Don't put that camera away when it gets dark, get outside and take pictures! A whole universe of wonderful images awaits you.
Our cameras get most of their workouts during the
daytime, when there's plenty of light from the sun to illuminate our
subjects. If we shoot at night, it's usually with a flash attached to
brighten things up, or with long exposures using man-made light from our
streets or houses. That big dark sky above us at night seems to offer
little that we can see or photograph... but that's not the case. There's
not just a whole other world of things to take pictures of in that dark
sky -- there's a whole universe of opportunity. This how-to article
will provide the basics to let you start taking dramatic photos of the
sky at night.
Astrophotography is the name given to the process
of taking pictures of anything not on the Earth, but out in space. Most
of us probably think of images from the Hubble Space Telescope when the
word is mentioned, but making good astrophotos doesn't require billions
of dollars or a ride in the Space Shuttle. This article will show you
how to start making photos with equipment almost every photographer has
on hand, progress to the next level with some simple equipment you can
buy inexpensively or make yourself, and finish with what you can do if
you decide to invest in some specialized (and sometimes expensive)
equipment.
The first thing to remember when getting into
astrophotography is that the earth rotates on its axis once per day.
Obvious? Sure, but what it means for astrophotography is that you're
never trying to capture a "still" object. The earth's rotation makes the
stars, planets, and the moon appear to move across the sky all night
long. The moon and planets also have their own motions, which complicate
matters. And since most of the things we'd like to take photos of are
awfully dim compared to a typical subject in the sunlight, we're going
to have to use long exposure times to gather enough light to see them,
and they'll be moving (or appearing to) the whole time. There are only
two ways around this constant movement: ignore it, or compensate for it.
Fixed-Position Astrophotography
Let's start with what we can do by ignoring the
earth's rotation. Shooting these kinds of photos only requires a camera
capable of long exposures (a "B" or Bulb setting, or exposure settings
from 30 seconds to several hours). The obvious example is "star trail"
images.
The earth rotates on its axis at about 1/2-degree
per minute -- so the stars will appear to rotate that same amount
overhead. If we set a camera on a fixed tripod, open the shutter, and
let the earth rotate us under the stars, we get an image that shows the
stars as "streaks" or trails as they appear to move across the sky. The
longer we leave the shutter open, the longer the trails will be in our
final image. The image shown here was exposed for about 60 minutes,
giving 15-degrees of arc to each star trail. Notice in the image that
near the center there appears to be almost no star-trailing, and as you
move further out the star trails appear to get longer. That's because
the camera was pointed right at the center of the earth's rotational
axis: Polaris, or the North Star. Since this is the center of rotation,
there's the least amount of apparent movement near this axis. As you
move further away from this point, stars will appear to move more and
more in a straight line until you reach the "celestial equator"
(90-degrees from the North Star in the sky), where the trails are
perfectly straight and will appear longer.
Here's how to take star-trail photos:
• Use a sturdy tripod, and secure it in place or weight it down if you can. Any movement of the tripod will show "squiggles" in your star trails.
• Use a fairly wide-angle lens for best results, the 35mm equivalent of 20-50mm focal length is a good place to start.
• Choose a medium-speed film or a digital-camera ISO of 400-800. That's high enough to record even fairly dim stars, but it shouldn't introduce too much grain or noise.
• Set a medium aperture of f/5.6 to f/11. The stars won't change much in brightness no matter which aperture setting you use, but smaller apertures will reduce the brightness of "skyglow" from nearby towns or other light sources.
• Select a dark location away from city lights if possible. Include something interesting in the foreground (such as the trees above) to give scale to the image and to help show the sky's apparent rotation against the earth.
• Make sure you have new or freshly-charged batteries in your camera. Holding the shutter open for long periods drains batteries fast! If your camera has a DC car-power adapter, or a battery pack, use them.
