March: Clusters and Galaxies
M46 and M47 in the constellation Puppis are neglected by northern observers because they are so low in the horizon. Only for a few hours each night during the coldest winter months can these clusters be spotted, hovering above the trees. Like a pair of mismatched jewels, the faint 400 stars of M46 contrast with the bright but sparse 50 stars of M47. M46 (upper right) shows its age of 300 million years with some yellow suns at a distant 5,400 light-years, while younger M47 (lower left) is only 80 million years old and still has a few blue-white brilliant stars at 1,600 light-years. But there are three surprises in this view. In the lower right, the loose open cluster NGC 2423 can be defined from background stars. In the left center, a tiny cluster NGC 2425 can be seen. Best of all, superimposed on the north (right) edge of M46 is the 1.1-arcmin planetary nebula NGC 2438, which is just a foreground object only half the distance to M46. Imaging. Open clusters can appear boring in images, especially if the field of view is so small that the cluster cannot be demarcated from the background. Generate more visual interest by framing this region with a short focal length telescope or a camera lens. But if your field is too large, the planetary nebula in M46 can become insignificant. Either a single-shot color camera or routine RGB imaging works best for open clusters. Luminance layering adds unneeded complexity for bright clusters. If your telescope has imperfect color correction, refocus between filter changes to keep stars sharp. If you use single-shot color, image close to the meridian to reduce blur from atmospheric refraction at low altitudes.