May 19, 2009: Preliminary eyepiece tests of Televue zoom Naglers

Jay Reynolds Freeman

Recently I took delivery of excellent used examples of both the 2-4 mm and 3-6 mm Televue zoom Nagler eyepieces. On the evening of May 19, 2009, I gave them a preliminary try-out, and did a quick comparison with a high-end conventional eyepiece in the same magnification range.

The telescope I used for my tests was a 70 mm f/8 Vixen fluorite. I keep this instrument set up on a simple old-style (pre-dovetail-bar) Vixen altazimuth mount. I have had the 70 mm for many years, and it has one of Vixen's wonderful fluorite doublets, with magnificent optical quality. Furthermore, it is small enough to cool down quickly and to be less bothered by poor seeing than my larger instruments, so it is well suited to this kind of testing.

I took it out in my yard in Palo Alto and aimed it at Polaris. The sky was clear, and after letting everything come to temperature equilibrium the seeing was such that the Airy disc itself was always visible but rarely steady, and one diffraction ring was visible but generally broken up and in motion.

With this telescope, the 3-6 mm zoom Nagler gives magnifications from 93 through 187, and the 2-4 mm gives 140 through 280. The comparison eyepiece was a 3.8 mm Pentax XP orthoscopic, which gives 147x.

In a sense, there is not a lot to tell. The zooms both worked. They could easily be zoomed without disturbing the telescope and without jigging the target out of the field of view, even when used with a small telescope on a very light-duty tripod. They gave crisp central images throughout their ranges of focal length, remained nearly in focus while zooming, and were nearly parfocal with each other as well. I could hold Polaris B steadily with averted vision with all magnifications in all three eyepieces.

For a more careful comparison, I set both zooms to 3.8 mm focal length and played back-and-forth with each other and with the Pentax. Polaris B was almost lost in the glare of the much brighter A component, and thereby provided a sensitive differential test of the amount of close-in glare -- forward scattering in the optical path -- that the three eyepieces provided. I could detect no noticeable difference in the difficulty of holding the companion between any pair of these three 3.8 mm eyepieces.

The apparent field of view of the Naglers at 3.8 mm was noticeably larger than that of the Pentax, and both zooms held the same apparent field of view at all focal lengths.

At 3.8 mm focal length, at f/8, both zooms exhibited no field curvature, and gave round, undistorted star images all the way to the edge of the field. So did the Pentax. All three had an eye relief that I would describe as small. The zooms actually had considerably more eye relief than the orthoscopic, but I usually find myself using the latter by centering the target and then backing my eye off from the optimum position, so that I can only see the central portion of the field of view. That works fine, so the small eye relief of the orthoscopic is not a big deal.

I bought the zoom Naglers for several reasons:

First, I hoped that they would be adequate substitutes for a whole bunch of high-magnification Pentax and Takahashi orthoscopics, so that I could reduce the clutter in my eyepiece box. The comparison with the Pentax suggests that I got that one right.

Second, I hoped that they would be easy to zoom, so that it would be easier and faster to switch magnifications to follow changes in seeing than by changing individual eyepieces. They are indeed easy to zoom, so that makes two good guesses.

Third, my experience with lunar and planetary observation suggests that relatively small differences in magnification sometimes make a great deal of difference in how much one can see. These zoom eyepieces provide a continuous change of magnification in a range of focal length where individual eyepieces generally have focal lengths that are widely spaced, proportionally speaking. Thus even if it had turned out that the eyepieces were not quite up to scratch compared to non-zoom units, the ability to select any focal length might have given them an advantage in everyday use. The eyepieces appear to match at least one first-rate Pentax in optical quality, so a win here seems likely as well.

There is more testing to be done. I am anxious to try the zoom eyepieces out on the Moon and planets, but though Saturn was well placed on the evening of this preliminary work, an undriven telescope is not the best choice for observing anything critically at 280x, so such tests will have to wait for my next major astronomical outing. (My yard has trees, bushes and street lights, so I have to move the whole telescope for just about every different object; an aligned and driven mounting is not practical there.) Furthermore, at f/8, a 2-4 mm zoom eyepiece spans the magnification range from "probably too much" clear out to "double ridiculous". The real utility of this eyepiece will be for fast Newtonians on driven mountings. I will be curious how well it works at f/4.

But it certainly looks as if both of these eyepieces deserve a place in my kit.

And no, I am *not* selling my Pentax and Takahashi orthoscopics.

-- Jay Reynolds Freeman, Deep-Sky Weasel