Framing Bode's Galaxy and the Cigar Galaxy (M81 and M82) together

This pair is a framing exercise more than a size exercise. Each galaxy alone is modest (M81 is 21′ × 10′, M82 9′ × 4′ in the SEDS catalogue), but they are 36.9′ apart, and the frame has to hold the gap as well as both disks. NASA notes that in binoculars M81 appears in the same field of view as M82, and the pairing is not only visual: the Cigar’s starburst is attributed to gravitational interaction with M81.

The shape of the pair

M82 lies almost due north of M81. Drawing both catalogue outlines and measuring the smallest rectangle that contains them gives about 51′ × 12′, with the long direction close to north–south. That orientation is the key fact for framing: in the usual landscape setup (long side east–west) the 51′ span falls along the short side of the sensor.

Focal lengths for the pair

With the long side of the sensor turned to run north–south (portrait), the pair fits up to these focal lengths:

Sensor	Pair spans 80 % of the long side	90 %
APS-C IMX571 (23.49 × 15.70 mm)	1261 mm	1418 mm
1-inch IMX533 (11.31 mm square)	607 mm	683 mm
Full frame IMX455 (36.0 × 24.0 mm)	1932 mm	2174 mm

So a C8 with the f/6.3 reducer (1280 mm) and an APS-C camera holds both galaxies in portrait, while the same scope with a 1-inch square sensor (30.4′ field) does not. The setups table below is computed for M81 alone, which is why it shows the 900 mm and 1280 mm combinations as fitting.

One galaxy at a time

At 2032 mm on a 1-inch sensor even M81 alone (21′) slightly overfills the 19.1′ field. M82 alone fits comfortably at that focal length, with its 9′ × 4′ outline using under half the frame. If you plan separate frames, sampling becomes the deciding factor as it does for M51: 2032 mm with 3.76 µm pixels is 0.38″ per pixel.

Orientation details

SIMBAD gives M81’s major axis at a position angle of 157°, so its long axis runs roughly north-north-west to south-south-east, nearly parallel to the line joining the two galaxies. The data set used here has no position angle for M82, so the chart draws its outline unrotated; the framing calculator treats it the same way and reports north-up fit as unknown.

Season

At about +69° declination the pair is circumpolar from much of the northern hemisphere. It is highest at local midnight in mid-February, and NASA lists April as the best month for both galaxies. From 35° S it never rises.