Framing the Leo Triplet: M65, M66 and NGC 3628 in one field

The Leo Triplet is a group frame: three galaxies at about the same distance, which NASA gives as roughly 35 million light-years, close enough together that a modest telescope shows them in one field. NASA’s M65 page notes that M65 can be spotted with a small telescope in the same field of view as M66 and NGC 3628.

Geometry of the group

From SIMBAD positions, M65 and M66 are 20.2′ apart, and NGC 3628 lies about 36′ from each, to the north. Drawing all three catalogue outlines (SEDS for M65 and M66, SIMBAD for NGC 3628) and finding the smallest rectangle around them gives about 41′ × 24′, with the long side close to north–south. The chart is centred on M66.

Sensor	Group spans 80 % of the frame	90 %
APS-C IMX571	1591 mm	1790 mm
1-inch IMX533 (square)	766 mm	862 mm
Full frame IMX455	2439 mm	2744 mm

These limits assume the camera is rotated to match the group. In north-up landscape orientation the outline box is about 33′ east–west by 43′ north–south, so a portrait orientation fits more tightly.

Catalogue sizes differ by a factor of two

M65 shows how much catalogues can disagree for edge-on galaxies. SEDS lists 8′ × 1.5′, NGC 2000.0 10′, and SIMBAD 16.0′ × 4.5′. M66 is similar: 8′ × 2.5′ in SEDS against 11.4′ × 6.0′ in SIMBAD. NGC 3628 has 24.4′ × 4.8′ in SIMBAD and 14.8′ in NGC 2000.0. If you plan with the larger SIMBAD outlines, the group’s extent grows, though not by much, because the gaps between galaxies dominate it. Either way, an APS-C frame at the C8-with-reducer focal length of 1280 mm holds the whole triplet.

Small galaxies, so think about scale

Each galaxy is small. At 400 mm with 3.76 µm pixels, the 8′ length of M66 covers about 250 pixels; at 1280 mm, about 790. Whether the extra pixels hold real detail depends on your seeing, which the sampling calculator checks for a given focal length and star FWHM. NASA’s M66 page also describes the interaction within the group: asymmetric spiral arms and an off-centre core in M66, likely caused by the pull of the other two galaxies, which is one reason to frame them together rather than singly.

When

The triplet culminates at local midnight around 11 March, and NASA lists April as the best month for M65 and M66. At about +13° declination it is placed well from both hemispheres: around 53° maximum altitude from 50° N and 42° from 35° S.