The eye is an extraordinary instrument, adapted and refined by millions of years of evolution to produce vivid, detailed images of the world around us. Tuned to the peak output of the Sun – a range of colours we call visible light – our eyes, in tandem with our brains, interpret a useful slice of the electromagnetic spectrum, while compensating for movement and changing light levels, to provide us with an effortless sensation of sight. But gazing at the Galaxy 's wonders wasn’t an evolutionary priority for the eye, which is primarily adapted to help us navigate the terrestrial domain, so we have developed tools, such as telescopes, to see deeper into the night sky.

However, just because the eye isn't a dedicated stargazing instrument, doesn't mean you can't extend its performance for better views of the night sky. Adapting to the darkness over a period of 40 minutes or so allows us to see much fainter stars, by switching our vision systems from photopic to scotopic. Photopic vision, or 'daylight' vision, utilises photosensitive cells called cones. They come in three flavours, sensitive to red, blue and green light, providing full colour views of the world around us. However, these cone cells are not sensitive in low light. Scotopic vision is achieved when highly sensitive rod cells - monochromatic photoreceptors - become active to improve sensitivity in dark conditions. This transition is slow to occur, and easy to undo rapidly by accidentally glancing at a bright light source. 

Cone cells are so concentrated near the centre of the eye's retina that rods are nearly absent there, resulting in a low-light blind spot at the centre of each eye. You can convince yourself of the existence and extent of this blindspot by closing one eye in a darkly lit room and staring at various objects around you. You'll notice that details at the very centre of your visual field are hard to perceive, and only become visible once you move your eye. Typically, the blind spot reaches anywhere up to about 15 degrees from the centre of the visual field, and any faint astronomical objects inside it will be virtually impossible to discern.

A technique called averted vision is used by skilled observers to overcome the low-light blind spot, which involves purposefully employing a different region of the retina where rod cells are found in relative abundance. Put simply, this involves looking slightly to the side of what it is you want to see. Try it indoors, with the lights off, by looking at a sheet of white paper across the room. When your eyes are dark adapted, you’ll find that the paper appears brighter if you focus your gaze to one side of it. The experience is rather strange at first, but becomes more familiar with practise. You will need to train yourself to 'see' the things you aren't looking at directly.

Averted vision is very effective when trying to spot extremely faint objects in the night sky, such as nebulae or other galaxies as seen through a telescope. When using both eyes, the negative effects of the blind spot are slightly, though not completely mitigated, but you can still appreciate the power of averted vision when look at the Milky Way, Andromeda Galaxy, or the many faint open and globular star clusters that populate the sky.

When using a telescope for deep-sky observing, averted vision becomes more critical. Your results will vary, but many experienced observers agree that the most sensitive part of the eye usually lies between the blind spot and the nose. This means it's often best to avert your gaze away from your nose, placing your chosen target on the nose-side of the blind spot. If you are a right-eyed observer, this means looking toward the right of the object; if you're a left-eyed observer, look toward the left. Everyone's retina is different, however, with varying distributions of photoreceptor cells, so it's worth experimenting to find out which direction works best for you.

This technique, in concert with the patience to achieve and maintain scotopic vision, will help you maximise the stargazing performance of your eyes, and unlock new observing experiences under dark skies. Try it yourself!

Image credits: Tom Kerss