How much aperture does it take?
Seeing The Andromeda Galaxy through a telescope
A large, soft, oval glow — brighter towards the middle and fading with no edge you can point at. Bigger than the full Moon, and almost nobody sees it that way because only the core is bright enough.
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.
Not visible
Detectable
Recognisable
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 | 90 mm | 170 mm |
| Recognisable | 48 mm | 160 mm | 240 mm |
| Detailed | 210 mm | 330 mm | 410 mm |
What more aperture buys
The dust lanes need about 250 mm and a genuinely dark sky. The two companion galaxies, M32 and M110, show in 80 mm.
When to look
Autumn evenings in the northern hemisphere.