Getting Lined Up

Trailed stars are the number one flaw in piggybacked shots. This is caused by poor alignment of the telescope mount with the north celestial pole, the point in the sky near Polaris, the North Star, about which stars appear to rotate as Earth spins on its axis. To track stars accurately, you must use an equatorial-type mount and align it so that its polar axis aims as close to the celestial pole as possible.

While computerized "Go To" telescopes can also follow the stars, they do so by constantly moving two axes. Piggybacking a camera onto one of those scopes can result in badly trailed stars, since the field will rotate during the exposure. Optional wedges or tripods for popular Go To models, such as those from Meade and Celestron, allow them to be tipped over and polar aligned, which is essential for piggyback shooting.

How accurate does this polar alignment have to be? It depends on the focal length of the lens you're using. Simply aiming along the mount's polar axis by eye on Polaris, nearly 1° from the north celestial pole, might be good enough. Wide-angle lenses, which cover relatively large areas of the sky, can be forgiving of errors in polar alignment. Long telephoto lenses, on the other hand, magnify everything, including any tracking errors. So they demand greater accuracy when aligning the telescope's polar axis, to within a fraction of a degree of the true pole. Some equatorial mounts have polar-axis finderscopes built in, and these make accurate alignment much easier than with fork-mounted telescopes.

Even with precise polar alignment, however, stars can still appear to trail. The culprit here is the telescope's motor-drive mechanism, which might not be tracking smoothly or simply is not turning at the correct speed. In that case, the solution is to manually "guide" the telescope with the aid of a high-power eyepiece fitted with illuminated crosshairs. The trick is to select a moderately bright star to guide on and keep that star perfectly centered on the crosshairs throughout the exposure. Dual-axis drives (with a motor and push-button speed control on each telescope axis) greatly ease the guiding process and keep your shaky hands off the mount.

What Exposure Should You Use?

Piggyback shooting of the Milky Way demands dark, moonless skies. To record the most stars and nebulosity, the best exposure is usually the longest one you can take before skyglow from city lights starts to wash out the details. There's more to setting the exposure, however, than just opening the camera shutter. The lens's aperture (opening) also controls the amount of light reaching the film or digicam sensor. Opening the lens to its widest aperture (that is, the smallest f/number, typically f/2) lets in more light and keeps the exposure time to a minimum, while still picking up stars as faint and as numerous as your sky will allow. Opening the lens by one f/stop, from f/4 to f/2.8 or from f/2.8 to f/2, cuts your exposure time in half.

Another option for shortening exposure time is to use a "fast" (sensitive) film or to switch your digicam to a high ISO setting. A film rating or digicam setting of ISO 400 will require only half the exposure time of ISO 200. If you're shooting with slide film, you can request that your photo lab "push" the film when it's developed — for example, to ISO 800 if the film is ISO 400. This will increase the picture's contrast, but at the cost of a slight increase in grain (coarseness).

Short exposure times may record less detail, but they'll minimize the streaking of stars due to poor or imperfect tracking and avoid aircraft flying through your field of view. So is a high ISO and a wide-open lens the best combination? Not necessarily. Wide-open apertures can reveal optical flaws in the lens, producing bloated and distorted star images, especially at the corners of the frame. Stopping down the lens (decreasing its opening) by one full f/stop to f/2.8 or f/4 reduces lens flaws and sharpens star images, at the expense of longer exposures.

Faster ISO speeds also have their drawback. They introduce grain with film or electronic noise with digicams. Fuji's Provia 400F offers remarkably fine grain with fast speed and is my choice for all-around piggyback film shooting. With digital cameras, try settings of ISO 400 or 800. Their electronic noise decreases if the ambient air temperature is chilly, such as on winter nights. It gets worse after the camera has been used to take several images and has warmed up, especially during summer. If that happens, turn off the camera for a few minutes to let it cool before resuming shooting.

Finally, film loses sensitivity over long exposures — a 20-minute exposure does not record twice as much light as a 10-minute exposure. By comparison, digicams maintain their full recording ability throughout an exposure. That's why a digital SLR can pick up in 3 minutes what it might take a film camera 9 to 12 minutes to record. That can make all the difference between images that are trailed and ones that aren't.