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Vixen ED115S Refractor

Posted by Lawrence Carlino   03/21/2007 12:00AM

Vixen ED115S Refractor
For refractor aficionados, the seemingly huge gap between the ubiquitous 4-inch ED and APO scopes and the much heavier and more costly instruments in the 5-inch range creates a dilemma. If reasonably-priced, high-quality scopes mid-way between the extremes were available, many potential buyers would be happy with the ideal combination of price, capability, and portability. Unfortunately, few telescopes in this”in-between” range exist. Orion (Synta) does offer a Chinese-made 120mm ED instrument, William Optics a series of 110mm triplets, and TMB a 115mm APO, but almost nothing else exists in category – save for the Vixen ED115S, a 115mm, dual-element f/7.7 instrument with an effective focal length of 890mm.



As somewhat of a refractor addict who has to try just about every moderately priced scope with ostensible APO performance, I purchased a Vixen ED115S directly from Vixen North America, as they were closing out their remaining Vixen product line at “wholesale” prices. Having had the scope for almost two months, I’ve been able to put it through its paces and carefully evaluate its performance. With a Takahashi TSA-102S and FS-128 on hand for direct “A-B” comparison, I managed a dozen or so observing sessions despite the cold and frequently overcast Western New York winter.

FIT, FINISH, and MECHANICS

When the ED115S arrived unscathed, I quickly discovered that it was an attractive and well-built instrument, and one that actually weighs less than many 4-inch scopes. At only 9.7lbs, with a tube length of 37” (dewcap in place), the Vixen is a good candidate for “grab-and-go” astronomy on a relatively light-weight mount. In fact, both a Universal Astronomics Unistar alt-az and Celestron CG-5 equatorial yield a damping time of less than two seconds and an easy, fully-assembled lift to the observing location.

This particular iteration of the Vixen comes with enough standard equipment to allow immediate use without the need to purchase essential “extras.” White tube rings and a matching Vixen/CG dovetail plate are included, as is an integral tube carrying handle, a very fine 7x50 finder on a quick-release dovetail, and the famous (or infamous) Vixen flip-mirror star diagonal. My particular “bundle A” telescope also came with the dedicated f/5.2 focal reducer for ccd imaging and photography.

The Vixen’s focuser is rather “old school”: a 2-inch rack-and-pinion unit that is very smooth and precise, but lacks the rotational feature and dual-speed capability that has become so popular. There is almost zero image shift as the drawtube is racked in and out, save for a tiny region where the images displaces by about 5 arc-seconds, creating no real problem. Because of its rather long tube, the telescope has limited “in travel” that may not allow most binoviewers to work properly, but with a 2-inch or 1.25” star diagonal in place, there is no difficulty in getting any standard eyepiece to focus.

Certainly, Vixen has done an admirable job in the area of fit and finish: the gloss white tube is smooth, uniform, and free of any defects. Despite its light weight, the ED115 exudes an impression of quality and careful workmanship.

THE OPTICS

Without engaging in the seemingly endless debate of what constitutes “APO” performance, suffice it to say that a well-made ED doublet is light-years ahead of even the best achromat in color correction. While no 2-element lens of this aperture (4.53”) and f/7.7 focal ratio can be made totally color free, the proper selection, mating, and quality of the glass elements can produce a superb result. That appears to be the case here, for the Vixen yields outstanding performance.

Star testing the ED115 on Polaris revealed a beautifully corrected optical system with absolutely no detectable spherical aberration and not even the tiniest hint of astigmatism. Out-of-focus Fresnel rings were well-defined and uniform, attesting to the fine precision and smoothness of the optics. Noticeable color on both sides of focus disappeared as Polaris was brought to a sharp point. At high power, the intensity of the North Star’s Airy disk and perfectly symmetrical first diffraction ring made for an impressive view, and the pinpoint sharpness of the companion star was very satisfying.

Though Vixen doesn’t specify the type of glass used in the ED115, the ED element is almost certainly FPL-53 as defined by the scope’s actual performance. Despite cranking the telescope to magnifications of over 250x on double stars, Venus, Saturn, and the moon, I could detect none of the violet “wash” that is characteristic of achromats or ED scopes of lesser quality. Only at 300x or more on Sirius and Rigel, was there the slightest hint of deep indigo. Instead, the Vixen’s color correction has been skewed to allow a slight red excess that manifests itself in the high-power first diffraction ring of brighter stars. Because the human eye is relatively insensitive to faint reddish light, this is hardly noticeable to the average observer, though my own abnormally high red sensitivity does make it a mild annoyance.

Overall, however, the excellent correction of the optical system yields images that are tight, of high contrast, and very impressive. I get the feeling, that if Takahashi produced ED doublets instead of fluorites and triplets, they would produce a telescope with a similar level of performance.


OBSERVING


With the ED115, 4 and 5-inch Takahashi’s, a 12-inch Starhopper Dobsonian, and a Celestron 6 SCT, at the ready, I set out to test the comparative performance on a variety of objects.

Double stars often provide a viable test for the quality of an optical system, and the ED115 acquitted itself very well. Castor is a good example of a brighter multiple star that needs excellent image quality to be seen as it truly should be. I swapped the mediocre Vixen flip-mirror diagonal for an Astro-Tech 2-inch dielectric unit to allow the Vixen to perform to its full potential. At 133x, using a Meade 6.7mm UltraWide Angle eyepiece, the scope revealed the two off-white main components with sharply defined Airy disks, plenty of dark sky between, and delicate, symmetrical first diffraction rings. When the air was steady, no false color could be noted. However, as the seeing deteriorated, tiny flecks of red and blue flashed into view momentarily and then disappeared. With the Takahashi TSA-102S triplet (having essentially perfect color correction), the true colors of the duo remained unaffected.

