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Something Pink for Valentine's Day
updated: Feb 15, 2014, 11:00 AM
By Chuck McPartlin
Last time, we visited the bright Winter Circle of stars around Betelgeuse,
the red supergiant star in the shoulder of Orion, the Hunter. It's actually
a bit too orange for Valentine's Day, so let's look at something flagrantly
pink - the Great Nebula in Orion. You can find it in his sword, as shown
below. It's easily visible as a cloudy spot in binoculars, which is
appropriate, since nebula is Latin for cloud. You won't see color, though,
unless you take an image, since your eye just isn't sensitive enough at
low light levels.
The Orion Nebula, also known as Messier 42, M42, or NGC 1976, is a type
of object called an emission nebula. The ultraviolet light from massive
stars formed within the nebula causes the hydrogen gas making up the cloud
to fluoresce, emitting a distinctive pinkish wavelength which dominates
the light we receive from the nebula. There's also some blue reflection
from dust particles, and greenish emission from ionized oxygen. Dark clouds
of cool opaque gas intrude around the edges.
The entire area of the sky encompassed by the Winter Circle is full of
dense molecular clouds made up of primordial hydrogen, plus oxygen and
silicate and carbon dust burped off by earlier generations of stars. Some
are dark, but where stars have coalesced we see the telltale glow of
infrared and visible emission.
M42 is so photogenic that, sooner or later, almost everyone with an
interest in astrophotography takes a shot at it. You don't even need a
telescope; only a camera capable of taking time exposures with some degree
of low-light sensitivity. Just remember that if you're not moving the
camera to compensate for the Earth's rotation, your star images will show
trails unless you limit your exposure time. You'll get decent results if
you follow the "rule of 700", limiting your unguided exposure time in
seconds to about 700 divided by your lens focal length in millimeters. Thus,
a 55 mm lens would be limited to 30 seconds, a 100 mm lens to 7 seconds, and
a 300 mm lens to 2 seconds. Use high ISO to capture as much light as you can,
although higher ISO means higher noise levels. Stay away from light pollution
if you can. These days, that's hard.
Here are some single shots from the wilds of Noleta, with a fair amount of
light pollution in the southern sky where Orion spends his time. These
images were taken with a Nikon D5100 DSLR, using a 55-300 zoom lens set to
f5.6, at ISO 400. The camera was piggybacked on a tracking telescope to allow
longer exposures. This first shot was at 55 mm for 1 minute. You can see the
three belt stars at the top.
Zooming in a bit, here's 100 mm for 5 minutes. M42 is more prominent, and you
can see hints of the Flame Nebula (NGC 2024) near Alnitak, the leftmost belt
Next is 300 mm for 3 minutes. The field of view is getting smaller, of course
If you can shoot through a telescope, you can start to see much more detail in
the Orion Nebula, and you'll want to be tracking the sky. You can use a video
camera or webcam, and capture images. Whether with video or DSLR, image processing
to align and stack multiple short images can give you the equivalent of a much
longer exposure, reduce noise levels, and bring out fainter details.
This is a capture from a black and white video camera which internally stacked
256 images, about 8.5 seconds of exposure time.
The camera doesn't have the dynamic range of your eyeball, so the central core of
the nebula is burned out. You can see lots of structure in the cloud, but grayscale
just doesn't match the gotcha of color. Here's a color videocam shot, integrating
for 28 seconds.
More wispiness is visible, and it's easier to differentiate the gas and dust, but
the resolution is pretty low, and an even larger area of the core is burned out.
Even extensive image processing doesn't help much.
Composing a single image from several DSLR shots of varying exposure times is a way
to get around the dynamic range issues, and you get much higher resolutions. The
tradeoff is spending more time on the computer processing.
Here's a 5 second shot through a 5.1 inch, f/6.8 refractor acting as the lens for
the Nikon DSLR, now at ISO 6400. The central region has not blown out.
Here's a 30 second shot. More nebulosity is visible (plus some geosynchronous satellites),
but the central region has blown out.
Combining several images at a range of exposure times lets you have the best of both
worlds. Here is the result from combining a single 2 second image, a single 10 second
image, a stack of five 20 second images, and a stack of ten 30 second images. Using
layers and masks, the details in the central region can be preserved, while also showing
a large area of the dimmer nebulosity.
There are plenty of astrophotography tutorials available on the web, and the Santa Barbara
Astronomical Unit, your local astronomy club sponsored by the Santa Barbara Museum of
Natural History, has lots of experienced astroimagers who will be glad to show you the
ropes at the Tuesday evening telescope classes.
References for Cloudy Evening
The Astronomical Unit
The Orion Nebula
Astrophotography Techniques by
Photoshop Workflow Tutorial by Tom
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