Framing the Andromeda Galaxy (M31): focal lengths, rotation and companions

The Andromeda Galaxy is the widest galaxy most northern imagers will ever frame. The SEDS Messier catalogue gives it 178′ × 63′, close to 3° by 1°, and NASA’s Hubble Messier page describes its angular size as about six times the apparent diameter of the full Moon. That size decides almost everything about how you shoot it: the focal length, the sensor orientation and whether one frame is enough.

The focal-length ceiling for one frame

The limit is set by the galaxy’s long axis against the sensor’s long side. Using the exact field-of-view formula with the catalogue size:

Sensor (imaging area)	M31 fills 90 % of the long side at	50 % at
APS-C IMX571 (23.49 × 15.70 mm)	408 mm	227 mm
Full frame IMX455 (36.0 × 24.0 mm)	626 mm	347 mm
1-inch square IMX533 (11.31 × 11.31 mm)	197 mm	109 mm

These figures assume the camera is rotated so the galaxy lies along the long side. The setups table below shows the same thing from the other direction: a 250 mm Petzval on APS-C leaves generous sky around the galaxy, the 400 mm FRA400 is already close to full, and at 550 mm the outline no longer fits.

Why rotation matters here

SIMBAD lists the major axis at a position angle of 35°, measured from north towards east. With the camera in the usual landscape orientation (long side east–west, north up) the galaxy runs diagonally across the frame, and its bounding box is much taller than the 63′ minor axis suggests. The framing calculator reports this as “fits rotated” but not “fits north-up” for focal lengths near the ceiling. Turning the camera by about 55° from landscape puts the disk along the long side and buys back most of the frame.

For a square sensor such as the IMX533, rotation changes less: the frame is the same size both ways, so the diagonal is what matters, and the limit is set mostly by the 11.3 mm side.

Catalogue size versus what a long exposure shows

The 178′ figure is a catalogue outline, not a hard edge. SIMBAD’s dimensions for the same galaxy are larger, 199.5′ × 70.8′, and the faint outer disk keeps going in deep images. If you want dark sky around the galaxy in the finished picture, plan for the larger value: at 199.5′ the APS-C ceiling drops from about 408 mm to about 364 mm. The target framing calculator lets you add a margin as a percentage of the frame for exactly this reason.

Two companions in the same field

M32 and M110 sit inside any frame that holds the whole of M31. NASA describes M32 as one of two satellite galaxies orbiting M31, the other being M110. On the chart above both are drawn at their SEDS sizes (8′ × 6′ and 17′ × 10′). M32 lies about 24′ south of M31’s centre and M110 about 37′ to the north-west, so a frame that clears the main disk also clears them.

Going longer: mosaics

Above the ceiling you either crop the galaxy or build a mosaic. With an APS-C camera at 550 mm the field is 146.8′ × 98.1′; covering the 178′ × 63′ outline plus 10 % margin with 15 % overlap needs two panels side by side with the camera rotated. At the 900 mm and 1280 mm setups in the table, the core alone fills the frame, which is a different picture: dust lanes and the bright bulge rather than the whole galaxy. The mosaic planner works out panel counts for any combination.

When it is well placed

M31 is highest at local midnight around the end of September and well placed in the evening through autumn; NASA’s page gives November as the best month. From mid-northern latitudes it passes almost overhead (the table above lists 81° from 50° N), while from 35° S it never rises more than about 14°, low enough that atmospheric extinction and haze become the main problem.