How much aperture does it take?
Seeing The Hercules Cluster through a telescope
A round grey fuzzball at 50 mm. At 150 mm the edges break into individual stars, and at 200 mm it becomes a swarm — the moment most observers name as the one that hooked them.
Through three telescopes, under a suburban sky
Each circle is the true field of a 10 mm eyepiece. Same target, same sky; only the aperture changes.
Not visible
Recognisable
Detailed
The aperture it takes, by sky
Millimetres of aperture to reach each level. Observing heuristics, quoted at Bortle 4 and adjusted per class of sky — somebody else's eyes, somebody else's night.
| Level | Milky Way blazing | Milky Way, barely | No Milky Way |
|---|---|---|---|
| Detectable | 24 mm | 80 mm | 120 mm |
| Recognisable | 60 mm | 120 mm | 160 mm |
| Detailed | 130 mm | 190 mm | 230 mm |
When to look
Summer evenings in the northern hemisphere.