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M46: A Cluster With a Bonus

by Robert J. Vanderbei

Last Thursday the skies were clear in New Jersey and winds were not quite calm but manageable. I took the opportunity to grab a picture of the open star cluster called Messier 46 (M46) using my four-inch refractor telescope and my (monochrome) StarlightXpress CCD camera.

This cluster passes directly overhead for people in the Southern Hemisphere who live at latitude 15 degrees south.

Hence, for those of us in the Northern Hemisphere, the cluster never rises high in the sky.

From my home, the highest M46 gets is 35 degrees up from the southern horizon. Three nights ago, it reached its maximum “altitude” at about 6:45 p.m. local time—a little before sunset.

When I began imaging at 9:30 p.m. it was just 22.5 degrees above the horizon, and when I quit an hour later it was 13.5 degrees up.

But finding it is worth the effort, because this star cluster comes with a bonus.

Along the same line of sight is a planetary nebula called NGC 2438. This nebula was discovered by William Herschel in 1827 (that’s 50 years after Messier added the cluster as the 46th entry in his famous catalog of non-comets).

The nebula is small in apparent size—it would take about 30 of them placed side by side to span the width of a full moon.

It is also very faint to the naked eye, much fainter than it appears in my picture.

In fact, to Herschel and to visual observers today, the nebula appears more like this:

Do you see it? It’s there. You might need to turn the lights off to see it.

The first, more dramatic picture was taken using a pair of narrow-band filters. Here’s how they work:

Planetary nebulae are examples of so-called emission nebulae. This means that they emit light of only a few very specific wavelengths. In visible light, the strongest emission wavelengths are called H-alpha, which is deep red, and O-III, which is blue-green.

For a couple hundred dollars, I bought myself a pair of these filters. They let almost 100 percent of the nebula’s light through, but they block almost all of the light pollution, including glow from the moon if it’s up.

They also block most of the starlight, making the stars in the picture less bright relative to the nebula.

The faint—but visually accurate—image was taken with standard red, green, and blue filters. For each of those filters, the total exposure time was 2.5 minutes.

The brighter image is a combination of the red, green, and blue filtered images plus H-alpha (added to the red) and O-III (added to both the green and the blue). For each of the two narrowband filters, the total exposure time was 17.5 minutes.

Robert VanderbeiRobert J. Vanderbei is chair of the Operations Research and Financial Engineering department at Princeton University and co-author of the National Geographic book Sizing Up the Universe. Vanderbei has been an astrophotographer since 1999, and he regularly posts new images on his astro gallery website.


  1. Julian
    April 29, 2011, 7:06 am

    Awesome,I have just started observing the stars from my friend an avid star gazers telescope.We meet over the weekends and watch the stars.

    Holistic Doctor