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
SkyTools
3, because the astronomy matters.
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