Framing the Whirlpool Galaxy (M51): focal length and sampling

For the Whirlpool the question flips. With M31 you ask how short you can go; with M51 you ask how long you can usefully go. The SEDS catalogue gives 11′ × 7′, NGC 2000.0 the same 11′ and SIMBAD 9.0′ × 7.8′: about a third of the Moon’s width.

How small 11 arcminutes is on a sensor

Setup (from the table below)	Field	M51 spans
250 mm + APS-C	322.8′ × 215.8′	3 % of the long side
550 mm + APS-C	146.8′ × 98.1′	7 %
900 mm + 1-inch IMX533	43.2′ × 43.2′	25 %
2032 mm + 1-inch IMX533	19.1′ × 19.1′	57 %

To make the galaxy fill half the long side, an APS-C IMX571 sensor needs about 3700 mm and a 1-inch IMX533 about 1770 mm. Those are SCT and long-Cassegrain focal lengths. With a small refractor the galaxy is a target for cropping rather than for filling the frame.

The real limit is sampling

At 2032 mm a 3.76 µm pixel covers 0.38″. If your stars measure 2.5″ FWHM, that is more than six pixels across each star, well past the 2–3 pixel band most sampling guidance aims for. The image is not sharper for it; the same light is spread over more pixels, each with its own read noise. Two common answers are binning 2×2 or 3×3 at capture or in software, or a reducer: the Celestron f/6.3 reducer in the 1280 mm setup gives 0.61″ per pixel with the same camera. The sampling calculator shows the binning options for your own pixel size, and the sampling guide explains where the 2–3 pixel rule comes from.

The companion: NGC 5195

M51 is two galaxies. NASA describes NGC 5195 as the small, yellowish galaxy at the outermost tip of one of the arms, passing behind the Whirlpool. Frame for the pair, not for the catalogue ellipse of the main spiral: leave the companion’s side of the frame with as much room as the other. The chart above draws only the catalogue outline of M51 itself.

Orientation and composition

SIMBAD’s position angle for M51 is 28°, so the outline is slightly taller than wide with north up. Because the galaxy is nearly round in the sky, camera rotation matters less here than with edge-on galaxies; a square sensor such as the IMX533 frames it the same way at any rotation.

When to shoot it

The galaxy is highest at local midnight in mid-April; NASA notes it is easiest to spot in May, when it is high in the evening sky. At +47° declination it passes within a few degrees of the zenith from mid-northern latitudes (87° from 50° N) but reaches only about 8° above the horizon from 35° S.