• Use a cable release or remote release, set manual focus, focus on infinity (put a small piece of masking tape on the lens' focus ring to hold it if you can), and open the camera's shutter. Leave it open as long as possible. Longer exposures mean longer star trails, but also pick up more "sky glow.
• Use a sturdy tripod, and secure it in place or weight it down if you can. Any movement of the tripod will show "squiggles" in your star trails.
• Use a fairly wide-angle lens for best results, the 35mm equivalent of 20-50mm focal length is a good place to start.
• Choose a medium-speed film or a digital-camera ISO of 400-800. That's high enough to record even fairly dim stars, but it shouldn't introduce too much grain or noise.
• Set a medium aperture of f/5.6 to f/11. The stars won't change much in brightness no matter which aperture setting you use, but smaller apertures will reduce the brightness of "skyglow" from nearby towns or other light sources.
• Select a dark location away from city lights if possible. Include something interesting in the foreground (such as the trees above) to give scale to the image and to help show the sky's apparent rotation against the earth.
• Make sure you have new or freshly-charged batteries in your camera. Holding the shutter open for long periods drains batteries fast! If your camera has a DC car-power adapter, or a battery pack, use them.
• Use a cable release or remote release, set manual focus, focus on infinity (put a small piece of masking tape on the lens' focus ring to hold it if you can), and open the camera's shutter. Leave it open as long as possible. Longer exposures mean longer star trails, but also pick up more "sky glow.
Star-trail images can be spectacular, but due to
the motion of the stars, they can be somewhat "abstract." What if you
want to take an image of the stars as we actually see them?
As mentioned before, the stars appear to move at
about 1/2-degree per minute across the sky. If we use a wide-angle lens
(which has a wide field of view), you can leave the shutter open for a
short while before that movement becomes visible in the image. The stars
will still be "trails" in your picture, but the trails will be so small
they're indistinguishable from single points. In the image above, I
used a 14mm lens (with a field of view of 104-degrees horizontally and
81-degrees vertically), and exposed for 30 seconds. In that time, the
stars moved less than 1/4-degree, taking up less than 1/400th of the
horizontal field of view. In the full-sized image you can see that
they're little trails, but just barely. This kind of image shows the sky
as we appear to see it with our eyes. Only the brightest stars will
show up clearly this way, but as you can see from the image above you
can clearly make out the constellation Orion, the Pleiades star cluster,
and other constellations and stars.
Here are some tips for shooting non-trailed fixed-position star images:
• Use a very wide-angle lens -- the wider the better. At 28mm (35mm film equivalent) focal length, you can expose for about 20 seconds without significant trailing. At 50mm, you can only expose for about 10 seconds, which is only long enough to record the very brightest stars. At 14mm, you can shoot for 30 to 40 seconds, which should show more lower magnitude stars.
• As with star-trail images, include something interesting in the foreground! The image above was shot on a night when the nearly-full moon was just rising in the east, and the camera was aimed west, away from the moon. The moon illuminated the foreground without washing out the sky.
• The same setup rules apply as with star-trail images; use a sturdy tripod, a remote release, ISO 400-800, and f/5.6 to f/8.
• Use a very wide-angle lens -- the wider the better. At 28mm (35mm film equivalent) focal length, you can expose for about 20 seconds without significant trailing. At 50mm, you can only expose for about 10 seconds, which is only long enough to record the very brightest stars. At 14mm, you can shoot for 30 to 40 seconds, which should show more lower magnitude stars.
• As with star-trail images, include something interesting in the foreground! The image above was shot on a night when the nearly-full moon was just rising in the east, and the camera was aimed west, away from the moon. The moon illuminated the foreground without washing out the sky.
• The same setup rules apply as with star-trail images; use a sturdy tripod, a remote release, ISO 400-800, and f/5.6 to f/8.
So far we've only looked at what we can do with a
fixed tripod, with our camera still while the sky rotates above us.
