The weather was wonderfully clear last night and the seeing pretty good, certainly good enough to evaluate this sample in real life.....and it did not disappoint, in fact, quite the opposite.
I looked at Vega, Jupiter, Saturn, and, later, Mars. I started with mono-vision then later ditched that in favor of bino-viewing. I used my AP 2" diagonal, the viewer was an excellent sample Denk II with 1.4x/2.0x/2.8x (or there about) powerswitch/OCS system. I evaluated the scope at full (120mm), 80mm, and 52mm apertures. Eyepieces were my trusty, good old Celestron 10mm, 15mm, 17mm, and 22mm jobbies in pairs for bino viewing and a Silver Top 2X barlow for mono vision.
I did mono vision mostly to do star testing on Vega and quickly settled in with the 10mm plossl w/2x barlow at full aperture, yeah, a little low but enough to do the job. I did not use a green filter as I wanted an ...unfiltered experience (HAH!). Vega is a tough test and yup, a bunch of color all around a nice little yellow airy disk surrounded by a couple of colorful diffraction rings. The center zone was obvious sweeping around focus, showing a "hot spot" outside, close to focus. But the spherical correction was rather good, with the two outer rings being uniform in thickness and brightness and very similar on each side of focus, with the exception of the overlying CA, which I had to look through. There was no astigmatism or coma to be seen. This is all consistent with the DPAC and indoor star testing, except outside, the star test was "cleaner". At low magnifications, say under 35x, the "CA" was almost invisible on Vega.
Stopping the aperture to 80mm, radically improved the "CA" and star test. At 120x, the airy disk was much whiter with a single diffraction ring with notably less overlying "CA"...but it was still there. The intra/extra focus diffraction patterns were almost identical and much sharper. Importantly, as I carefully got really close to focus, the donut pattern simply fell, uniformly, into and then out of the airy disk. I could detect no real spherical error. Not a perfect star test at 80mm aperture (could see some zoning way out of focus), so it's just really, really good. Again, no astigmatism or coma anywhere. This performance is entirely consistent with the images in DPAC, which shows no really meaningful SA at this aperture but some zoning.
Stopping the aperture to 52mm, with the hole in the dew shield cap, was a lesson in perfect star testing. At focus...still at 120x (!), the airy disk was white with just the faintest of blue around it. Moving a little off of focus did show the blue a bit and different tinting in the diffraction rings inside and outside of focus but it all got quickly swallowed by the airy disk at focus. Just perfect really.
I performed a brief star test at higher magnification on Vega (~165x) at full aperture and got the same result so I started in first on Jupiter. I was surprised just how sharp the image was at full aperture with plenty of surface and belt detail. The image was however, dominated by the out of focus color, giving the planet a very yellow appearance (note in the DPAC images at full aperture, the red and green are fairly close to each other in focus, this means the yellow is too). I had to take my time carefully focusing for sharpest belt detail. The atmospheric dispersion (AD) around the planet seemed much more pronounced as well in comparison to the SW 120ED I had out too. And yes, there really was no real comparison between the two as the 120ED was obviously the optically superior scope with notably sharper, more finely detailed and color free views (though there was some AD). But the 120ST didn't suck out loud either, by itself, or in comparison to the 120ED on Jupiter. If I had to project, I'd say the 120ST would be sharper on Jupiter than my C5 despite the obvious differences in CA as the the 120ST is notably smoother in figure with better SA (at least in yellow/green). That might be an interesting comparison....someday.
At 80 mm aperture, again, the CA and AD was much improved and Jupiter looked really good actually with plenty of belt detail. It was interesting to just reach up and pull the mask off of the objective as I could get immediate A-B-A comparisons ( yes, the mask is snugged right on top of and touching the glass ). Pulling the mask quickly off, I had to retouch the focus a tad, but there was obviously more CA, however, the image was still rather sharp and with more detailing as afforded by the larger aperture with its greater resolving power and brighter image. I didn't do any aperture comparisons at similar exit pupils, though I did later on Mars with the Denk power switch.
With its predilection for a yellow tint, the 120ST did very nicely on Saturn at full aperture, being nicely sharp with Cassini's division obvious and the AD and CA seemingly more in the background compared to the views of Jupiter. The 120ED was still the better optic, but the differences were not as pronounced as with Jupiter. The 120ST was very nice really, again, I was surprised at how nice. At 80mm aperture, Saturn was first rate for the aperture. The residual CA was just not bothersome to me and I quickly ignored it as the Image was very sharp and that's what drew my attention. At 100x I could pick out Cassini's division. With at 52mm aperture the image was pristine, color free, and very sharp. Despite the small aperture I was taken aback at the quality of the image. But I could not pick out the division though there was the gap between the planet and the rings and maybe the planet's shadow on the rings as a really thin black "hair" .
Now Mars is where it got interesting as the 120ST did very well actually at full aperture. Which makes sense as Mars is rather mono-chromatic with its predominantly red color, which is where the best correction of this sample 120ST is. And it showed at the eyepiece with nice sharpness of the planet's border against space and surface detail. Syrtis major was obvious and it showed nice texture around its borders as it spanned the globe. The pole cap was white-ish blue with a red/blue fringe but rather distinct. The blueish rim to the planet from its atmosphere was smudged out a bit though, especially in comparison with the 120ED which showed it distinctly. The 120ED was again, the better scope showing even finer gradations in color tint and finer textures in the surface detailing such as the ice cap border and the borders of Syrtis Major. At 80mm aperture, I was a bit floored by how nice and sharp and pleasing the image was even compared to the 120ED at full aperture. This was one time I tried to match exit pupils but with the 120ED. I got ~110-115x with the 80mm stop compared to ~165X with the 120ED and I have to tell you, the differences were there in terms of fine detail resolution, but the differences were by no means stark. Never underestimate or under-value the abilities of a 3" scope for planetary use. And the 120ST was a complete hoot on Mars at 52mm aperture as well with the white pole cap and Syrtis major both easily seen.
Ok, so a lengthy report from a long but very enjoyable session.
I have to admit that I got this sample because I just had a hard time believing some of the glowing reports (and I don't mean from the CA) concerning the optical quality of this cheap, fast achromat. I'm a refractor snob, even an achromat snob. I had a strong, up front expectation that it would be, at best, "serviceable", only usable at low power, with a bad edge and strong zoning with astigmatism on top of it all. I was wrong. Yes, it does have a moderate zone covering about 60mm of the center, but it is well defined but blends smoothly with the rest of the figure and does not mess that much with overall figure. Yes it does have lots of "CA". BUT, but there was no astigmatism or coma in sight at any aperture or magnification I used. All of which means I get rather sharp views at full aperture, even with the CA and center zone. As a bonus, the scope is a complete hoot to use at 80mm aperture as well giving up nothing visually to any 3" achromat I've had (I may just compare it to my 80ED....someday).
So my time with this sample Orion 120ST has made allowed me to become .......a humbled achromat snob.