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 mount.

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 operate.

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.

Guiding errors

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).