ORION 120ED vs. ASTRO-TECH 127EDT
As a confirmed refractor enthusiast, I’ve searched for decades in trying to find the optimum combination of aperture, optical performance, portability, and price for my “dream scope.” Having gone through several dozen achromats, ED’s, and APO’s in apertures ranging from 60 to 152mm, I have been happy with a number of choices, but usually at significant expense. Scopes in the 4-inch range, though very impressive when well crafted, seemed to lack the necessary light grasp for deep-sky observation and high-power planetary viewing. Six-inchers, on the other hand, required a heavy mount that limited mobility and cut down on the frequency of use. Something in the 5-inch range appeared to be the best compromise.
However, unlike the ubiquitous 100-110mm scopes, the 120 to 130mm spectrum has been characteristically lacking in choices. Pity.
In the past 15 years, I’ve owned a Brandon 130mm triplet APO, two Vixen 130ED’s (no longer produced), two Takahashi FS-128 fluorite doublets, a TMB Signature Series 130mm triplet, an APM (TMB) 130mm, f/6 triplet, an Orion 120mm ED doublet, and an Astro-Tech (Meade?) 127mm triplet. Other possibilities from Astro-Physics, Stellarvue, Pentax, William Optics, TeleVue, and Borg were either wait-listed or too expensive for consideration.
Of the telescope that I’ve actually owned, the Tak FS-128 has been the decisive winner in optical quality, but its long tube made mounting problematical and limited its “grab-and-go” potential. What I really needed was a scope that approached the Tak’s optical quality with lighter weight and significantly lower cost.
When the Orion (Synta) 120mm ED doublet became available, I was severely tempted to try one, but estimates of theoretically less-than-optimum color correction put my desire on hold. With the recent introduction of the Orion Eon 120ED (really a gussied-up version of the original), the price of the scope dropped to $1599, and I took the bait.
The Astro-Tech 127mm ED triplet became available in a rather ususual bit of serendipity: Originally designed to be imported under the Meade label and having suffered a long period of development and QC difficulties, a handful of these bargain APO’s were brought in under the Astro-Tech banner at less than $1700 and quickly sold out. This instrument is available in Europe as a “Meade” and in Australia as a “Maxvision” scope.
The Orion 120mm ED doublet arrived as an optical tube assembly without a finder, tube rings, or star diagonal. As I already had a pair of light- weight 116mm Orion rings, a 9x50mm Celestron finder, and several 2-inch and 1.25” dielectric star diagonals, the scope could be quickly configured to fit a CG-5 equatorial or Universal Astronomics Unistar mount. With the finder slipped into the telescope’s integral dovetail slot and the tube rings and Vixen-style mounting bar attached, the 120ED weighed in at a svelte 11.5 pounds. This lack of heft, combined with a tube length of 36 inches, was a real benefit in creating a nearly vibration-free set-up on both equatorial and alt-az mounts. Many 4-inch refractors are heavier.
Fit and finish of the telescope is typical of Orion products fielded from China: decent, but rather drab in a metallic dark gray finish. A friction-fit, non-retractable dew cap and single-speed Crayford focuser complete the ensemble. This focuser is no FeatherTouch, but it is reasonably smooth with a solid feel and no backlash or image shift. Certainly, the Eon 120ED that has now superseded the original version adds considerable cachet with its gloss black tube, dual-speed focuser, heavy duty tube rings, and retractable dew cap. However, as far as I can determine, the optics are identical. The same ED doublet lens is also available under the Skywatcher label in both the U.S. and Canada with different levels of trim, standard accessories, and color schemes.
The Astro-Tech 127EDT, in comparison, is a relative monster. Weighing 19 pounds with tube rings and finder attached, this triplet is a 127mm, f/7.5 set up with an efl of 950mm. With its retractable dew cap pulled in, its overall length is just 34 inches, but extending the cap fully makes it a bit longer than the Orion scope. The AT’s tube is finished in an attractive gloss white: not particularly smooth or uniform, but without noticeable defects. Trim is typical “Meade blue,” and a dual –speed Crayford focuser graces the back end. A very nice “chrome look” illuminated reticle 50mm finder is standard equipment as is a carrying case, 2-inch dielectric star diagonal, 20mm and 10mm wide-angle eyepieces, tube rings with a bridge for mounting accessories, and a Vixen/CG dovetail mount. The dust cap is a nicely machined thread-on unit. For $1699, it’s quite a deal.
