Shooting the Sky
So often we talk about astrophotography in terms of taking images through a telescope. But that's not how most of us actually begin. Our first adventures in astro-photography usually involve placing a camera on a tripod and taking a picture of the night sky. While this "Astrophotography 101" may not seem as engaging as telescopic images, it can yield some very impressive photos of the sky. So for those just starting out, here are some thoughts on this simple, but effective, technique.
First of all, a camera-on-tripod setup is not going to be able to follow the diurnal motion, so an exposure longer than about 10 seconds is going to show elongated star images. The closer your target lies to the north celestial pole, the less pronounced would be this star trailing. That's because objects nearer the pole move a smaller distance in a given time increment compared to objects nearer the horizon. Obviously, this star trailing becomes more pronounced if you use a zoom or telephoto lens, since you've now magnified everything.
On the other hand, there are times when you may want to purposely show star trails. Many of us have taken a long exposure aimed at Polaris in order to show how the stars rotate about the pole. We knew it all along, but it's as if we want to prove it anyhow! Also, stars show color more prominently when they appear as a line than as a pinpoint. Thus if you want to demonstrate how won-derfully colorful the night sky can be, allow the stars to trail on your exposure. Finally, if you leave the shutter open for a long period during a meteor shower, you're likely to record some meteors as streaks of light cutting across the uniform arcs formed by the stars.
A northern lights display is probably the one case where we really don't care whether we get star trailing. Here the main focus is a colorful, shifting aurora, and no one will pay much attention to the background stars. To record an aurora well, you need a minimum exposure of about ten seconds, depending on the strength of the aurora and the speed of your film. Digital cameras are more sensitive than film, so they can yield good results with shorter exposures. Obviously, try various settings to find out which works best for your camera. Typically we tend to go overboard on exposures of the northern lights. To achieve more dramatic shots, we burn in the aurora, along with the sky background. It's a temptation I find hard to resist! This gives bright, detailed images of auroral streamers, but against an almost day-like sky. A camera can capture more detail than the human eye can discern, but we need to use this capability to end up with images that still look reasonably natural.
Long exposures with just a camera can yield some especially impressive images. You've no doubt marveled over the incredible richness of the summer Milky Way in the direction of Scorpius or Cygnus. But here we do want pinpoint star images. The secret is to use an equa-torially mounted telescope as a platform for the camera to carry it along with the apparent motion of the sky - better known simply as "piggyback photography". A four or five minute exposure can reveal a profusion of stars. A piggyback adapter to fit a camera on top of your telescope is a pretty easy do-it-yourself project, or you can buy a commercially-made adapter.
If your telescope is reasonably well polar aligned, you need only aim it and turn on the drive motor when doing wide field photography. However, with a telephoto or zoom camera lens, you will need to also guide the telescope in order to make the minute corrections that will keep the stars from trailing. Here a guiding eyepiece with a crosshair reticle is invaluable to help keep the telescope platform tightly aimed at a chosen guide star. If you are imaging a comet, then it's best to guide on the head of the comet, rather than a field star. That's because comets move quickly with respect to background stars; it's common to see nice sharp images of comets with the stars showing as short trails.
While piggyback photography is much easier with a motor-driven scope, it's also possible to manually guide and move a non-driven scope, provided you don't use a telephoto lens on the camera. Speaking from personal experience, this "armstrong guiding" method is ex-hausting once exposures run longer than about three minutes. Nonetheless, the results can be very satisfying.
Finally, longer exposures raise the issue of "fogging" or burning in the background. A black sky is what we like to see, but in using long exposures to capture more detail, you start to get a bright sky background that detracts from the overall image. At some point, loss of contrast with the target object negates the benefit of a long exposure. That point, of course, is determined by the degree of sky glow. If the sky was light-polluted, there's not much you can do about the background if you hope to capture an object. So take lots of images at different settings to get the most pleasing rendition, balancing target image quality against sky fogging.