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[SAL]

This is my take on the general differences: The 120ST is an achromatic refractor. This means its objective lens has more chromatic aberration than the ED glass has. The larger aperture of the 120mm will allow you to see dimmer objects through the scope than the 80mm lens will. However the view will have more false color (especially at medium to high magnifications) than an “ED” refractor produces. To further complicate the comparison, in achromatic refractors (like the 120ST) the faster the focal “f” ratios (lower f ratings), the more false color the scope will produce. The Orion 120mm f8.3 refractor will capture the same amount of light as the 120ST, but produce less false color. The 80ED lens arrangement allows much higher magnifications without false color, but will not capture dim objects as well. So why would anyone choose to own “fast” achromatic scopes then? They provide more spacious wide fields of view, which are excellent for observing larger objects at low to medium magnifications (large star clusters, big swaths of the Milky Way, etc.) Also they are also popular for astrophotography I am told (though I am not well-versed in astrophotography). The slower achromatic refractors are better suited for planetary observations. Those with medium focal ratios are a compromise between the two and are described as more “general purpose” scopes (like the 120mm f8.3). They allow higher magnifications with less false color than “fast” a chromatics. But they are not nearly as color-free as the “ED” lens is. Lastly, larger (heavier) scopes require sturdier (more expensive) mounts and tripods. It really comes down to what types of objects you like to view. If you can, try to attend a “star party” so you can actually look through different types of scopes. This will give you a much better feel for their different performance parameters. Hope that helps explain it a bit. Others may have different thoughts...(Scott)