RigTonightHow to photograph the Andromeda Galaxy (M31): focal length, filters and exposure
The Andromeda Galaxy is the nearest large spiral galaxy to the Milky Way, about 2.5 million light-years away, and the most famous galaxy in the sky. It's also one of the biggest deep-sky targets: its disc stretches about 3° across, six times the width of the full moon. That makes it a perfect target for short focal lengths, and a challenge to fit at longer ones.
When and where
- Position: RA 0h 43m, Dec +41° 16′, in the constellation Andromeda, a short hop from the Great Square of Pegasus.
- Season: best from August to January in the northern hemisphere, highest in the evening in October and November.
- Height: almost overhead from mid-northern latitudes. From the southern hemisphere it stays low in the north; at 33° south it barely reaches 16°.
Framing
M31 spans about 190′ × 60′. Its two bright companions are M32, close to the core, and M110, a little further out.
| Focal length (APS-C sensor) | Field | What you get |
|---|---|---|
| 135 mm | 10° × 6.7° | A small galaxy in a big star field |
| 250–300 mm | 5.4° × 3.6° at 250 mm | The whole galaxy and companions with comfortable space |
| 400 mm | 3.4° × 2.2° | Edge to edge along the long side of the sensor |
| 600 mm and up | 2.2° × 1.5° at 600 mm | The core and dust lanes; the whole disc needs a mosaic |
Rotate the camera so the galaxy runs along the long side of the sensor. At 400 mm on APS-C there's very little room, so many imagers use a 0.8× reducer or a full-frame camera.
It needs dark skies
M31's bright core is easy, but its spiral arms and dust lanes are faint and spread out. Like all galaxies, it shines in ordinary starlight across the whole spectrum, so:
- Shoot near new moon. A bright moon swamps the arms.
- Skip narrowband and dual-band filters for the main image. They block most of the galaxy's light.
- Avoid light-pollution filters where you can. They dim the galaxy and shift its colour. From a bright site, more total exposure and good gradient removal usually beat a filter.
- Go darker if you can. An hour under a Bortle 3 sky can beat a whole night in the suburbs. See The Bortle scale explained.
Exposure
- Subs of 60–180 seconds with a cooled camera, or 30–120 seconds with a DSLR, depending on your f-ratio and sky.
- Aim for at least 3–4 hours in total; the dust lanes and outer arms keep improving well beyond that.
- The core is much brighter than the arms. A few short subs (10–30 s) blended in keep it from burning out, just as with the Orion Nebula.
The extra touch: hydrogen regions
With a mono camera, an Hα filter picks out the pink star-forming regions dotted along Andromeda's arms. Shoot it on moonlit nights, then blend it into the red channel of your broadband image.
Example rigs
RigTonight's Tonight tab shows how many dark, moon-free hours Andromeda gets from your site tonight, and the Targets tab draws your own camera frame on it.