Optics Performance Of Orion Astroview 120ST
The 120ST is a 120mm (4.72”) f/5 achromatic doublet refractor. Being a large f/5 achromat, there is quite a lot of chromatic aberration displayed on the Moon, planets, and bright stars.
This scope is really meant for low-power, wide-field deep-sky viewing, and not much else. However, images at high and moderate magnifications of the Moon and planets are acceptable for the casual observer. Although they have severe chromatic aberration, the optics in the 120ST I used were certainly at least diffraction-limited, perfectly acceptable if not above average for a fast achromat.
The 120ST’s focuser is a rather simple, though thankfully all-metal in construction, 2” rack-and-pinion. While it’s great for holding heavy eyepieces or cameras, precise focusing can be somewhat difficult. A 1.25” adapter (though not a compression-ring design) is also included. The focuser also has a slot and thumbscrew to accept any finderscope or reflex sight with a Vixen/Synta-style compatible foot.
Unless you are mounting it as a guide scope on a really large instrument, for pretty much all use cases you’ll want 116mm inner diameter tube rings, which Orion sells, and a Vixen dovetail plate for them (probably an 8” long unit).
The 8.6-pound weight of the 120ST makes it suitable for a lot of lightweight mounts or even a photo tripod, though the weight could climb to or even exceed 12 pounds once a finderscope, 2” diagonal, 2” eyepiece and tube rings/dovetail are installed – consequently, I would recommend something rated to carry 15 pounds at the minimum.
I would recommend using the 120ST on an alt-azimuth mount due to its primary use as a low-power, wide-field telescope. Orion’s own VersaGo III, Vixen’s Porta II, and Explore Scientific’s Twilight I are all excellent choices for this. If you insist on an equatorial mount, however, anything from the EQ3/EQ4 class on up will work fine, such as the Orion AstroView or Sky-Watcher EQM-35 – though something like the Orion SkyView Pro or Celestron CG-4 will be even steadier.
How is the Scope In Use
With a 1.25” diagonal and low-power eyepiece, the 120ST is capable of achieving a field of view as large as 2.7 degrees – that’s about as wide as the Andromeda Galaxy, or about 5 and a half times the apparent width of the full Moon! With a 2” diagonal and eyepieces, the maximum possible field expands as large as 4.5 degrees – though the low magnification required to achieve this will result in light loss, so you’re really limited to around or a little over 4 degrees. This makes it great for rich-field viewing of star clusters and large nebulae.
Astrophotography Abilities Of 120ST
Orion advertises the 120ST as capable of astrophotography, and indeed it is – but it is not great for the task. The severe chromatic aberration will result in bloating and halos around stars. You could shoot LGRB or narrowband with a monochrome camera to get around this, but it’ll require refocusing for different colors and overall is probably not worth the effort. For optimal results, a field flattener would also be a good idea. Any mount in the EQ5 class range is probably sufficient for the 120ST provided you use autoguiding. Overall though, you’d be much better off with a smaller apochromatic refractor that uses ED glass and preferably a triplet lens.
If there was any astrophotography task the 120ST is particularly suited for, it would probably be as a guide scope on a large astrophotography rig – probably for a scope with at least 1500mm of focal length, at the least. The 120ST’s large aperture and long focal length (for a guidescope) of 600mm would make it great for autoguiding provided your setup is capable of carrying it – and of course you’d need guidescope rings and piggyback clamps.
The Orion 120mm f/5 Refractor Telescope really excels at the primary job it’s meant for – low-power deep-space viewing. While it’s certainly not optimized for much else, I would say it would still make a good “grab n’ go” telescope for general use as long as you are not too concerned about sharp high-power views. It is probably best for the experienced amateur, however – if you’re a beginner we recommend perusing our Best Telescopes and Best Refractor Telescopes articles before settling on anything.