When you go outside it will take around 20 to 30 minutes for your eyes to be mostly adapted to the dark – it will be several hours before they reach their peak night vision. Make sure you turn off any outside lights, or inside lights that you can see in windows, and choose a place in your yard where street lights or other lighting is not directly within your vision. Every bit of light shining in your eyes is going to affect your night vision. The more sensitive your eyes are to light, the brighter the stars will appear and the more you will see.
Once you’ve reached your peak night vision you need to preserve it! Avoid using a bright torch to read your charts or adjust equipment. There are special red LED torches available for just this purpose, although recent research suggests that low white light has no more adverse affect on night vision than low red light. Red light has been used for so long to avoid spoiling night vision that it’s hard to convince anyone of this though, so red light is still in common use. You should make it one of your earliest purchases so you are able to check star charts in the dark without ruining your night vision. You should also avoid unnecessary trips inside. Stay out in the dark for as long as you’re viewing if you can.
Using Averted Vision
Using averted vision is related to your night vision. Averted vision is using your peripheral vision instead of looking directly at something. The eye is very complex and its complete workings are beyond what we need to understand here, but we will go over some basics to understand why using averted vision is so important in astronomy.
Light enters your eye through the pupil which opens and closes depending on the amount of light it’s exposed to. In darkness the pupil will open to its full extent to allow the maximum amount of light to enter. The light passes through the lens and onto the retina at the back of the eye.
The retina has two types of photoreceptor cells that detect the light – rods and cones. Cones are responsible for colour vision and fine detail, being able to detect fast changes in images. They’re less sensitive to light, requiring bright light to generate a signal. The cones are packed into the fovea centralis – the central rear area.
Rods on the other hand are extremely sensitive and are used for vision in low light levels. Rods use only one pigment for detecting light (as opposed to cones that use three) and so do not detect colour. They fill the outer area of the retina – around 120 million rods to the 6 million cones in the central area.
The rods are coated with a pigment called rhodopsin that is extremely sensitive to light. When exposed to bright light rhodopsin photobleaches instantly, making the rods less sensitive. When it darkens, it takes time to produce more rhodopsin, which is why it takes a lot longer to adapt to the dark after being in light than it does to adapt to bright light after darkness.
In very low light levels the cones are not active, as they are not receiving enough photons to activate. This means the rods are almost solely responsible for your night vision and is why you see little colour in near darkness. The other effect of low light with the cones not operating is that you have a near blind spot straight ahead. The cones are all packed right in the centre – when they’re not working you will have very poor vision dead ahead.
This is where averted vision comes in! The rods are more sensitive to light, so you need to direct the light to them. You do this by averting your gaze slightly. You may have noticed this previously – you see a dim light off to the side but when you look at it, it vanishes. When you turn your gaze away, there it is again! The ideal amount to divert is around 8° to 15°.
There is one further complication. Each eye has a blind spot! This is where the optic nerve leaves the eye – there are no rod or cone cells at that point. This blind spot is around 15° to the temporal side or the outside of the eye. Right where we might be trying to use our averted vision. You don’t normally notice it because the brain uses both eyes for information and it’s also very small – the eye averages the information around it rather than making a black spot. If you look up blind spot in Wikipedia there’s a demonstration test you can take.
The result is that averted vision only works to one side. If you are using both eyes, either naked eye viewing or binoculars, you can compensate for this by averting your vision up instead of to the side.
For those using a telescope, if you use your right eye on the eyepiece, divert your gaze to the right, and if you use your left eye, divert your gaze to the left. You don’t need to avert your gaze much remember, just around 12°. If you’re looking through an eyepiece, look toward the rim of the eyepiece instead of directly into the centre.
This is a little tricky and will take practice. Don’t be put off if it doesn’t appear to work first time, when you get it right the difference is amazing. It can be the difference between seeing a small white dot and a whole nebula.
Measuring the Night Sky
The sky appears as a dome over our heads, known as the celestial sphere. Measuring distances across this dome is done in degrees of arc – the dome, or sphere, spanning an arc of 180 degrees from horizon to horizon. A distance of 90 degrees would be from the horizon to directly overhead.
This is a handy way of finding things in the night sky if you know the distance between them in degrees of arc. You might be told for example that Jupiter is currently 5 degrees from a particular star, or that if you go 10 degrees west from this star you will find a faint galaxy.
We can approximate measurements in the sky using our hands. If you hold your hand out at arms length toward the sky and extend your little finger, the tip of your finger is approximately 1 degree. Your three middle fingers held together will cover about 5 degrees of sky. Your fist measures 10 degrees and if you stretch you fingers out as wide as you can you will span around 20 degrees from the tip of your thumb to the tip of your little finger.