Notes from Under Sky

The Vixen LV 8–24 Zoom

In which I review this eyepiece

I got this eyepiece as a birthday gift about a week ago as I write this (11 September 1999). It supposedly shares an optical design with the Tele Vue unit, which sells typically for about $20 more, as well as the Meade zoom, which for some reason sells for about $40 more. Meade used to use a Swarovski design that was supposedly superior; perhaps that explains the higher price, even though they now all share a design. I wonder if the other units have the same Lanthanum lens, though.

It looks like the mechanism is the moving Barlow at the tail end of the eyepiece. I shone some light in it, but though I tried working out the reflections until my head spun, I couldn't divine the layout of the eyepiece. Vixen's web site shows that its LV eyepieces are either three groups comprising five elements, or four groups comprising seven elements, but I have no idea which it is for the zoom. At the very least, though, that means six air-to-glass interfaces, where standard Plossls and symmetrics, as well as orthos, have only four. (Some so-called modified Plossls have an extra element, making for six interfaces, or so I hear.)

I've only had a brief opportunity to test it, but I used it 7 September 1999 on Mars and M22. Mars is down to, what, about a 7-second disc? I examined it at 278x using both the 9 mm ortho and the zoom lens set at just above 8 mm (it's only marked at 8, 12, 16, and 24 mm settings). No question, hands down, the ortho wins. Not a huge difference, but definite. Despite so-so seeing, dark markings in the northern hemisphere were clearly visible through the ortho; they were evident through the zoom, but much less clearly. I did not try the 8 mm setting compared with a Barlowed 15 mm Tele Vue Plossl.

I then compared it at 24 mm with my 25 mm SMA, the Kellner variant (?) that comes with the C5+, this time on M22. The transparency was awful, perhaps limiting magnitude about 4. It was invisible through the 6x30 finder (yes, I took out the internal stop). I was hoping to see if the optimal magnification for this object was somewhere between those yielded by 8 and 24 mm, but unfortunately, best visibility was to be had at 24 mm, yielding 52x. At this magnification, I'd give only a slight edge to the zoom over the SMA; it showed somewhat less off-axis astigmatism. (A daytime comparison between the two, though, gave the big edge to the zoom—the SMA leaks a lot of light. The inside of the barrel is painted black, you see, but it is flat, unlike all of my other eyepieces, which are corrugated.)

Fans of the optimal magnification debate will be happy to hear, though, that the zoom did have one benefit in that regard. While the cluster as a whole was shown to best effect at lowest magnification, contrast features near its center were better seen with about a 10 mm focal length, or 125x. Having to switch eyepieces might have made it difficult to recapture the cluster visually, but this way, I could keep my eye on the cluster and watch it slowly fade, while the internal mottling, though not quite resolved, made itself more evident. It was cool to watch the sky darken magically before your eyes!

Two nights later I tried it out on Jupiter and Saturn, then about 45 degrees up the eastern sky. The image was warbly while the scope was cooling down, but settled down acceptably in about 10 minutes. Jupiter was first. At 24 mm, of course, Jupiter was a sharp but small ball with the two main belts clearly visible, but not much else. Three of the four moons were also visible at this wide view (40 degrees AFOV doesn't bother me that much). The 25 mm SMA was about even with it in performance.

At 8 mm, the image was a bit fuzzy around the edges, but perfectly acceptable. The 9 mm ortho was better at somewhat lower magnification; equatorward bulges on the NEB became sharper-defined festoons. Saturn showed similar differences in contrast, but surprise: the zoom was a bit brighter, which doesn't make a whole lot of sense to me. Despite that, the detail on Saturn was considerably sharper through the ortho. The Cassini Division was sharply visible all around the gradually widening rings—beautiful! Through the zoom, Cassini was visible but not quite as distinct. This was during a 20-minute period of seeing I'd rate as a 9, nearly stock solid.

The zoom is not quite parfocal with itself (!), but the extent of refocusing is not great and, as has been mentioned previously in this thread, it does not cause one to lose sight of the target. It would be interesting to see this eyepiece perform under dark skies. Still, even without using it there, it's a keeper!

Copyright (c) 1999 Brian Tung