We're limited in what we can shoot that way. Most of the really
interesting things in the sky are quite dim, so we need much longer
exposure times in order to see them in our images... but longer exposure
times mean the stars leave trails in our images.
However, we can get longer exposure times without
trails by mounting our camera to something that compensates for the
Earth's rotation. If we could have some kind of rig that rotated around
one axis (like the Earth does), align our rig's axis of rotation with
the Earth's axis, and then rotate in the opposite direction the same
speed as the earth rotates (about ½-degree per minute), then our camera
would stay pointed at the same spot in the sky as the Earth rotated, and
we could do longer exposures without having the stars trail.
Hmm, that sounds pretty complicated, though. How
can we make such a rig? Actually it's very simple, and all you need are a
couple of boards, a hinge, and a couple of bolts... it's called a "Barn
Door" mount.
A Barn Door is simply two pieces of wood mounted
around a simple hinge. At the end of the boards opposite the hinge, you
put a screw-bolt through the bottom board in a threaded nut, so that it
pushes the top board when you turn it. Using a simple formula, you can
calculate how many turns per minute you need to give the screw to match
the earth's speed of rotation. Line the hinge (the Barn Door's axis of
rotation) up with the North Star (the Earth's axis of rotation), and you
can very closely counteract the movement of the stars across the sky
for fairly long lengths of time. With a Barn Door mount, you can use
longer focal lengths (up to around 135mm) and exposure times of up to
about 15 minutes and not see any star trailing -- all for about $10-$20
in materials.
Designs for Barn Doors vary from the ridiculously
simple and inexpensive to deluxe versions with battery-operated drive
motors (to save you from having to turn the screw manually). A simple
single-arm version will track accurately for 10-15 minutes, while
only-slightly-more-complex double-arm versions can track accurately for
an hour or more! The Internet is rife with design possibilities; here
are a few of the many that are worth looking at (I built the second one
in the list below, and used it for the Milky Way photo above):
Anthony Galvan III's Simple Barn Door Mount
Dave Trott's Motorized Double-Arm Drive
Dave Trott's Motorized Double-Arm Drive
The next step up in "tracking" mounts from a Barn
Door is a German Equatorial Mount. A GEM (sometimes acronyms are very
fitting) in its simplest form is a tripod head that has two axes that
rotate 90-degrees from each other. When you align the main axis with the
Earth's celestial pole, it can be driven to counteract the Earth's
rotation. The other rotational axis lets you position the camera or
telescope anywhere in the sky while still maintaining the first axis'
alignment with the Earth. Commercial equatorial mounts vary greatly in
size, quality, and price; a decent-quality mount with a drive motor (to
keep you from having to turn it by hand) that will support a camera and
short lens can be purchased for under $200, while a higher-quality mount
that will support long lenses or telescopes (and often includes a
computerized control that will automatically point it to specified
celestial objects) can cost between $1,000 and $10,000.
Equatorial mounts can allow you to use longer
focal lengths (using either camera lenses or telescopes) to "zoom in" on
galaxies, star clusters, and nebulae. The image at left shows two
telescopes mounted together on a mid-range (in both price and capacity)
GEM, with the camera mounted on a 540mm FL telescope. Why two
telescopes? Because once you start using focal lengths longer than 250mm
or so, you must track the sky VERY accurately. Even a tracking error as
little as a tenth of a degree can ruin an image, making stars look like
little oblong streaks rather than nice, sharp points of light. Since
even rather expensive GEM mounts are driven by mechanical gears,
imperfect machining causes those gears to be slightly inaccurate, and
produce tracking errors. To get a good image, these tracking errors have
to be corrected. This is called "guiding."
