Astrophotography is all about the night sky. The moon, the Milky Way, the shooting stars, planets and nebulae are all targets with the right telescope. But the right technique is needed to capture these sometimes faint subjects.
Consider your location
The most important thing to take into account is your location. You want to find as dark a patch of sky as possible. A dark sky means fainter objects can stand out. In light polluted areas near cities, only the brightest stars and planets are visible. One of the best tools is the DarkSiteFinder website. Looking over the map, you can see how the light pollution of cities affects the surrounding sky.
The eastern United States has practically no areas of prime night sky viewing. It’s not until the Plains states start that we find good patches of darkness available. Likewise, Western and Central Europe are entirely light polluted. So a bit of travel is necessary if you want to photograph something other than the moon.
Timing is crucial
The second factor is timing. Certain objects, like the moon, are easily predicted. Also try to choose a night when cloud cover is minimal. A tripod is necessary if you want to capture crisp details on the surface like craters. Shooting handheld will blur these fine details, especially with a telephoto lens. Ideally that telephoto has a focal length of 200mm or greater, to make the moon even larger. For such a bright object a shutter speed around 1/125th of a second and ISO 200 should suffice. But experiment as needed for the conditions of the evening. And look into when the moon is going to rise or set. The brightness of the moon will ruin most views of the dark night sky. If you want views of the Milky Way or other fainter objects, look to shoot at least an hour before moonrise or an hour after it sets. The same goes for the sun. The best times are 90 minutes after sunset or 90 minutes before sunrise.
Capturing meteor showers
Other subjects like meteor showers are seasonal. The American Meteor Society has a great calendar showing when and where showers are best viewed. But individual shooting stars can be seen any time of the year. As the earth revolves around the sun, bits of space debris occasionally strike the atmosphere and burn up. The darker the sky, the greater the chances of catching one in a long exposure image.
Star trails can make for spectacular photos if you understand how to set up properly. Star trails capture the motion of the Earth’s rotation. These photos are either long exposures done over an hour or more. Another method is creating a composite photo. A picture is taken every few minutes and the elements that have changed (like the positions of the stars) are added to the baseline background. Some cameras, like mirrorless cameras in the Olympus line, have this feature built in, but usually software like Photoshop or StarStaX is required. For the first method, the longer the exposure time, the better. ISO should be kept low to avoid noise in the photo. Exposure time is a matter of the local conditions and time of night. Anywhere from a few minutes to a few hours might be needed.
Whether using the composite or long exposure method, a tripod is mandatory. Lastly, try to locate the North (northern hemisphere: Polaris) or South (southern hemisphere: Sigma Octantis) pole star, if possible. Because star trails are an effect of the earth’s rotation, the pole star will appear as a fixed point the other stars are spinning around. You can use the pole star as a guide for composing your final image.
If you just want to capture the stars, then you have to take into account the rotation of the earth. Because long exposure times are needed in astrophotography, you’ll capture unwanted star trails with just a few seconds of exposure with a long telephoto length. So how to tell what exposure time is needed?
#1 exposure hack for astrophotography
There’s actually a handy formula to calculate that: Divide 500 by the focal length of the lens as if using 35mm film. So this means you need the crop factor of your digital camera. If you don’t know that, it will be in your manual with the image sensor specs. If your crop factor is 1.5x, then multiply the focal length you want to use by that number. If you want a 15mm wide angle field of view, then your formula is: 500/(1.5×15) = 22.22 seconds of exposure. Anything greater will show star trail blur from the rotation of the earth. Remember this number will reduce as you increase the focal length (zoom in).
The night sky can be a bit challenging at first, as our usual methods of photography don’t work very well here. The right location can be challenging to find and much longer exposure times are needed. A bit of math is also required to figure out which objects are out when, how to avoid other objects, and how to photograph the ones we want. But the results can be incredibly rewarding. Astrophotography is a great way to expand your portfolio and camera know-how, and everyone should spend a night or three with the camera pointed at the night sky. Read more on How to Photograph Natural Phenomena at Night.