Wonders in the Sky Open Cluster M46 Back | Up | Next

Prime-Focus Tracked
Open Cluster M46

Because this cluster and planetary nebula are so bright, they can be shot in a single 1 minute exposure from a reasonably dark-sky observing site.

This long of an exposure at a long focal length requires that the telescope be on an equatorial mount that is correctly polar aligned and tracking the stars.

M46 is a beautiful large open cluster located in the constellation of Puppis.

It contains a surprise - the small planetary nebula NGC 2438, seen here on the north side of the cluster at the top.

The stars of the cluster shine with a combined brightness of 6th magnitude. M46 is about 27 arc minutes in diameter, making it almost as large as the full moon.

Planetary nebula NGC 2438 is about 1 arc minute in diameter and shines at magnitude 10. The planetary nebula is not really part of the cluster, however. It lies well in front of the cluster.

A 6th magnitude star can be seen near the center of the planetary, but it is not the true central star that powers the nebula. The central star is very faint at 17th magnitude and is not visible in this photo. The bright 6th magnitude star is probably a member of M46, that just happens to be in a chance alignment with the planetary nebula.

NGC 2438 appears blue-green in this image, glowing in the light of Oxygen III emission. This is the brightest wavelength in the nebula to which the camera is sensitive.

This image is a single 60-second exposure taken with the Stellarvue SV70ED at f/6.

Image Data

  • Lens / Scope: Stellarvue SV70ED ED doublet refractor
  • Focal Length: 420mm
  • F/stop: f/6
  • Exposure: Single 1-minute exposure
  • Mount: Orion Sirius polar-aligned German-equatorial mount
  • Guiding: None
  • Camera: Unmodified Canon EOS 1000D (Digital Rebel XS)
  • Mode: JPEG
  • ISO: 1600
  • White Balance: Custom, set on sky background
  • In-Camera Noise Reduction: Off
  • Filter: None
  • Temp: 35F
  • Start Time: 10:20 p.m.
  • Date: March 24, 2009
  • Location: Maxwell, NJ
  • Calibration: None
  • Processing: Standard in-camera JPEG processing. Cropped and saved in IrfanView.



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