Guiding an exposure involves looking through a
separate telescope from the telescope or lens being used to take the
picture. A telescope eyepiece with crosshairs embedded in it is used --
the photographer centers a star on the crosshairs, starts an exposure,
and then carefully watches the star through the guidescope. When
mechanical errors begin to move the star off the center of the
crosshairs, the mount's drive controls can be used to put it back dead
center. Keeping the guide star on the crosshairs during the exposure
will ensure that tracking errors are minimized, and long exposures (up
to several hours) can be made very accurately. Obviously, this can get
quite tedious -- you must keep your eye glued to the guidescope for the
entire exposure, and delaying too long in making corrections will ruin
an image. Technology has offered a solution to manual guiding tedium -- a
CCD guide camera. Commercial guide cameras are available, but even
inexpensive digital Web cams can be easily modified to act as guide
cameras. Attached to a computer, the camera takes an initial exposure,
and notes the pixel position of a guide star (specified by the user).
Then the shutter is opened on the imaging camera, and the guiding
sequence started through the computer. It takes a new image every few
seconds, and again notes the pixel position of the guide star. If it's
moved from its original location, the computer sends signals to the
equatorial mount to move it back to where it was, just as you would do
manually. This "auto-guiding" can easily be more accurate than manual
guiding, and doesn't require the photographer to sit at the guidescope
for hours on end. The links section at the end of this article has
resources for making your own auto-guider from an inexpensive Web cam.
With a good-quality GEM mount, even fairly short
focal lengths can provide stunning "close-ups" of many celestial
objects. The image at right is of the Andromeda Galaxy, taken with a
540mm telescope. While Andromeda (also known as M31) is one of the
brightest galaxies we can see from earth, it's still very dim by
photography standards. To make this image, the total exposure time was
12 hours!
Once you start using longer focal lengths, a
whole universe of celestial objects become potential targets. But those
longer focal lengths, while magnifying those smaller celestial objects,
also magnify tracking errors -- making it more and more difficult to get
good images of them. Taking "deep-sky" multiple-hour exposures of tiny,
dim objects at long focal lengths will require investment in a
high-quality GEM mount (which is expensive!), sophisticated tracking
methods and equipment, and lots of practice. The details of this kind of
work are beyond the scope of this beginning article. If you start with
star trails or perhaps build a Barn Door mount, you can make wonderful
night-sky images without investing a lot of money in equipment. But
beware -- astrophotography can be addictive! Once you start getting
beautiful night-sky pictures, it's quite likely you'll want to move up
to higher levels, which means a significant investment of time and
money. The results are very satisfying and beautiful -- but when your
spouse starts asking why you need to spend $5,000 on some fancy tripod,
don't say I didn't warn you!
There's one night-sky target we haven't covered
yet. I saved it until last because it's one of the easiest to take
pictures of... our Moon. While the moon appears to move across the sky
at nearly the same rate as the stars, it's very bright -- about the same
brightness as a daylight scene on Earth (since the light we see from
the Moon is just reflected sunlight, that makes sense!). Since it's so
bright, it doesn't require the long exposure times that are needed for
stars, galaxies, etc. But the Moon can be a great target to photograph.
The issue with taking pictures of the Moon is
really focal length of the lens you're going to use. On 35mm film or
full-frame digital cameras, it takes a focal length of about 2000mm to
fill the frame with the Moon. APS-C DSLRs need about 1200mm to do the
same. While you can get satisfactory Moon pictures with shorter focal
lengths, the closer you can come to those frame-filling ideals, the
better.
Proper exposure values for the Moon are easy to
figure out -- just use the "sunny 16" rule. At f/16, use a shutter speed
that is the reciprocal of your film or sensor's ISO value; at ISO 200,
you'd use 1/200th sec. at f/16. Adjust the shutter speed up or down
according to how many f-stops above or below f/16 your lens is operating
at. Use a sturdy tripod, use your camera's mirror lock-up function (if
it has one), and use a remote or cable release. Focus at infinity, and
shoot away. It's best NOT to use your camera's auto-exposure modes; most
camera exposure meters see all that black surrounding the Moon, and add
exposure time to compensate, which over-exposes the Moon itself.
