This is a longer blog post reporting on my findings with the Houdini 30 prototype we received on the last day of 2025, compared to the legendary Nagler 31—one of my all-time favorite eyepieces.
The weather has been awful for astronomy recently, but in early February I finally managed to get some observing time with both eyepieces in the 25-inch f/5 Dobsonian telescope.
Nagler 31
I have long been a big fan of the Nagler 31. I acquired it more than 20 years ago for use with the 25-inch telescope, and I remember writing at the time that this eyepiece delivered an entirely new experience—almost like having a new telescope.
The unit in the photo on the left is my own—as you can see, it shows signs of significant use, and I've filled the annoying undercut in the barrel with some copper strip.
I normally use the Nagler 31 with a Paracorr 2, but for this test I deliberately used it without a coma corrector to keep the magnification comparable to the Houdini 30 and to better understand how much difference the Houdini’s built-in coma correction makes. At f/5, coma becomes visible from about halfway out in the field, but most of the view remains quite usable—it’s just not tack-sharp as it is in the center.
Houdini 30
The Houdini 30 is the longest focal-length eyepiece in our series of coma-correcting eyepieces. It features an 80° apparent field of view rather than the usual 86°, as this was the maximum achievable within a 2-inch barrel.
The optical layout of the Houdini 30 is similar to that of the Houdini 20, with eight elements in six groups. However, the internal lenses are larger, and there is roughly 100 grams more glass in the eyepiece.
I gave the manufacturing team at KUO a very challenging set of requirements based on the Houdini 30 optical design:
- Use the largest lenses possible in a 2-inch barrel (47 mm clear aperture), as this reduces vignetting at the edge of the field.
- Keep the body diameter as small as possible (maximum 65 mm) and design a conical top to facilitate binoviewing.
- Keep the weight as low as possible (maximum 700 grams).
The prototypes meet all of these requirements with flying colors. The body diameter is 64.5 mm, and the weight is just 580 grams—only 10 grams more than the Houdini 20, despite the substantial increase in glass volume. The team at KUO has delivered something truly special that exceeded my expectations.
Initial Field Testing
In early January, quick tests of the Houdini 30 prototype at home with 12-inch f/4 and 8-inch f/4 Newtonians already indicated a very sharp field and comfortable eye positioning.
However, it wasn’t until February that I was able to properly validate the eyepiece at my dark-sky site using the 43-inch f/3.6 and 25-inch f/5 Newtonians.
In the 43-inch f/3.6, using the Nagler 31 without a coma corrector is simply not an option—well over half the field turns into “warp speed,” with comet-shaped stars. This made the 25-inch f/5 Newtonian the ideal testbed for a direct comparison between the Houdini 30 and the Nagler 31.
The Comparison
Both eyepieces are about 140 mm tall and feature a 2-inch barrel, but the Houdini is noticeably slimmer and lighter.
The Nagler 31 has an 83 mm diameter and weighs 975 grams.
The Houdini 30 has a 65 mm diameter and weighs 580 grams.
Both eyepieces share an identical 42 mm field stop and therefore deliver the exact same true field of view. This raises an interesting question: how can a 30 mm, 80° eyepiece show the same field as a 31 mm, 82° eyepiece? It suggests that the Nagler may not actually provide a full 82° apparent field (visually it appears closer to 80°), or that distortion reduces its effective true field.
In actual use, the views confirm that the true field is essentially identical, with the Houdini 30 showing just a hair more than the Nagler 31. Magnification is about 100×, with a true field of roughly 45 arcminutes.
Eye positioning with the Houdini 30 is easier than with the Nagler 31. This surprised me, as I have always considered the Nagler quite comfortable. The Houdini 30 takes that comfort to the next level. Once you’ve experienced the Houdini 30, the Nagler 31’s eye positioning feels comparatively finicky, and partial blackouts (kidney-beaning) are easier to induce.
In an f/5 Newtonian, only the central portion of the field in the Nagler 31 is truly sharp, meaning stars near the edge cannot be brought to proper focus and show noticeable comet-like tails. The Houdini 30's built-in coma correction allows stars to be focused anywhere in the field.
Observing
Test objects included the Orion Nebula, Jupiter, M35, Castor, M81–82, and M51.
At around 105×, the central sharpness was very similar between the two eyepieces on all objects. The Orion Nebula showed exquisite color, and Jupiter revealed some very nice banding with details. M35, with NGC 2158 at the edge, was a good test of the peripheral sharpness of the 45' field. Castor was a beautifully split double star, and M51 showed its delicate spiral structure.
Most detailed comparisons were made on M81 and M82, which were beautifully framed together. To evaluate eyepiece transmission, I followed a chain of faint stars between the two galaxies. The Houdini 30 consistently went slightly deeper than the Nagler 31. Faint stars were easier to detect, and a few stars at the detection limit of the Houdini 30 were not visible in the Nagler 31. It’s unclear whether this is due to glass selection—Houdini glass types are chosen for maximum transmission—, advances in coating technology since around 2000, or the small power difference in the center (106× vs. 103×).
Both eyepieces cleanly resolved a cute double star near M81, likely STF 1387, with a separation of about 9 arcseconds.
Conclusion
I am very impressed with the Houdini 30 prototypes. KUO has pushed the eyepiece body design to the practical limits in terms of diameter and weight. The eyepiece is compact and light enough for binoviewing, provided your interpupillary distance is at least 65 mm. (The picture shows them in the 6" f/8 binoscope observing the Sun).
Eye positioning is very comfortable and forgiving, with no blackouts—even more so than the Nagler 31. Transmission is also outstanding: in my tests, the Houdini 30 revealed marginally fainter stars than the Nagler.
The field is sharp from edge to edge, allowing stars near the edge of the field to be brought cleanly into focus. Thanks to the integrated coma correction, even fast Newtonians can now take full advantage of the entire 42 mm field stop!
In short, this is an exceptional eyepiece that gives its legendary predecessor a serious run for its money. It is poised to become the new reference standard for 30 mm wide-field eyepieces.
It's probably my favorite Houdini of the series so far!
