The Deepsky: Top-100

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Deepsky Top-100 (1): M46 and M47

I vividly remember a cold winter night I spent at my favourite dark sky site a few years ago. It was already dark when I arrived. Before setting up the telescope I scanned the night sky with my naked eyes. The sky was perfectly transparent, and the naked eye limiting magnitude was 6 to 6.5. It was freezing. Due to the wind it felt like minus 15 degrees Celsius. After 10 minutes, just above the tree line, I noted a bright spot. When I looked at this object using averted vision, a slightly dimmer object appeared to the left of it. I didn't have a clue what these objects where. A fellow observer told me it was M 46 and M 47.

I immediately got out my 7x50 binoculars. M 46 was just a hazy glow of unresolved stars, a bit like a planetary nebula, while M 47 was already partially resolved. It was time to get out the telescope. Here's a finder chart for M 46 / M 47. Limiting magnitude is 6.5 and the red circles represent a field of view of 4 degrees. Start from Sirius in Canis Major.

Generated with SkyTools 2 by Capellasoft

Click here to download a printable finderchart (PDF)

Later that night I turned my telescope at M 47 first. It’s a bright (magnitude 4.4) open cluster with a diameter of 25'. There where 6 or 7 bright stars and maybe 30 others, with a great variety in magnitude. The cluster looked very irregular. You could see some knots and strings of stars. It didn’t impress me at first, until I moved on to M 46, which lies only about 1.5 degrees to the east of M 47. I was shocked by the sheer contrast between these two open clusters. In this open cluster with a magnitude of 6, I counted about 60 to 70 stars, seemingly of an almost equal brightness. The round cluster had a diameter of 25 arc minutes. At 100x and 133x the 11th magnitude planetary nebula NGC 2438 was very easy to see. I spent quite some time this night looking at M 46 and M 47.

Here's a map with M 46 and M 47 in a 3.5 degree field of view. Limiting magnitude is 9.5.


Generated with Skytools 2 by Capellasoft

The day after we visited our dark sky site, I tried to observe M 46 and M 47 from my light polluted backyard. With my naked eyes I could not detect them. Using my 7x50 binoculars, I only managed to spot M 47. The dimmer M 46 was impossible to find! Then I got out the telescope. M 47 was easy with the telescope but M 46 could only be seen as a very faint glow. Averted vision showed a few stars in the background glow. This object was completely washed out by the heavy light pollution in the southern direction.

That night I made this sketch of M 47. As you can see, there wasn't much left of the fainter stars.


For further details on the sketch and an observing report follow this link to my deep sky log.

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