What Aperture Actually Determines
Aperture — the diameter of the primary lens or mirror — determines two things: light gathering (how faint an object you can see) and resolution (how much detail you can distinguish). A 70mm aperture gathers about half the light of a 100mm. For astronomy, this means the difference between seeing a galaxy as a fuzzy smudge vs. resolving its spiral arms.
Magnification is not fixed — any telescope can theoretically magnify to any level. But magnification beyond what the aperture can support produces empty magnification: big, blurry images with no additional detail. The practical maximum useful magnification is 2x the aperture in millimeters. So a 70mm scope maxes out around 140x; a 100mm at 200x.
The 70mm Refractor (Casual Use)
Celestron Frontier 70mm ED or SvBony 70mm APO
Weighs under 1.5kg, fits in a carry-on bag, sets up in 3 minutes. Best for: nature observation (birds, whales), lunar eclipses, wide-field Milky Way scanning. Will show Jupiter's moons, Saturn's rings, and the phases of Venus clearly. Will NOT show detailed planetary features or faint deep-sky objects.
Best for: someone who wants to occasionally look at the moon, the bright planets, or nature, and values portability over performance. Not a serious astronomy instrument but a good outdoor companion.
The 100mm Tabletop Dobsonian (Serious Starter)
Heritage 100p or Zhumell Z100
Weighs 2.5kg, tabletop mount (requires a stable surface — a folding stool works), sets up in 5 minutes. This is the sweet spot in hobby astronomy. Jupiter's cloud bands, Saturn's Cassini division, hundreds of deep-sky objects. The 100mm aperture is where serious astronomy begins.
Not collapsible — requires a carrying case. At 35cm length, it barely fits in a large backpack. The right choice for someone who has caught the astronomy bug and wants to actually see things.
For Birding Specifically
For birding, a good spotting scope (65-80mm) beats a telescope for practical reasons: waterproof, quick focus, binocular-style eyepieces, and close focus down to 2 meters. The Kowa TSN-550 (77mm, $600) is the benchmark. For casual nature observation, the Celestron Land & Sky 60mm ($60) is a reasonable entry point.