How much aperture does it take?
Seeing The Whirlpool Galaxy through a telescope
Two faint grey patches close together. From a dark sky and 250 mm the spiral arms appear, and the bridge to the companion galaxy is visible — one of the few places an amateur sees structure in another galaxy.
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
Not visible
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 | 36 mm | 140 mm | 220 mm |
| Recognisable | 80 mm | 200 mm | 280 mm |
| Detailed | 210 mm | 330 mm | 410 mm |
What more aperture buys
This is the target that most rewards a drive out of town. The difference between Bortle 6 and Bortle 4 is larger than the difference between 200 mm and 300 mm.
When to look
Spring evenings in the northern hemisphere.