I want to tell you about how to make the best out of your eyesight and how to see some of the fainter things in the night sky, with or without a telescope.
When we look up at the night sky, the light from whatever the celestial wonder is has to hit our retina to register on our consciousness – it does this by passing through our eye lens and our pupil.
Our pupil is the limiting factor here – at best, when we are young, it’s some 7mm wide and it constricts with age to be about 5mm. So, to capture more light, we need a bigger area than the 5mm hole of our pupil – we can use a pair of binoculars or a telescope.
Most binoculars are described with two numbers, say 10 by 50. This means that they magnify by a factor of 10 and are 50mm wide. These would capture 100 times more light than our naked eye since the area of the lens is 100 times bigger than our pupil and so we see much fainter things. (Click image to see more...)
The same is true of telescopes – it’s the area of the collecting lens or mirror that counts, telescopes are simply light buckets whose job is to collect more of those wonderful photons and feed them into our eyes.
There is another technique we can use to see fainter, with or without optical aid, known as “averted vision”. Instead of looking directly at something, try looking out of the corner of your eye so that you see it in your peripheral vision – you’ll be surprised how well this works.
This works since our eyes have two types of light sensitive cells – rods and cones. The cones are sensitive to colour and detail but are pretty insensitive to light, needing more light to switch on – these are centred on the fovea, the central part of your eye. Rods are less sensitive to colour but are more sensitive to faint light and are mainly located in our peripheral vision. They are actually 20 to 40 times more sensitive.
If you look between eight and 20 degrees to one side (I find image noseward is best) you’ll
see much more, without buying any optics. It works with binoculars too – just look slightly upward instead.
Have you heard the phrase "If you want to see in the dark - eat your carrots "? There is a grain of truth in it as there is a chemical in the cells of your eyes that are sensitive to light that is called Rhodopsin - this chemical breaks down under light to retinal and opsin, now to help the chemical recombine and also to be made in the first place requires vitamin A and this is found in abundance in vegetables like carrots...
The last technique is to make sure your eyes are fully dark adapted. Your pupil takes time to fully dilate and the chemical processes in your eye take time to acclimatise to faint light (think of it as taking time to clear the effect of the light from the cells after they have been charged up by bright light). If you wait about 20 minutes in a dark place without looking at any bright lights, your eyes will be most sensitive but be careful, a quick glance at a bright light can be enough to ruin the effect and set you back another 20 minutes or so.
One way around this is to use a red torch to find your way around - you'll find that the red light is much less impactful on your night vision - the red light doesn't trigger the pupil to contract - you can find a good one here, click to see more:
Some people also say that’s why pirates wore an eye patch – so one eye was always dark adapted although that sounds a bit like a tall tail to me...