How much aperture does it take?
Seeing The Orion Nebula through a telescope
A grey-green fan of light wrapped around four close stars. It is the one deep-sky object that survives a city sky, and the only nebula most people ever see colour in.
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.
Detectable
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 | 6 mm | 60 mm | 110 mm |
| Recognisable | 30 mm | 100 mm | 150 mm |
| Detailed | 125 mm | 200 mm | 250 mm |
What more aperture buys
The four stars at the centre are the Trapezium; a 100 mm splits all four. Larger apertures add wings and texture rather than colour.
When to look
Winter evenings in the northern hemisphere; summer in the southern.