After experimenting with astrophotography with a fixed camera, either with just the camera itself using a fast ISO setting to allow short exposures, or using the camera body attached to a telescope to take photos of the Moon at prime focus or using eyepiece projection, a sensible progression would be to try piggy-back photography. This is the term applied when an equatorially mounted telescope is used to guide a camera mounted on the same equatorial platform. Camera lenses commonly used vary from 28 mm for wide-angle shots of the Milky Way to 300 mm for telephoto shots of large nebulae such as the North America nebula (NGC 7000) and the Eta Carinae nebula (NGC 3372).

The basic requirements for piggy-back photography are:

• a digital SLR camera with a bulb (B) setting;
• various lenses to provide for a range of subjects (eg 50, 135, 300 mm) or a zoom lens (zoom lenses are usually of lesser optical quality);
• an equatorial mount with a motor drive on at least the R.A. axis; and
• brackets to rigidly hold the camera and lens.

An autoguider is an optional but very useful addition - it will guide much more accurately than is possible manually and it removes the tedium of guiding.

Hand-operated slow motion controls on both axes could be used for lens focal lengths up to about 135 mm, but hand tracking in right ascension at the accuracy needed for a 300 mm lens will become tedious for exposures of more than about 10 minutes. Also, the operation of the controls could cause unacceptable, unintentional movements.

The guiding tolerance at the plane of the CCD to ensure visually circular star images is 0.04 mm. (Gordon, 1985). Assuming a 135 mm lens is being used on the camera and the guidescope has a focal length of 1000 mm, the tolerable drift at the illuminated reticle of the guiding eyepiece is 0.04 x 1000 / 135, that is, 0.3 mm. Now, most guiding eyepieces have a 0.2 mm square box etched on the reticle, so the drift should be limited to 150% of the box size. Similarly, for a 50 mm lens, the drift can be up to 4 boxes and for a 300 mm lens, it must be limited to 65% of the box. (Different values for the tolerable drift will be obtained for different focal length guidescopes.)

Remember that digital SLR cameras have built-in filters that exclude the red light of H-alpha, so do not expect to record bright red nebulae unless your camera has been modified. Images will need to include dark frames to enable electronic “noise” to be subtracted from the images, and even then, exposures will need to be relatively short with multiple exposures stacked if necessary. Advanced users will also use “flat fields” to enable them to remove the effects of dust on the detector and vignetting.

A problem likely to be encountered is flexure. As the right ascension axis rotates, gravitational forces on the guidescope and camera vary with time and differential movement can occur unless everything is very rigid. For a 35 mm SLR camera with a 35 or 50 mm lens, a robust ball and socket bracket will be adequate, but for the same camera with a telephoto lens, a support for the lens as well as the camera will be necessary to reliably achieve untrailed star images.

Always remember that in astrophotography there are no hard and fast rules regarding exposure. Until you know through trial and error what is right for your location, your equipment and your CCD or CMOS chip, always bracket exposures and record what is best. In fact, record everything!

Piggy-back photography can yield magnificent pictures of constellations, nebulous regions such as Orion, Cygnus and Carina and also comets.