good tool for celestial locating for the beginning astronomer
good camera for SLR beginners
to pick a lens for Milky Way photography
camera settings to use
methods for astrophotography
Stacker home page
Stacker Users Manual (HTML version)
Deep Sky Stacker tutorial
GIMP home page
workflow of the entire process
of all of John's articles
Services Web Site
|Here in Central New Jersey in 2015
the sky is polluted with ordinary air pollution as well as an
overabundance of light, which is another kind of pollution that
obscures pictures of stars and such at night. It all makes it hard to
even see stars, much less get an image of the not-so-bright ones.
Photographers are shelling out thousands for trips to the Hawaiian
Islands, the Atacama Desert in South America and other exotic places
near and far where the sky is very dark and clear.
But it's also possible to photograph constellations, star clusters,
nebulae and even entire galaxies from right here in Hamilton using
relatively inexpensive equipment, some free software plus a little
know-how. The trick is to
- Find a good, dark spot
- Pick a clear-as-can-be night
- Use the Rule of 600 to obviate the effect of the Earth's rotation on
longer exposures, and take multiple raw-image photos of the same
- Stack those photos one on top of the other and save a single,
resultant TIFF image file using Deep Sky Stacker, a free utility
- Add some contrast to the TIFF using Photoshop or GIMP, the free photo
editor, and save it as a JPEG.
An SLR (single lens reflex) camera in manual mode allows for precise
shutter duration, canceling of all automatic functions; and a “raw
image” mode that preserves as much of the sensor's data as possible.
They say it's possible to use a point-and-shoot camera, but this
article assumes you have access to an SLR and some familiarity with it,
as well as some knowledge of what there might be of interest in the sky
on any given night. Any lens may be used but the best ones allow for
wider aperture settings to let in lots of light. A tripod is necessary
because of the long durations required for the exposures and also
because the camera must stay pointed at exactly the same location over
the course of the multiple exposures. A red flashlight helps to avoid
stumbling around in the dark and to check camera and lens settings.
A good spot on a clear
The main source of light pollution in Hamilton is the city of Trenton.
It lends a reddish cast to images, especially those shot in a northerly
direction. Go out after 2:00 A.M. when most good folks are off the road
and safe in bed, and try to find a spot where few local lights can be
seen, especially those facing the camera. Any clouds that pass in front
of the camera will obscure the resulting exposures, though on some
nights the clouds may be few enough that it's not a problem. But clouds
may also add drama to nighttime landscape photos that include the sky.
It all depends on the light and the photographer's intentions.
Duration of exposures, the
Rule of 600 and focusing at night
SLR lenses are of two general types, prime and telephoto. Telephoto
lenses, like the Nikon 70-300mm f/4.5 VR Zoom Lens, have variable focal
lengths and can be used for wide-angle as well as distance photography.
Prime lenses, like the Samyang 24mm f/1.4 Wide Angle Lens, are not
adjustable that way and are used at the single focal length they
provide. Long exposures are necessary in night photography because
there's less light and so it takes longer for the camera's sensor to
gather enough of it to create a visible image. As the Earth rotates and
the tripod and camera stay in place, if the shutter is left open too
long, the stars form a streak in the image instead of a point. What's
needed is to leave the shutter open just long enough for the stars to
appear as points. For wide-angle images, shutter duration can be longer
than for more close-up, telephoto images.
This is where the Rule of 600 comes in. It turns out that if you divide
600 by the focal length of the lens (from 70 to 300mm, or 24mm in the
above cases) and round it down, you get an optimum shutter duration in
seconds for use in eliminating star trails. So, for example, 600 ÷ 70 =
8.5714 and therefore, 8 seconds is a good time to leave the shutter
open when shooting at 70mm with the 70-300mm lens. Keep your feet in
place and use the camera's timer mode to lessen the possibility of
shake. Shoot from 6 to 17 camera-raw exposures (required by Deep Sky
Stacker, JPEGs won't do here) all at the same settings, one immediately
after the other. Then put the lens cap on and shoot another four or
five with the same settings so that DSS can have some “dark” images to
use to make a darker background for the spaces between the stars. There
is a link at the left to a PDF file with good camera and lens settings
to use to get started.
Focusing at night can be daunting because of the low light. Jerry
Lodriguss has an exhaustive tutorial on the subject, linked at the left.
Deep Sky Stacker (DSS)
There are loads of Deep Sky Stacker tutorials available, both video and
as text-with-screengrabs. Find a link to the left to download the
latest version of the Windows-only utility. After your night
photography session, and before you begin with DSS, look closely at the
source images from your camera and make a note of which set of star
images and of dark images you are associating with each view you
photographed. You don't want to confuse the issue by mixing in images
of different views, and there may be some you should discard for
blurriness or in case a plane left a streak.
At one point in the DSS process, you get to choose the star detection
threshold- that is, how many stars will show in the resulting image.
The lower the threshold, the more and fainter stars will get to be in
the result. For the other settings, go with the recommended DSS
defaults for your first tries. Be prepared to run the program more than
once for each set. Save the result as a 16-bit TIFF file to use with
the next step. Experiment with the settings options DSS offers.
GIMP2 (the GNU Image
Find another link at the left to a workflow tutorial that includes an
essential video on using the Photoshop or GIMP Curves and Levels Tools.
PS Elements won't do because the Curves tool is absent, but the free
GIMP has Curves and Levels tools that work exactly like those found in
the very expensive PS. At this stage, you use these tools to add
contrast to bring out the fainter stars, to add some blue or purple to
the background and finally to bring up the native colors of stars in
images that would otherwise appear more black and white. Save the final
image as a high-quality JPEG.
If you visit this article online, you can see a shot of the Orion
Nebula. Just follow the link at the left to John's articles.
February 11, 2015