Framing the Triangulum Galaxy (M33): the 400–600 mm sweet spot

M33 sits in an awkward middle ground. It is too large for a typical SCT and too small to fill the frame of a 250 mm Petzval, so the choice of focal length changes the picture more than it does for most galaxies. NASA calls it the third-largest galaxy in our Local Group, after the Andromeda Galaxy and the Milky Way.

Three catalogue sizes, one galaxy

The catalogues on record disagree by more than usual:

Source	Size
SEDS Messier catalogue	73′ × 45′
NGC 2000.0 (Sinnott 1988)	62′ (largest dimension)
SIMBAD	60.3′ × 35.5′

The framing numbers on this page use the SEDS outline, the largest of the three. The difference is not academic: with an APS-C sensor, a 73′ galaxy reaches 90 % of the long side at about 1000 mm, while a 60′ galaxy would reach it at about 1200 mm. If you want visible sky beyond the faint outer arms, plan with the bigger number and add margin in the target framing calculator.

Focal lengths that work

Computed for the SEDS outline with the camera rotated to suit:

Sensor	30 % of long side	50 %	90 %
APS-C IMX571	332 mm	553 mm	996 mm
1-inch IMX533 (square)	160 mm	266 mm	479 mm
Full frame IMX455	508 mm	848 mm	1526 mm

In the reference setups below, the 550 mm Esprit with an APS-C camera puts the galaxy at half the frame, a common composition with room for the surrounding star field. The 400 mm and 250 mm setups make it small (36 % and 23 %), and the 900 mm setup with a 1-inch sensor no longer fits it.

Orientation

SIMBAD gives a position angle of 22° for the major axis, close to north–south. With the camera in landscape orientation and north up, the long axis of the galaxy runs across the short side of the sensor. At 550 mm on APS-C that still fits, but near the 1000 mm ceiling a portrait orientation (camera turned about 70° from landscape) is what makes the galaxy fit.

Going in close: NGC 604

At long focal lengths the frame stops being about the whole galaxy. NASA’s page identifies NGC 604 as the largest star-forming region in M33, a compact target in its own right inside the disk. With an SCT at 1280–2032 mm, a crop on NGC 604 and the inner arms is a different project from the whole-galaxy frame; check the image scale against your seeing with the sampling calculator before committing to the longest focal length, because the 2032 mm setup samples at 0.38″ per pixel with 3.76 µm pixels.

Season and altitude

M33 is highest at local midnight in mid-October, two weeks after M31, and NASA gives November as its best month. At +30.7° declination it climbs to 71° from 50° N and nearly overhead from 35° N, but only about 24° from 35° S, which puts it in the thicker, brighter air near the horizon for southern observers.