Spectacular photos of the Milky Way, such as this one of
the Southern Cross and Eta Carina region, can only be made if the
camera is tracking the stars, counteracting Earth's rotation.
The need for guiding exposures of the night sky
Anyone who tries photographing the night sky with just a camera
and tripod soon finds that this is not well possible, since the
stars move. Unless you use very short exposures and a wide-angle
lens, the stars will produce visible trails on the photo.
The Earth rotates eastward at a slightly shorter period than
one day. As a result, for an observer fixed on the rotating planet,
all stars appear to move westward. In order to compensate for the
apparent star motion, you need a camera mount that tracks the stars.
This mount rotates at the same rate as Earth and along the same
axis, but in the opposite direction to counteract Earth's rotation,
and as a result the stars will appear fixed in the view of a camera
mounted on the mount.
Earth days and sidereal day
Around midday every 24 hours, the sun appears due south or north
for any given location on Earth. But the Earth rotates around its
axis slightly faster than once per 24 hours, because Earth also
orbits the sun, and as a result every day the sun appears slightly
further east among the stars. Thus, Earth has to rotate a little bit
further in order to have the sun at the same location in the sky.
However, Earth's orbit is elliptical, and its velocity changes a
bit throughout the orbit. It goes slower when it is farther from
the sun. Therefore the sun will not always be exactly due south at
the same time every day throughout the year, so we use the average
period for a day.
There are 365.24 days per year, and in one year every bit of
extra rotation to have the sun appear due south (or north) in the
sky has added up to an extra day, thus Earth rotates 366.24 times
around its axis in one year. The rotation period in seconds (which
I'll call T) can therefore easily be approximated:
24h = 24 * 60 * 60 = 86400 seconds
T = (365.24 / 366.24) * 86400 seconds = 86164.1 seconds
which is about equal to 23h 56m 04s, the so-called sidereal day.
After this time, the stars appear at exactly the same location in the
sky as before. Therefore, this is the rotation period a camera
mount should have in order to track the stars correctly.
Mounts like these from Orion are relatively compact and light and
you can bring these with you if you go on holidays, even by airplane. They are cheap
and easy to setup and use.
Types of mounts
A mount refers to the setup to track stars, whether this is a
camera, telescope or something else. There exist different types
of mount, the most widely used being:
- equatorial mount which has one axis of rotation aimed
exactly to either the north or south pole, so that the mount's polar
axis is parallel to Earth's rotation axis. This mount requires only
one motor, to rotate the polar axis once every sidereal day. This
axis controls the right ascension. There is another axis which sets
the declination ('vertical') angle, but this doesn't need to be
motorized if you just want to take wide-field photos with a camera.
- Horizontal mount or Dobsonian mount, which has one
axis pointing at the zenith, controlling the azimuth angle, and
another axis horizontal, controlling the elevation angle. Such a
mount requires computer-controlled motor adjustment, since the
rotation axes of the mount and Earth differ. A Dobsonian mount
for photography is not useful, since the star field also rotates as
the mount tracks the stars. Therefore, you'd need three
computer-controlled motors as opposed to just one for the equatorial
Considering the above, I highly recommend you to use an equatorial
mount. Mounts come at different qualities and prices. The
company Orion sells relatively
cheap mounts for around $200 that perform very well. If you are
going to be very much into astrophotography, you will want a better
and more expensive mount, but for starters the cheaper mounts
perform well, especially if you only do wide-field photography.
If you don't want to spend any money at all, you can also make
your own mount such as the low-tech barn-door mount. This
mount consists of two wooden plates connected with a hinge. It is
a simple form of equatorial mount, where the hinge axis is aligned
with the rotation axis of Earth. A threaded rod with a wingnut that
you turn very slowly at the correct rate will rotate one of the
plates on which you mount the camera, at the sidereal rate. Very
decent photographs can be taken this way, but it is a pain to
Even relatively cheap equatorial mounts will work well for minute-long exposures
with portrait to telephoto lenses. This photo of the Large Magellanic Cloud in Dorado was made
using a 135mm telephoto lens on an Orion Astroview mount.
All mounts have errors in guiding. The most common guiding error
is a periodic error, caused by imperfect gears and worms. The guide
will run fast for some time and then run slow, while on average
guiding the stars very well. But when using your camera with a
zoomlens or behind a telescope, periodic errors are substantial and
may ruin your exposure. In such cases you will have to manually
control the guiding, which requires a telescope of some sort. In
practice, using a cheap mount will allow you to make nice photos
when you use lenses shorter than about 100mm or so.
Another common guiding error occurs when the mount is not aligned
exactly to the pole. Most modern commercial equatorial mounts have
a polar alignment scope which you can use to align the mount to the
pole to within 10 arcminutes or better. The better you have the
mount aligned to the pole, the longer exposures you can make without
the stars drifting through the field of the camera.
Different ways to use a camera
The easiest way to get rewarding photographs (wide-field) is to
mount the camera directly on the mount and let it track the stars.
If you are new to astrophotography, I recommend you to use this
method first, because it takes no special equipment other than the
mount, and the photos can be amazing.
If you have a telescope, there are several different techniques
with which you can photograph deep-sky objects and planets and such.
You can use the telescope as a very long telephoto lens (the
afocal projection method); or you can mount the camera behind an
eyepiece, with which you can photograph much smaller objects such
as planets and galaxies well. A telescope also allows you to
manually correct guiding errors if you mount the camera on the
frame of the scope (the piggy-back method).