In this fascinating world of astronomy, our eyes are the most powerful observational equipment, as much as the telescope, if not more. Averted vision, is the concept of not looking directly at the object, but at an ‘averted’ angle. As strange as this might sound, it enhances our vision of the night sky and helps us understand more about faint objects, which are usually difficult to perceive with our normal vision. Let us look more into this vision, and train ourselves with this technique.
The Relevance and History of Averted Vision
The practice of averted vision is not recent, people have used it for 2500 years! The renowned Greek philosopher, Aristotle, noted that stars appear brighter when viewed indirectly. It was also not limited to just astronomical observations, the seafarers and travellers also used it as a method during the night to identify faint lights from other ships and the nearby shores, as a means to navigate in the dark. We can detect stars with 4 astronomical magnitudes fainter when using this technique, compared to direct vision.
The Science Behind Averted Vision: A Look at the Eye Structure and Optic Nerve
To understand the workings of averted vision, one has to first understand the structure of the human eye. The centre of the retina, the inner layer of the eye which detects the light, is called the fovea. It is the most used part of the retina and is packed with cone cells - the type of photoreceptors which perceive colours when the image is bright.
As we go about 15°-20° towards the periphery of the eye, we find a region densely packed with rods - the cells which are sensitive to faint light and motion and don’t perceive colours, only brightness. To take advantage of this region, we try to focus the light from any faint source at some angle from the fovea.
There are also these ganglion cells, which convert the light sensed by the cells into electrical signals. At the fovea, each cone is connected with one of these ganglions, but as we move towards the sides, around 100 rods might connect to one of those, sending information sensed via the same cell. This leads to the lower resolution of the image produced at the sides, compared to the centre.
There is an important point to note, we make sure the object being observed at an angle is closer to our nose and not away from the nose because there is a high chance the blind spot in our eye - where the optic nerve leaves the eye - might cause the light to not be detected at that angle.
Studies also suggest that 8° is the perfect angle for averted vision, as opposed to the region of densely packed rods at 20°. There is not a single reason found, but it can be a combination of the balance between the resolution and the rod density, and our habit of using the region near the centre more often than the periphery.
Dark Adaptation: The Key to Achieving Night Vision
There few more tricks to take advantage of the dense region packed with the rods. We call this set of etiquettes Dark Adaptation. There are two main changes in the eye when a person steps out from a bright room into darkness - the pupils dilate, and the secretion of the chemical rhodopsin increases. The latter almost takes 40 minutes to settle down, after which one will be able to detect faint light much better in the dark atmosphere.
During the time our eyes get adapted to the dark, we should not expose our eyes to any bright light, including street lights and mobile lights. It might cause the process to reset, making it necessary to start it over again. We can use red-light torches (or white torches covered with red paper) or red-tinted phone screens to check the star map, which doesn’t damage this process.
How to Use Averted Vision: A Step-by-Step Guide
To employ averted vision, begin by locating the celestial object you wish to observe directly. Then, subtly shift your gaze about 8 to 16 degrees to the side. While maintaining your gaze at this averted position, concentrate on the area where the object is located. It may take some time and patience, but gradually, you’ll start to discern more details than you could with direct vision.
- Choose a place with clear sky, and with not much exposure to light - even street light.
- Wait for 30lights0 minutes for our eyes to get adapted to the darkness.
- Identify the location of the object we want to observe directly, using a map or its coordinates.
- We can view slightly faint objects with our naked eye, but it would be better if we could set up a telescope, which would enable us to detect fainter ones.
- Now try to place the object of interest around 8°-16° away and make sure the it is towards the nose, i.e., the our vision is ped to a place above the object.
- Moving the telescope back and forth also might help in detecting it, because the eye is more sensitive to motion than static objects.
Congrats, you are successfully using averted vision. Using this technique on purpose is not so easy, so don’t give up if you are not able to do it the first time. Mastering this technique will unlock a lot more opportunities and details than before!
Observing with Averted Vision: An Illustrative Example
With our naked eye, the Andromeda Galaxy (M31) or M41 open start cluster serves as an excellent example to practice averted vision. While Andromeda’s core is bright enough to be seen directly, its faint outer spiral arms are an ideal candidate for averted vision. By using this technique, you’ll be able to perceive these spiraling arms, prospirallingricher and more detailed observation of the Galaxy.
Other objects to try out with a telescope are the blinking planetary nebula - NGC 6826 or the Cigar galaxy, with a magnitude of about 8.
Averted vision is a smart trick. It uses a special feature of our eyes to help us see things in the universe that are hard to spot. So, the next time you’re looking at the stars, remember: sometimes, the best way to see something is by not looking directly at it.
References
https://www.skyatnightmagazine.com/advice/how-to-master-the-art-of-averted-vision
https://starwalk.space/en/news/averted-vision
https://en.wikipedia.org/wiki/Averted_vision
https://c01.purpledshub.com/bbcskyatnight/2023/03/23/explainer-master-the-art-of-averted-vision/
https://adsabs.harvard.edu/full/1977JRASC..71..327B - Excerpt of Aristotle stating about averted vision in his book Meteorologica