The scope arrived in good condition and perfect collimation, save for a broken switch on the finder illuminator. This was promptly replaced by the good people at High Point Scientific – an excellent example of top-notch customer service.
Also somewhat problematical was the focusing mechanism. It created some very unsettling grinding sounds when racking in and out with the coarse focus knob, though the fine focus worked smoothly. I invested a solid two hours fiddling with the numerous recessed hex-head tensioning screws in order to get the Crayford functioning properly. When the optimum tension was finally achieved, the unit operated with a smooth, “weighty” feel, though it still had a tendency to slip when the scope was pointed near the zenith. This is one area that could use improvement.
Mounting the AT127 was no challenge, but the Unistar mount afforded marginal stability because of the telescope’s weight and length. A Celestron CG-5 with its 2-inch tubular steel legs worked very nicely, however. A 2-second damping time and an overall weight allowing single-stage movement of the combination for reasonable distances was the result.
Though the 120ED is not a full 5-incher, its 4.74” clear aperture puts it pretty close. The air-spaced ED doublet objective lens uses one element of Ohara FPL-53 glass, about as close the true CaF2 as one is likely to get. The mating element is unspecified, but it does seem to be a good choice in the elimination of chromatic aberration without resorting to a triplet design.
At f/7.5 with an efl of 900mm, the telescope SHOULD display a good deal of false color, but surprise! It doesn’t. In fact, at least for visual use, spurious color is limited to a small, faint violet halo around Vega at 250x plus, and just a touch of violet on the lunar limb coupled with a barely noticeable deep red adjacent to crater shadows at the same magnification. Racking Polaris through focus, I found the extra-focal fresnel rings to be beautifully defined and concentric with the outermost ring a deep red. Inside of focus, the outer ring was a cyan cast, and the observed pattern was slightly less well defined but still close to the outer configuration. Spherical aberration and astigmatism were virtually absent. In focus, the Orion presented a tight, well-defined Airy disk with a faint concentric first diffraction ring at 150x. Only when the air became turbulent did tiny flashes of violet and red become fleetingly visible.
The AT127ED triplet has a center lens element made of an undisclosed type of ED glass, presumably something like FPL-51 in its optical characteristics. The lens is produced in China by Kunming Optical, and is ostensibly hand crafted.
This full 5-incher also star tested well, producing a yellow outer Fresnel ring and cyan inner rings with Polaris racked outside of focus. Inside of focus, the outer rim took on a cyan hue with the overall ring pattern less well defined than that of the Orion doublet. Though correction for spherical aberration and astigmatism appeared to be quite good, I believe that the Orion scope has slightly better overall correction. This proved to be true especially in regard to false color as the AT scope did display a more prominent violet halo around Vega and a very thin deep indigo on the lunar rim at 150x. Not bad at all and hardly noticeable, but not quite as good as the 120ED.
However, in-focus stellar images did serve up a very tight Airy disk and thin first diffraction ring. Fainter stars delivered the highly-desired “diamonds on black velvet” appearance that refractor enthusiasts crave.
Both of these telescopes proved to be better than the minimum “diffraction limited” criteria by a good margin.
LUNAR AND PLANETARY OBSERVATIONS
In observing the moon through several cycles in most of its phases, I found that both the Orion and Astro-Tech scopes produced startlingly clear and impressive images of the lunar surface. Because of the relatively close (but not exact) focal lengths of the instruments (900 vs. 950mm), it was difficult to generate the same magnification with eyepieces of the same type. To equalize power, I used a Celestron zoom eyepiece. However, because of the scopes’ identical f/ratios, the same ep (for example, a TeleView Nagler 7mm) provided equal image brightness, albeit at slightly different power.
