How much aperture does it take?
Seeing The Double Cluster through a telescope
Two rich knots of stars in the same field, each a hundred or more points of light. Best at low power in a wide field — a small telescope beats a large one here.
Through three telescopes, under a suburban sky
Each circle is the true field of a 25 mm eyepiece. Same target, same sky; only the aperture changes.
Recognisable
Detailed
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 | 6 mm | 40 mm | 70 mm |
| Recognisable | 18 mm | 60 mm | 90 mm |
| Detailed | 65 mm | 110 mm | 140 mm |
When to look
Autumn and winter evenings in the northern hemisphere.