Framing the Pleiades (M45): cluster size, dust and focal length

The Pleiades are the opposite of a galaxy target. There is no faint disk to hunt for: the cluster’s brightest stars are visible to the naked eye, and NASA lists the cluster at apparent magnitude 1.6. The framing problem is deciding how much of the surrounding dust to include.

The catalogue size and what it covers

The SEDS Messier catalogue gives a single dimension, 110′, close to two degrees. That describes the cluster of bright stars; the dust that reflects their light is not a sharp-edged object. NASA’s page explains the blue-white wisps near the star Merope as streams of dust particles, with smaller particles slowed by the star’s radiation pressure more than larger ones. In deep exposures this material spreads well beyond the cluster, so a frame sized exactly to 110′ crops it.

Focal lengths

Treating the cluster as a 110′ circle:

Sensor	Fills 50 % of the short side	70 %	90 %
APS-C IMX571	245 mm	343 mm	442 mm
Full frame IMX455	375 mm	525 mm	676 mm
1-inch IMX533 (square)	177 mm	247 mm	318 mm

Because the cluster is round, the short side of a rectangular sensor is what fills first. In the reference setups, the 250 mm Petzval with an APS-C camera shows the cluster at about half the frame with plenty of dust-field around it; the 400 mm FRA400 is at 82 %, already tight once the outer dust is counted; the 550 mm Esprit no longer fits the catalogue size.

Bright stars and the frame

The cluster’s bright stars are the main design element, and they also decide how the frame will look. Fast optics and long exposures saturate them quickly, and stars near the edge of a frame show any field curvature or tilt in the optical train first. A short-focal-length Petzval or a refractor with a matched flattener keeps the whole field usable; the small refractor comparison lists image circles as published by the makers, which tell you whether a full-frame sensor will be covered to the corners.

Composition

There is little reason to rotate the camera for orientation, since the catalogue size is a circle. Rotation is about where the dust goes: the wisps around Merope and the other bright stars do not spread evenly, so check a wide field in a planetarium program and use the field of view calculator to see how much sky your setup covers before choosing.

Season and nearby targets

The Pleiades culminate at local midnight in mid-November, and NASA names December as the best month. They are visible from both hemispheres: the maximum altitude is 64° from 50° N and 31° from 35° S. The cluster shares the autumn and winter sky with M31 earlier in the evening and with the Orion Nebula later in the night.