful inspection, not quite as good as the Radians. The seeing has been below average lately, but Saturn still provided a good test. I used a Baader Neodymium filter to improve contrast and reduce fringing for planetary viewing. Here is where the Swan fell noticeably behind. The 18 Radian and 18 Ultima clearly showed more detail on the ringed planet, even allowing for their slightly higher magnification. There was more light scattering in the Swan making the Casini Division a challenge, perhaps due to the un-blackened lens edges. I repeated the test with each eyepiece in line with a 2x Ultima barlow, with the same results. My companion backed up these observations. Not a big difference, but noticeable. Comparing the 20mm Swan to a 25 mm Sirius isn’t really a fair comparison, but they did seem on par with one another for planetary viewing. On a positive note, barlowing the 20 mm Swan did improve its off-axis sharpness. Such a combination may be a good selection for globular clusters, but I would stop short of recommending it for planetary viewing.

I also looked at light transmission. This is hard to judge in the field, and I had to make some “field adjustments” to compensate for the different magnification between eyepieces. The best I can say is that the light transmission is very good, with all eyepiece types performing very close together.

Performance in a 6” F5 Reflector
My second telescope is a Celestron rich field Newtonian with good optics. It only has a 1.25 inch focuser, so the 20 mm Swan could potentially be a workhorse eyepiece for this telescope. Unfortunately, the eyepiece was a disappointment in a short focal ratio telescope. The softness towards the edge was exaggerated, with the sweet spot only extending 30% out from center. Furthermore, the stars blossomed into blurry V shapes at the very edge. The blurring reduced the visibility of fainter stars, diminishing the visual impression of the night sky.

I then needed to separate out the difference between the Newtonian’s inherent coma and the effect of the eyepiece. Using a 30 mm Ultima with 52 degree apparent field produces an image with almost the same angular field as the 20 mm Swan with 72 degree apparent field. Swapping eyepiece back and forth, it was clear that the Ultima had far less coma at the same off-axis point than the Swan. Both the 30 mm and 18 mm Ultimas showed softening of star images at about 50% diameter, but only gradually so until the last 10%. The 18 mm Radian was better than all of them, showing even less coma, albeit the true field wasn’t wide enough to really show much coma anyway. I spent some time observing with the Swan, but was not satisfied with the views. Switching back to the Ultimas was refreshing in the F5 instrument, despite their much smaller apparent field of view.

Discussion
Wide field eyepieces are more than a luxury. They allow higher magnification while still adequately encompassing an object, thereby darkening the sky. It is logical that you can get away with less eyepieces in a complete if you have wide field eyepiece designs, somewhat justifying the extra cost. And finally, they provide a much more immersive viewing experience. My personal impression is that there is a big difference between the 60 degrees of a Radian eyepiece and the 52 degrees of a Plossl or Super Plossl.

For users with longer focal ratio telescopes, say F7 and above, it would be interesting to make additional comparisons with the WideScan series of eyepieces, the Antares W70 series, and perhaps even the forthcoming Meade 5000 series. Even without further test results, I believe the William Optics Swan series are a nice step up from modest plossls. With longer focal ratios like F10 SCTs and F12 Maks, the performance of the Swans should improve making these a serious bargain.

I’ll keep this eyepiece for a long while, until I can afford a 24 mm eyepiece with green lettering on the side. The Swan eyepiece series may go on to solidify a good reputation with long focal ratio telescope owners, the way Celestron Ultima and UO Orthoscopics have for many telescope types. A few more data points with the other focal lengths in the Swan series should shed more light on their value. The longer focal length Swan eyepieces in the 2” format use a different lens design, and I would caution readers from extrapolating this test report to those designs.

Strengths:
Quality Construction
Lightweight with good ergonomics
Comfortable viewing with good eye relief
Good light throughput
Adequate off-axis sharpness with slow (F8+) telescopes
Barlows well
Nice, immersive views
Moderate price

Weaknesses:
Less well suited for planetary viewing
Sharpness drops dramatically with fast (F6-) telescopes
Lens edges not blackened

Chad Moore
Bryce Canyon, UT

Click here for more about this subject. -Ed.