Turning to some very tight and challenging doubles, I found that the Vixen more than lived up to expectations. At 254x, with a TeleVue 3.5mm Nagler Type 6 in the drawtube, the scope easily resolved Alnitak, Eta Orionis, and the devilishly tough Eta Geminorum (Propus, mag 3.3 and 8.8 with 1.4 arc-sec separation)). The intensity and tightness of the images approached those rendered by the 4-inch Tak, and the resolution was actually a bit better because of the Vixen’s greater aperture. Again, however, the change in focus brought about by degraded seeing conditions showed the biggest disadvantage of the ED doublet: intermittent flashes of false color.

The biggest surprise, however, occurred when I turned the telescope toward Sirius. Amazingly, the elusive “Pup,” Sirius B, could be clearly seen in the Vixen at 133 and 178x in brief moments of steady seeing. Using the Tak FS-128 and a 12-inch Dob to confirm the position angle, I found that the tight concentration of the bright primary’s disk in the ED115 made the split possible and definite, where the companion could just be glimpsed with the smaller Tak TSA-102. As expected, when the air became even mildly turbulent, none of the telescopes could resolve the classic pair.

The brilliant Venus, as it descended in the western sky, became a potent test of the Vixen’s optical quality. Despite a fair amount of atmospheric turbulence, the scope served up a tightly defined gibbous disk at 178x with a 5mm Nagler Type 6. No violet halo could be noted, but atmospheric refraction did produce the expected red and blue on opposite limbs of the planet. During brief flashes of steady seeing, very faint dusky markings of questionable shape could be noted, as could brighter areas at the Venusian poles.

Saturn, now high in the southeastern sky, was a much more attractive target. At 127x, the ringed planet was a stunning sight in the Vixen. The ED115 snapped to a sharp focus and easily revealed the Cassini Division, ring and planetary shadows, the evanescent Crepe Ring, and South Equatorial Belt complex. With the power boosted to 178 and 223x, the subtle shadings of the polar area became visible, as did four of Saturn’s moons. In comparison to the Takahashi TSA-102 triplet, the Vixen rendered a slightly “warmer” tone to the image, but its 27 percent greater light grasp allowed for a significantly brighter image at a given magnification. I came away with the impression, especially when I used a 6-inch Celestron SCT to assess color purity, that the authenticity of color is slightly better in the Tak triplet, but the difference in hue is difficult to detect.

The first-quarter moon provided another good test of the ED115’s abilities. Again, the “snap” to focus delivered by the Vixen was very satisfying, and like the Takahashi’s, it seemed to cut through atmospheric turbulence in a manner that seems the exclusive domain of high-quality refractors. Lunar features stood out in stark contrast at 127x using a 7mm Nagler Type 6. No hint of limb violet could be detected, and the glare from the offset lunar orb was well controlled by the scope’s excellent baffling. Detail in the Hyginus rill was beautifully defined as were the complex central mountain peaks in Theophilus. The lunar north, with its smattering of seas and craters, provided my very favorite lunar illusion: observing the rugged surface from an orbiting spacecraft. Shadows, at this power, were dead black and sharply defined, and the telescope’s optics “disappeared,” leaving only a very impressive vista.

With the magnification run up to 223x with a 4mm TMB/Burgess planetary eyepiece, however, an almost undetectable amount of deep red could be glimpsed adjacent to crater and mountain range shadows when the seeing conditions turned poor – again pointing out the one (minor) shortcoming of even the best ED doublet.

Deep-sky observation is not the strong point of a relatively small (in aperture) telescope, but the Vixen did provide some very fine images. With a 40mm Orion Optiluxe eyepiece, the ED115 provided an expansive 2.8 degree field-of-view – enough to please most RFT fans and easily frame the Pleiades, Beehive, and most other large deep-sky objects,

Cruising at 68x with a 13mm Nagler Type 6, the scope served up a beautiful image of the Great Orion Nebula, showing a tad more detail than the 4-inch telescope and somewhat less than the 5-inch Takahashi. At 133x and 178x, both the “E” and “F” stars in the Trapezium were reasonably easy to pick out with the Vixen, more so than with the TSA-102. It seems that in this 4 to 5-inch size category, 12 or 13mm of additional aperture DOES mean something in deep-sky viewing, as the jump from 102mm to 115mm to 128mm provided a noticeable improvement in the brightness and visibility of everything from bright nebulae and globular clusters to faint smudges of NGC galaxies.
Obviously, size does matter, and any of these fine refractors are easily outclassed by a 10 or 12-inch Dobsonian where light grasp is concerned.

CONCLUSIONS

Overall, I’m quite impressed with the ED115’s performance. It fills the niche between the plethora of 4-inch APO’s and the much more expensive 5-inchers. Though its tube length is a bit long, the Vixen’s sub-ten pound weight allows for a variety of mounting options and excellent portability.

Optically, the telescope is about as good as it gets for an ED doublet of moderate focal ratio in this intermediate size class. Superb contrast and tight stellar images attest to its fine optical correction. The almost total lack of the dreaded violet halo found in achromats is offset a little by the slight red excess and fleeting glimpses of false color when the seeing is poor, but calling the telescope an APO (depending on one’s personal definition) wouldn’t be too much of a stretch.

Is the Vixen ED115S worth its $2995 retail price? Given the lack of competition in its size category, I’m inclined to say that it is. A rotatable dual-speed focuser would be a nice addition, as would a retractable dewcap, but otherwise, the scope comes well equipped. At the current “wholesale direct” price of $2145 with the f/5.2 focal reducer for imaging, it’s a bargain that is almost sure to please. It’s like a middleweight boxer with a heavy punch – a rare and very good combination.

Clear and steady skies,
Larry Carlino

Click here for more about this subject. -Ed.