Framing the Orion Nebula (M42): focal lengths, M43 and the bright core

M42 is bright, close and large. NASA describes it as the closest large star-forming region to Earth, 1,500 light-years away and visible to the naked eye below Orion’s belt. That combination makes it forgiving to frame and demanding to expose.

Which size to use

The SEDS Messier page lists two sizes for M42: 85′ × 60′ in its data table and 66′ × 60′ in its text. The other catalogues on record fall in the same range: NGC 2000.0 gives 66′, Sharpless 60′ and Lynds 90′ × 60′. This page uses the 85′ × 60′ table value. None of these catalogues gives an orientation, so the chart draws M42 as a circle of its largest dimension rather than guessing which way the long axis runs.

Focal lengths for the whole nebula

Sensor	Fills 50 % of the frame	70 %	90 %
APS-C IMX571	450 mm	630 mm	810 mm
Full frame IMX455	688 mm	963 mm	1239 mm
1-inch IMX533 (square)	229 mm	320 mm	412 mm

“Fills” here means the tighter of the two directions, assuming the camera is rotated so the long axis of the outline lies along the long side of the sensor. Because the catalogues give no orientation, a cautious plan treats the nebula as an 85′ circle; then the short side limits, at about 570 mm on APS-C for a 90 % fill. In the reference setups, the 550 mm Esprit with an APS-C camera is a balanced frame (61 %), and the 400 mm FRA400 leaves room for the fainter nebulosity around it (44 %). At 900 mm and beyond with a 1-inch sensor, the frame shows only the central region.

M43 comes free

M43, de Mairan’s Nebula, is 7.9′ from M42’s position in SIMBAD and 20′ × 15′ in the SEDS catalogue. It always lands in the same frame as M42. How well the two separate in the image is a question of scale: at 250 mm with 3.76 µm pixels (3.10″ per pixel) the 7.9′ gap is about 150 pixels; at 550 mm (1.41″ per pixel) it is about 340.

The bright core is the real problem

The Trapezium, the group of massive young stars at the heart of the nebula, sits inside the brightest part of M42. NASA’s page notes that the Trapezium stars were born together in this region. For imaging, that bright centre sits inside much fainter nebulosity. If sub-exposures long enough for the outer regions saturate the core, one approach is to capture a short set for the core and a long set for the rest and combine them; the integration time guide explains how signal-to-noise scales with the time you put into each set.

Season and altitude

M42 culminates at local midnight in mid-December, and NASA gives January as the best month. Near the celestial equator (Dec −5.4°), it is reachable from both hemispheres: about 35° maximum altitude from 50° N, 50° from 35° N and 60° from 35° S.