(c) Skyhound

I took this image with iTelescope T21 on the night of March 26th, 2016. This is a combination of one ten-minute exposure in the Red filter, and three five-minute exposures in the Blue. I used SkyTools 3 to plan the observation. In addition to calculating the best times to expose, SkyTools helped me determine that I should expose longer in the blue filter than in the red to get a color-balanced image. The Blue images were stacked via DeepSkyStacker. All other processing was done in Photoshop. The green channel is a 50-50 combination of the red and blue. North is to the left and east is down.

In a telescope, NGC 2371-2 appears as two small hazy spots of light. These are the brightest portions in the above image. The great visual observer and discoverer of Uranus, William Herschel, discovered this planetary nebula in 1785 using his 18.7 inch telescope. He considered it to be two separate objects, so each lobe was included in the NGC catalog with its own NGC number. It has been called the Peanut and the Gemini nebula. The latter name refers to the fact that it is a binary nebula, and in the constellation Gemini (the twins). 

The visual extent of the brighter lobes of the nebula span about 44" and have an integrated magnitude of ~12-13. The central star is magnitude 14.8, which is within reach of an eight-inch (20 cm) telescope for an experienced observer at a dark site. The lobes can be glimpsed in a six inch (15 cm) telescope, but only a faint haze is visible under the darkest conditions. The southwest lobe is the brighter of the two and the most likely to be visible in small scopes. 

The trick to spotting the Gemini nebula is to use as much magnification as the conditions will permit. The difficult part is to find the correct spot in the sky to center on in the lower-power eyepiece before moving to higher magnification. A chart that shows the star field as seen in the eyepiece is essential.

This nebula is an amazing sight  when viewed in a large-aperture instrument (> 16-inches, 41cm). In such apertures the bipolar structure is seen clearly. Again, use as much magnification as conditions will permit. 

A good challenge for large instruments is to detect the faint bridge of nebulosity that stretches between the lobes, and the faint halo that surrounds the entire object.  For a real challenge, have a try at the faint patches that lie at right-angles to the lobes (see image).  I am not aware of anyone successfully observing them. 

Here is my log entry for the nebula in my 18-inch Dob: 

Wow! VERY cool!

 25 mm -- Obvious, apparently double.
12.4 mm -- Bipolar with central star.
12.4 mm x 2 -- Very cool. Twin lobes around a central star. The western lobe is brighter. The eastern lobe is larger. Some haze between, around star. Haze surrounding?

Like all planetary nebulae, NGC 2371-2 is the result of a star shedding its outer envelope on the way to becoming a white dwarf. According to Jim Kaler of the University of Illinois, "The whole structure is illuminated by a hot ... central star of 118,000 Kelvin, the star shining with the light of perhaps 700-1400 Suns. The star ... should shortly start to dim toward the realm of the white dwarfs, as the nebula -- expanding at the good clip of 43 kilometers per second -- slowly dissipates into the mists of interstellar space."

Behind the Scenes: How I Plan my Observations

Note that we have made the window as small as possible to fit the space here.

The SkyTools 3 Nightly Planner is set up (above) for February 16th in Death Valley for a 5-inch (12 cm) Refractor. The location has a naked-eye limiting magnitude of 6.9. The Skyhound observing list is loaded and the Gemini Nebula is selected. 

The red-dashed line in the graphic at the top displays the altitude of the nebula during the night. The teal line is the altitude of the moon. The shading is the actual brightness of the sky taking into account moonlight, twilight, and light pollution. 

By looking at the red line we can see that the best time to observe this planetary is between 21:00 and 22:00. Looking at the table, the best view will be at 21:25, and the good views will last until 23:18, when the moon rises. The nebula is detectable in this telescope under these conditions. These calculations are unique to SkyTools, which samples the visual contrast of the object as seen in a given telescope during the night. There is no other software that can do this.

The location of the Gemini nebula is fairly easy to find to the southwest of Castor.  I like to start at Castor and use the two bright stars that form a nearly equilateral triangle to the southwest as a guide to tell you which direction to sweep. Once you get to the finder or wide field eyepiece, the nebula can be found between the widest pair of three 7th magnitude stars that lie in a straight line.

Above is the simulated view in the same telescope with a 12.5 mm eyepiece.  Open to view a larger version. The arrow shows the direction that stars will drift if the drive is off, for orientation. Even if the nebula isn't visible in a lower-power eyepiece, it is fairly easy to find the correct spot in the star field before inserting one of higher power.

Greg Crinklaw Developer of SkyTools 

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