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M46
open cluster in Puppis
M46 is a rich, fairly faint open cluster in Puppis with a small planetary nebula, NGC 2438, that appears to lie inside it, though it is actually in front. It is a pretty pair with the nearby brighter cluster M47.
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// Key facts
- Catalog
- M46
- Type
- open cluster
- Constellation
- Puppis
- Size
- 27′ across
- Brightness
- Magnitude 6.1
- Distance
- about 5,000 light-years
- Peak month (any location)
- Jan
- Typical prime months
- Nov to Mar
- Difficulty
- Intermediate
- Filter
- No filter (broadband)
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How to image M46
- M46 and M47 are only about 1.5 degrees apart and fit a single wide frame.
- At 27 arcminutes, use 500 to 800 mm for M46 alone.
- NGC 2438 is tiny, so only a very long focal length shows its shape.
Framing: what focal length?
The focal length at which M46 spans about 60% of the long side of the sensor, leaving room around it. Treat it as a starting point; the right framing is a matter of taste.
| Sensor | Focal length |
|---|---|
| 1-inch sensor (13.2 mm) | about 1010 mm |
| APS-C (23.5 mm) | about 1800 mm |
| Full frame (36 mm) | about 2750 mm |
How high does it climb from your latitude?
The highest point M46 reaches in the sky. Above about 60° is excellent, 30° to 60° is workable, and under 30° you are looking through a lot of air.
| Latitude | Highest point | Rating |
|---|---|---|
| 55°N (Scotland, Alaska) | 20° | Stays low |
| 45°N (Minneapolis, Milan) | 30° | Workable |
| 35°N (Los Angeles, Tokyo) | 40° | Workable |
| 25°N (Miami, Cairo) | 50° | Workable |
| 35°S (Sydney, Santiago) | 70° | High in the sky |
Also worth imaging
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