At powers in the 129 – 136x range, using a TV 7mm Type 6 Nagler, the nine-day-old orb was loaded with impressive high-contrast detail in both telescopes. The tiny chains of coalesced craterlets near Copernicus stood out boldly, and the miniscule Rima Birt near the Straight Wall was actually easy to pick out. The lunar north polar areas took on a three-dimensional appearance with pure black crater shadows, and the delicate bands on the floor of Archimedes displayed marvelous detail. Both telescopes performed in almost identical fashion, with the Orion giving a slightly “harder” image, while the AT127, possibly because of its greater aperture, seemed to create a more stark “big scope” vista.
With the power bumped up to the 250x + range (TV 3.5mm Nagler), the violet color error in the AT began to show. Though the image remained wonderfully sharp, a deep indigo rim could be seen on the lunar limb with perhaps a tiny bit of deep red adjacent to crater shadows. At similar magnification, the Orion displayed more red “spillover” near the lunar shadows, but it also generated a wonderfully sharp and satisfying image.
An even better test of the telescopes’ performance was the brilliant Jupiter. Though at low altitude, its unusually close proximity to earth made for a huge, detail-rich treat. I’ve always believed that the low contrast features of the giant planet provide the ultimate test of an optical system. A poor scope will show the equatorial belts and not much else, where a good one will extract minute detail in moments of steady seeing. Both the Orion 120 and AT127EDT passed this test with flying colors.
Even at powers of 90 to 100x, both telescopes furnished images of a hard, sharp planetary disk with multiple cloud belts and excellent color rendition. Pumping up the power to 180x and 190x with a 5mm TV Nagler Type 6 created a flood of detail within the North and South Equatorial Belts and revealed a distinct Red Spot Hollow and faded pink Great Red Spot. Both scopes showed delicate festooning in the Equatorial Zone and subtle detail in the polar regions. To me (and my sharp-eyed and experienced observing buddies, Tom and Rick) it appeared that the Orion provided slightly better contrast, but the AT seemed to have superior color saturation, particularly near the Jovian poles. Go figure! Neither instrument displayed any false color.
In comparison to the legendary Takahashi FS-128, both scopes did fall a bit behind. The Tak displayed an image with zero spurious color, deadly sharp contrast, AND amazing color saturation: not vastly better than the Orion and AT, but still noticeably superior.
Waiting patiently for nights with reasonably steady air, I was able to assess the performance of both telescopes on a variety of challenging multiple stars.
First and foremost was the devilishly tricky Antares. With fairly decent seeing conditions, both the Orion 120ED and AT127EDT were able to separate the tiny companion from the blazing low-altitude giant. At 180x, the Orion cleanly split the pair with dark sky between. The tightness of Antares’ disk and well-defined diffraction rings made it possible to hold the image without interruption. The AT, at 190x, also separated the duo, but seemed more susceptible to changes in seeing conditions.
Epsilon Lyrae, hardly a difficult test but an accurate gauge of performance, was beautifully resolved in both telescopes. Using the Celestron zoom and a 2x Orion premium Barlow to obtain 155x with the competitors, I looked in vain for traces of inaccurate color rendition or fuzziness around the quartet of stars: Nothing but brilliant white disks surrounded by faint first diffraction rings, with the Orion 120ED being just a tiny bit sharper. Very nice.
Likewise, both scopes separated the neat triple system Iota Cassiopeiae easily and turned the tight Zeta Aquarii into two white “headlights” at 200x. Compared to the superb Tak FS-128, the low-priced pair fell only a little behind in the overall quality of the image. The Tak always seemed to create a more intense Airy disk that almost appeared to “burn” in contrast to the dark sky background. The Orion came closest to duplicating the effect, and the AT127 fell just a bit behind.
Obviously, medium aperture refractors are at a serious disadvantage when compared to larger dobs and catadioptrics where deep-sky observing is concerned. Nevertheless, both the Orion and AT performed admirably on a variety of open clusters, globulars, nebulae, and galaxies. The exceptional light throughput and fine contrast created some striking images, particularly at low and moderate magnifications.