If your goal is to shoot frame-filling photos of
the Moon showing lots of detail, the best time to shoot isn't when it's
full: at full Moon, the Sun is lighting the Moon face-on. Just as with
your on-camera flash, straight-on lighting is flat, boring, and doesn't
provide any shadow details. Try taking Moon pictures at first or third
quarter (when the Moon appears half-lit, as above). At these times, the
Sun provides side-lighting, showing relief on lunar surfaces and
providing interesting shadows. Try not to shoot Moon pictures when it's
low on the horizon. At those positions you're shooting through the
thickest part of Earth's atmosphere, and the turbulent, moving
atmosphere makes it difficult to see fine details on the lunar surface.
Wait until the Moon is high in the sky, where the atmosphere is
thinnest, even if that means getting up at 2AM to shoot!
Another way to shoot pictures of the Moon is to
include it in landscape shots for dramatic impact. You can use much
shorter focal lengths than for the frame-filling detail shots, but stay
on the longer side (100mm or above) or the Moon will look very small in
the image. The same "sunny 16" exposure guide applies, with a caveat:
when the Moon is near the horizon, the thick layer of atmosphere you're
looking through to see it will dim its light, sometimes dramatically.
When it's near the horizon, start with about two stops more exposure
than when it's high in the sky, and bracket your exposures if you can.
For the moonrise image above I used about three stops more than the
"sunny 16" guideline. A full Moon will rise in the east about the same
time the Sun is setting in the west; a crescent Moon will rise just
before sunrise in the east or set just after sunset in the west.
Balancing the near-daylight exposure for the Moon with twilight can be
tricky, making sunrise and sunset the best times to shoot. Just be
careful not to over-expose the Moon or you'll wind up with a white blob
that shows no detail.
I hope this How-To has piqued your interest in
the possibilities of astrophotography. Don't put that camera away when
it gets dark, get outside and take pictures! A whole universe of
wonderful images awaits you.
Resources and Links
To do astrophotography well, you need to get to
know the night sky -- when and where the Sun and Moon will rise and set,
which constellations are in the sky at different times of the year, and
which celestial objects are visible. Here are some internet resources
to help get you up to speed:
NASA/JPL's Ephemeris
An ephemeris gives the positions of the Sun, Moon, and planets. The NASA/JPL site lets you enter your position on earth to get specific information for your location, at any time in the present, future, or past.
An ephemeris gives the positions of the Sun, Moon, and planets. The NASA/JPL site lets you enter your position on earth to get specific information for your location, at any time in the present, future, or past.
Jim Kaler's Constellation Maps
Provides maps of the constellations and their brightest stars, with different maps for the different seasons of the year.
Provides maps of the constellations and their brightest stars, with different maps for the different seasons of the year.
Sky and Telescope's Interactive Star Charts
An on-line star chart customizable for your specific location and time.
An on-line star chart customizable for your specific location and time.
The sky is full of interesting objects to
photograph besides stars -- galaxies, nebulae, clusters, and more.
French astronomer Charles Messier catalogued 110 of the brightest of
these objects in the late 1700's, and his Messier List is a good
starting point to find targets of interest. The Andromeda Galaxy above,
for example, is also known as Messier 31 (M31). You can get a list of
all 110 objects, along with information about where to find them in the
sky, at: SEDS Messier Catalog.
If you start going for long exposures at long
focal lengths on an equatorial mount, you're going to need to guide your
shots -- as mentioned in the text, a simple Web cam can be easily
modified to be used as an "autoguider." Ash's Astro Pages has several
articles and tips for using Web cams for guiding and astro-imaging.
If you're using a DSLR to do your
astrophotography, Michael Covington offers a comprehensive Web page with
lots of tips to help you get started: Michael Covington's DSLR
Astrophotography.
Paul LeFevre is a photographer, writer, and
astronomer living near San Diego, California. He can be reached at
plefevre@hughes.net.
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