DARK ADAPTATION
On a dark, clear night in an area with little light pollution when the moon is not full (and so not too bright), it is a wonder to observe the night sky with the naked eye. Your eyes contain two different types of cells in the retina known as rods and cones. Cones are able to make out colour and detail and only work in bright light conditions, but it is the rods that are used in dark conditions, and are what observers use during the dark nights. Rods are not colour sensitive, but are very sensitive to light intensity. Modern living with artificial light means that usually, our cones are active in our bright light conditions, while our rods are not required. To allow your eyes to optimise to the dark conditions necessary to observe the wonder of the stars, you must allow your eyes to adapt – this is known as dark adaptation and is the time taken to recruit the rod cells in your retina to the task of observing in low light intensity conditions. Your eyes can take between 20 and 30 minutes to become fully dark adapted. When this happens, you will be able to see more clearly and pick out fainter stars in the night sky. The process of dark adaptation takes time and must be restarted if you look at a bright light such as a mobile phone screen half way through. Red coloured light will not cause damage to your dark adapted eyes, and so all torches used during observation sessions should be fitted with a red filter.
AVERTED VISION
Since it is the rod cells in the eye’s retina that are used in low light conditions typical of observing the night sky, any light that enters the eye from faint stars will only be observed if it lands on a rod as opposed to a cone. The rods are found away from the centre of the retina, and so to see detail while observing faint objects, it is not useful to focus directly on them. Instead, astronomer’s use averted vision so that the light lands on a rod cell in the retina. They do this by looking slightly to one side of the object, and that way recruit the rods in the retina allowing them to see more detail. The rods are not useful when trying to see the colour of a star. To see colour, the cones must be activated. However, cone cells will only activate if the light is of sufficient brightness. When we observe the night sky, we see that the stars follow an arc across the sky. They rise and reach their highest point when they cross the North-South line (Celestial Meridian) that passes through the observer’s overhead point (Zenith). The light from the star must make its way through the atmosphere before it enters the observer’s eye. The atmosphere causes the light to change direction and it is this effect that leads to stars twinkling. The less atmosphere that the light has to travel through, the more clearly you will be able to observe the star, and so it is best to view stars, nebulae and galaxies when they are at their highest in the night sky. We say that the celestial object has ‘culminated’ when it is at its highest point in the observer’s night sky and the time of culmination depends on the observer’s location. The light from a celestial object at culmination will travel through less atmosphere and will be seen more clearly.
PLANNING
To ensure a good view of any object in the night sky then, you must pick a dark sight with a large view of the night sky. An example of the worst place you could pick would be between two skyscraper buildings in New York city: too much light pollution meaning your eyes never dark adapt and too little a view of the night sky due to the tall buildings. You should plan your observation session to coincide with clear skies and good weather. The amount of aerosols in the atmosphere such as smoke and dust can also affect your ability to observe, and so you should plan to carry out your observation away from sources of smoke or dust. The amount of atmosphere that the air must travel through must also be reduced and you can achieve this by viewing celestial objects when they culminate – i.e. when they are highest in the sky at the observer’s location. You can use packages such as stellarium to plan to observe objects when they culminate. For professional observations and photography, the amount of air that the light must pass through can be further reduced by carrying out observations at high altitudes. This is why a lot of professional telescopes can be found on mountain tops. Finally, to get really sharp images of your viewing target, you need good ‘seeing’ conditions. Good seeing conditions mean that the atmosphere is very steady – there is little turbulence in the air that the light must pass through before reaching your eye. With poor seeing conditions, there is a lot of turbulence in the air and this causes the light to be distorted resulting in an image of the target that is not sharp. You can carry out your observations away from heat sources as hot air is quite turbulent and will distort your image. When carrying out observations, it is useful to sketch what you see and keep a record of the date, time, moon phase, name of the target, weather conditions and a rating between 1 – 5 for the seeing conditions using a scale known as the Antoniadi Scale as follows:
I Perfect seeing without a quiver of turbulence at all.
II Slight shimmers; moments of stillness last several seconds.
III Average seeing; larger air tremors blur the view.
IV Poor views with a constant and disturbing swell.
V Bad views with severe undulations; so unstable that even quick sketches are out of the question.
https://www.skyatnightmagazine.com/advice/skills/a-guide-to-seeing-and-atmospheric-transparency/
So, to sum up some of the things to consider when carrying out observations, the turbulence of our own atmosphere is a severe hindrance. This is mostly caused by hot air rising and cold air falling. Indeed, when setting up a telescope to carry out some observations, it is best to do so away from anything that might cause fluctuations in the air temperature around the telescope such as concrete or buildings.
The moving air in our atmosphere distorts the path of the light travelling towards your eye or telescopes and causes the image to lose sharpness and can be quite frustrating.
Another factor that will limit the amount of light that your eyes can perceive is the level of light pollution where you are observing. Light from artificial sources in our urban environment can lead to an orange glow of light being reflected back off the atmosphere towards you, leading to it being impossible to see the fainter stars and celestial objects. Light pollution means that only light streaming towards Earth of sufficient brightness will be detected by the eye. The night sky is filled with a huge number of stars that can be detected by the eye in a dark location, and sometimes this can be overwhelming. It is good practice when first trying to learn the night sky to start with the brighter stars first. In this respect, the presence of light pollution is making your task simpler, as only those brighter stars will be visible in the presence of light pollution. As you become more familiar with the night sky, a good strategy is to go to darker and darker sites and so in that way, more and more of the fainter celestial objects are revealed to you without you feeling overwhelmed.
You too can start on that journey of understanding by taking a Science course. Find out more here.