The Astro-Tech, with just 11 percent more light grasp than its smaller Orion competitor, wasn’t expected to be significantly better at bringing in faint details, but I did notice a slight difference on some objects.
Using powers in the 100x to 130x range, I discovered that both scopes provided wonderful rendition of M 13, dozens of its brighter stars being resolved right across the core of the stunning globular. However, the extra aperture of the AT127 did make the resolution just a bit easier while making the use of extreme averted vision less necessary.
Likewise, M 57 easily showed its annularity in both instruments, but the nearby 13th magnitude star stood out more boldly in the Astro-Tech: A small difference, but a major improvement over the view afforded by my exceptional Tak TSA-102. The jump from 4 to 5 inches is a major one, with the 4.74” Orion and 5.00” AT both outperforming even the best 4-inch refractor where light grasp is concerned.
Through dozens of observing sessions, numerous “A-B” comparison tests, and gleaning opinions from knowledgeable fellow observers, I’ve been able to rank these 5-inch telescopes with a fair degree of confidence. However, it must be remembered that the particular scopes I’ve used may NOT be fully representative of the entire breed. In particular, a number of the Chinese-sourced instruments were obviously defective from the start and might not provide an accurate picture of true performance.
RANK – OVERALL OPTICAL QUALITY
1. Takahashi FS-128 : Superb image contrast, absolutely no spherical aberration or astigmatism. Visually, almost zero false color except for a tiny bit of violet excess around bright bluish stars at ridiculously high powers. One FS-128 just a bit better than the other.
2. Orion 120ED : Amazingly sharp and contrasty images. Very nice star test with some out-of-focus color but almost none that I can detect in focus. Great “snap to focus.” Views of the moon, Saturn, and Jupe generate “Wows!” at star parties. Image brightness is reduced because of its lesser aperture, but this a VERY good telescope.
3. Astro-Tech 127EDT and APM/TMB 130, f/6 (TIE) : This shouldn’t be the case, but the AT 127EDT, at one-third the price, was at least as good as the APM/TMB at revealing lunar and planetary detail, though it lagged a bit in color correction. Though the APM scope’s test data showed a Strehl ratio of 95.4 and peak-to-valley correction of .212 wave, it was evidently compromised by its fast f/6 optics. For imaging purposes, it might fare better than the AT127, but visually, I have to rate it a toss-up.
4. Brandon 130mm triplet : Sharp images with just a touch of false color. Views of the planets show a noticeable yellowish cast but excellent detail.
5. ?? TMB 130mm Signature Series : The example I received had serious optical defects and had to be returned. Most reports on this scope are generally favorable, but I can’t comment because of my limited experience.
6. Vixen 130mm ED doublet : Of the two Vixen 130’s I’ve owned, one was significantly better than the other. Very good color correction for a short f/ratio (f/6.7) doublet, but some spherical aberration and evidence of zones. Not bad, but not truly satisfying.
7. Vixen 140mm Neo-achromat : On a lark, I decided to try one of these. Color correction decent but clearly an achromat. Too much spherical aberration and zonal irregularities. Sloppy planetary images with a large violet halo. Powers above about 120x got ugly on double stars. Probably a defective unit. I returned it.
Both the Orion 120mm ED and Astro-Tech 127EDT have proven to be fine telescopes whose performance belies their reasonable cost. Though neither instrument achieves parity with the “gold standard” from Takahashi, particularly in build quality and (probably) consistency of quality control, both provide a viable alternative for those who can’t mortgage the farm to obtain a premium 5-incher. There is a lot to be said for the pure value represented and the consistent satisfaction generated by both scopes.
In particular, though I found the Astro-Tech 127EDT to be an amazing instrument, the Orion 120ED has turned out to be my most often used refractor: light enough to be truly portable with optics good enough to cut like a laser through unsteady air, this scope comes perilously close to delivering a premium APO image on every class of celestial object. Now, if they’d just make a 140mm version.
Clear and steady skies,
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