How to… master the art of averted vision
The Blinking Planetary Nebula can be used to test your averted vision. In this Hubble image, the nebula resembles a human eye
By Martin Mobberley
The eye is a remarkable detector but, unlike a CCD camera used for imaging, long exposures are not an option. So to see really faint objects you need to understand its limitations and the tricks that can be employed to coax the maximum out of its ‘short exposure’ capability.
However, the more you learn about the limiting factors, the more you appreciate how it is possible to use your retina to greatest effect. A few people seem to have truly exceptional night vision but, in the main, this is very rare. If you can’t see the faint objects that others can it will be down to a number of factors: local light pollution, impatience or inadequate dark adaptation.
As well as choosing the right level of magnification, collimating your telescope and realuminising your mirrors, mastering the art of averted vision by training your eyes will help you get the most out of your time observing the night sky.
The eye has two types of detector within the retina, the thin layer of cells that lines the back of the eyeball. These are called rods and cones. The rods are the low light detectors, whereas the cones allow full colour, high-resolution eyesight. The central 1° of the retina, the fovea, is packed with cones, and you are using them to read this sentence. Your brain creates the illusion that the whole magazine page is sharp, but in fact you are only seeing a few letters at a time at high-resolution and in full colour; your eye muscles are zipping everywhere else to create the illusion.
The electrochemical signals from the retina’s detectors travel via cells known as ganglion cells on their way to the brain. In the high-resolution, full colour, centre of the retina, one ganglion cell connects to one cone. But, as you go further out and low-light rods dominate, there may be 100 rod detectors passing their electrochemical signal into just one ganglion cell; it’s a case of grouping them to improve the signal-to-noise ratio.
Not surprisingly, with so many rod detectors teamed up, resolution suffers badly. While the foveal cones can resolve a 60th of a degree (one arcminute), the teamed-up rod system, well away from the centre, may only resolve 20 arcminutes. That’s not much finer than the size of the Moon, as seen with the naked eye.
The good news is that there is an optimum, ultra-sensitive,rod-packed region of the retina that you can bring into play. The eye is about four astronomical magnitudes (40 times) more sensitive at this crucial point than at its centre. So if you can hold, say, a 10th magnitude star steady in the visual centre of a 12-inch reflecting telescope’s field, you’ll be able to hold a 14th magnitude star steady on the rods 12° off centre.
To get to this sensitive area, you have to look to one side of the faint astronomical object you’re trying to see: place the object you’re looking at roughly 8° to 16° away from the eye’s centre – 12° is a good average value for the best part. At first this will seem incredibly difficult, but it will improve with practice.
This 12° offset should be arranged so that you appear to place the object nearer to your nose than the side of your face in the field of view. The reason for this strange requirement is that the eye has a blind spot where the optic nerve leaves the retina, and this blind spot is on the side of the eye, away from the nose.
When you first go outside and look through the eyepiece you will probably not see anything. This is because you are not ‘dark adapted’. When the human eye is plunged into darkness, two things happen. Firstly the pupil dilates (expands) to its maximum diameter. In young people this may be 7mm (0.2 inches) or so, but for astronomers in their 80s it may only be a few millimetres across. This is not a big problem at the telescope though, as the higher magnification produces a narrower beam of light which can pass through a small pupil.
The second development to happen in darkness is that the amount of the chemical rhodopsin in the retina increases dramatically, by many thousand-fold. So the combined effects of rhodopsin and using averted vision amounts to over 100,000 times more sensitivity than your central vision had in a fully illuminated room before you stepped outdoors. Dark adaption – waiting for the rhodopsin to do its job – cannot be rushed. You need to wait 40 minutes or more to feel the full effect.
So, if you are planning to observe a number of faint objects, save the faintest ones till last – you won’t be disappointed.
A step-by-step guide to using averted vision