Astrophotography

What astrophotography is

Astrophotography is any photography of objects in the night sky, from a starry landscape to a distant galaxy. It is one of the oldest uses of the camera: the Moon was photographed in the 1840s, and astronomers soon found that long exposures on a plate recorded stars too faint for the eye.

Well-known early names include John William Draper and Henry Draper in the United States, and Edward Emerson Barnard, who mapped the Milky Way on photographic plates. Today digital sensors and cheap tracking mounts let amateurs make images that once needed an observatory.

For learning it helps to split the genre into three branches: wide-field skies (the Milky Way over a landscape), lunar and planetary imaging (small, bright targets needing magnification), and deep-sky imaging (faint nebulae and galaxies needing long total exposure).

Wide-field Milky Way

This is the easiest entry point. You need a camera with manual control, a sturdy tripod and a wide lens, ideally 14–24 mm at f/2.8 or faster.

The stars move across the sky as the Earth turns, so exposures must be short enough to keep them as points. A common rule of thumb is the "500 rule": divide 500 by the full-frame-equivalent focal length to get the longest exposure in seconds. At 20 mm that is about 25 s; stricter variants give roughly half that (approximate).

Start around f/2.8, 15–20 s, ISO 3200, with white balance near 3800–4200 K. Focus manually: zoom in on a bright star in live view and turn the ring until it is a tiny point.

Darkness matters more than gear. Go far from city lights, choose a night near new Moon, and check that the Milky Way core is above the horizon (in the Northern Hemisphere roughly spring to early autumn).

The Moon and planets

The Moon is lit by full sunlight, so it needs settings closer to daytime: try ISO 100–200, f/8 and about 1/125–1/250 s, then adjust using the histogram (approximate). Using automatic exposure on a black sky usually overexposes it badly.

To fill the frame you need long focal lengths: at 600 mm on full frame the Moon is still only about a fifth of the frame width. The most dramatic detail appears along the terminator, the line between day and night, where shadows are long, so a half Moon often shows more texture than a full one.

Planets are tiny and shimmer through the atmosphere. Planetary imagers use a telescope and record short video clips, then keep only the sharpest frames, a technique often called lucky imaging.

Deep-sky imaging with trackers and telescopes

Nebulae and galaxies are so faint that they need minutes or hours of total exposure. A star tracker is a small motorised mount that turns the camera at the same rate as the sky, so single exposures of 1–3 minutes with a lens become possible.

For smaller objects, an equatorial telescope mount carries a small refractor or other telescope. It must be polar-aligned, meaning its axis points at the celestial pole, and many setups add a guide camera that corrects tiny tracking errors.

A good first target is a large, bright object such as the Orion Nebula or the Andromeda Galaxy with a 135–300 mm lens on a tracker. Start with ISO 800–1600 and 60–120 s per frame (approximate).

Stacking: many frames make one clean image

Every exposure contains random noise. If you shoot many frames of the same sky and average them, the signal stays the same while the random noise partly cancels. Quadrupling the number of frames roughly halves the visible noise (approximate).

Stacking software such as Siril, DeepSkyStacker or Sequator aligns the stars in each frame, then combines them. You can also add calibration frames: darks (same settings with the lens capped) to remove hot pixels, flats (an evenly lit frame) to remove vignetting and dust, and bias frames for sensor read noise.

After stacking, the image usually looks dark and grey. Careful stretching of the brightness curve, colour balancing and gentle noise reduction bring out the detail. Shoot in RAW so that the data survives this processing.

Common mistakes and good habits

Focus is the classic failure: autofocus rarely works on stars, and the infinity mark on many lenses is not exact. Check focus at full magnification and tape the ring if needed.

Other frequent problems: exposures too long for the focal length (stars turn into short streaks), dew forming on the lens as the night cools, and a tripod that shakes in the wind. A lens heater strip and a remote release help.

Be considerate at dark sites. Use a dim red light, avoid shining torches or car headlights at other observers, respect private land and park rules, and leave nothing behind. Light pollution is a shared problem, so shielded, warm outdoor lighting at home helps everyone.

An exercise to try

On the next clear night near new Moon, find a dark spot and photograph the same patch of sky with 10 identical frames: widest aperture, ISO 3200 and an exposure from the 500 rule.

Then stack the 10 frames in free software and compare the result with a single frame at the same zoom. Look at the dark sky between the stars: the stacked version should look much smoother, and fainter stars should appear.

Finish by framing a foreground such as a tree or a hill, shot during blue hour or lit very softly, so the sky has a place to sit.

More on StarScoped, our astronomy site: Astrophotography basics, The Milky Way.