How to image the night sky

With entries now open for the Insight Astronomy Photographer of the Year 2018 competition, Royal Observatory Greenwich astronomer Tom Kerss reveals his tips for capturing a magnificent nightscape.

As told to Lorin R Robinson.

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Royal Observatory Greenwich astronomer Tom Kerss
Credit: Tom Kerss


Beautiful photos of the night sky need not be a product of sleepless nights and big expense.

While fancy equipment and ample free time may increase your chances of capturing amazing shots of the heavens, much can be achieved with the equipment most photographers already have and a few minutes of experimentation - even if they’ve never considered trying astrophotography. 

The night sky is a unique low-light scene. All the usual rules apply, but each must be observed to the extreme.

A tripod is essential as exposure times will typically vary between a few seconds and couple of minutes.

Even a tripod will not deliver perfectly sharp stars unless the exposure is minimized because the Earth is constantly rotating.

Total compensation can be achieved only by using an equatorial tracking platform - a device that turns your camera at precisely the same rate as the Earth, but in the opposite direction and aligned with its axis.

Of course, star trails have a unique appeal and can be achieved with long exposures, ranging from 10 minutes to composites of several hours.

 

While equatorial mounts will help you take crisp nightscape images, star trails can be used to add a unique effect to astrophotos. Chap Him Wong won the 'People and Space' category in the 2015 competition with this amazing shot.
Credit: Chap Him Wong

 

Tom's top tips for nightscape photography

Whether or not you opt for equatorial tracking, here are several tips to help ensure dramatic images of the night sky:

  • Use a remote shutter or timer to ensure there’s no vibration when the shot is taken. The Earth’s atmosphere already deteriorates the crispness of the heavens - an effect astronomers refer to as “seeing” - so we have to make every effort not to introduce additional sources of blur.
  • Manually focus on a bright star (or the Moon) using live-view and 10x zoom. Get the star to be as point-like as possible. If shooting the moon, focus on the edge (known as the limb).Celestial objects are, for the purposes of optical design, infinitely far away. But since many lenses focus beyond infinity - not quite the physics-breaking miracle it appears to be - you can’t simply turn the focus all the way and expect point-like stars.
  • Your lens simply won’t be able to autofocus on stars, so leave it on manual. If you’re out on a cold night, consider using a small piece of tape to hold the focus. Thermal expansion inside the lens may push it slightly off the best focus position over the course of the session.
  • The stars are distant suns. If you want to capture their colours accurately, use daylight white balance. The camera is often able to see hues of pale blue, orange and, occasionally, deep red that your eye will not. These are the real visual colours of the stars, determined by their surface temperatures. Blue stars are very hot; red stars are relatively cool.
  • Shoot your images in RAW and a large JPEG. You can use the JPEG to preview and process from the RAW. Since stars are point-like high-contrast subjects, optical aberrations in lenses are a leading factor in spoiled pictures. Many software packages can correct somewhat for these when editing in RAW. However, the principal reason for shooting RAW is that 16-bit images can contain 281 trillion different colours as opposed to 16.8 million in an 8-bit JPEG. Incredibly subtle differences in shade can be extracted and emphasized in processing. Consider a light-polluted location from the Milky Way that appears just barely brighter than the surrounding sky. The two could be readily separated in a RAW images, allowing for the contrast to be greatly enhanced. In the corresponding JPEG, it would be more difficult to 'bring out' our galaxy without introducing banding artifacts.
  • If you’re taking long exposures, your camera’s sensor will become warmer than the ambient temperature. Give it time to cool down between exposures. Cooler sensors produce less noise. Controlling noise in astronomical pictures is best achieved by taking multiple exposures, but before exploring advanced techniques, give your noise reduction plugin a chance. Modern software is very good at reducing noise while preserving stars, even in a single exposure.
  • To minimize star trailing, use a wide-angle, fast lens. Short, low-magnification exposures produce the sharpest images. A fast lens will give you more stars and a brighter image, and a wide shot will be more forgiving of drift - a star is less likely to spill over from one pixel to the next over the duration of a shot. Many of my images are made with a Tokina 11-16mm F/2.8 used at 11mm.
  • Exposure times are a matter of experimentation. A bright Northern Lights display can produce a stunning image in just 5-10 seconds. The Milky Way typically demands longer exposures (30-60  seconds). Meteor showers are sporadic in nature, so why not open your shutter for a few minutes on bulb and see what you get?
  • If your lens is F/4 or faster then, in my experience, photos like the above can be achieved with ISO 400-800. Your success may vary because some sensors perform cleanly at higher ISOs. Most cameras have on-board long-exposure noise reduction tailored specifically to the sensor and will usually produce excellent results.
  • Astrophotographers are obsessed with minimizing noise while increasing the amount of real light they capture. A high signal-to-noise ratio (SNR) is always desirable, as most images require further processing—typically adjusting levels and making faint details appear brighter. Applying these changes to a low-SNR image will increase noise and spoil the result. For modern sensors, it’s often best to 'expose to the right' (ETTR) which, as the name implies, involves setting your exposure such that the histogram ends up pushing against the right-hand edge. You want the majority of bright luminance values to be represented so only the minority of very bright pixels are overexposed and the darkest pixels are thinly spread across the left side of the graph. Done well, this technique results in a high-SNR image from which a very clean result can be obtained.  It is particularly effective when extreme levels/curves adjustments are used on a single photo. 

Front image: River of Light by Martin Campbell
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