Have you ever noticed that you can see faint stars better not looking directly at them? The reason is the averted vision. How does it work, and how to use it “in the field”? Let’s clear this up!
What is averted vision?
Averted vision is a technique to view faint objects by looking not directly but slightly away from an object. It’s often used by astronomers to see much more in the night sky. This technique fits for naked-eye observations or viewing objects through binoculars or a telescope.
Averted vision involves using peripheral or side vision that allows seeing things outside of your direct line of sight.
How does averted vision work?
Averted vision is possible due to the human eye structure. Our retinas contain two types of detectors — rods and cones. The rods detect dim light in black and white, while the cones detect bright light and full colors.
The central part of the retina is primarily made of cones, and the rest of it consists of rods. This is why our eyes provide an ultra-clear detailed image of objects in the center of the field of view, but as soon as you look a little bit off to the side, an object’s image becomes blurred.
Such structure leads to a decrease in visual sensitivity in central vision at night. So to see a dim object at night, make the light fall onto the outer part of the retina, filled with the light-sensitive rods. In other words, look slightly away from an object. Unfortunately, rods (unlike cons) don’t detect various colors, so faint objects observed with averted vision will appear grayish-white.
Why is averted vision important?
Averted vision is the best way to see faint objects in the darkness. Using this technique, you can see sky objects invisible to direct vision.
Just imagine: the eye area filled with rods is about four astronomical magnitudes (or 40 times) more sensitive than the eye’s center made of cones. This means that if you can see a 2nd magnitude star in the visual center, you’ll be able to see a 6th magnitude star off the center using the averted vision!
How to use averted vision
You can use averted vision with or without optics. The method, in general, will be the same:
- Pick an object you want to see. If you use a telescope, choose an object of about magnitude 8 — for instance, the Cigar Galaxy. For the naked eye, pick something brighter, like open star cluster M41.
- Locate the object in the sky. A stargazing app will help you here — you can use the Sky Tonight app that shows hundreds of galaxies and star clusters for free.
- Once you’ve located the object, place it at about 8° to 16° away from the eye’s center (the exact angle is different for each person).
- In your field of view, keep the object nearer to your nose than to the side of your face. This way, you won’t let the object fall on the blind spot on the side of the eye.
- If you’re using binoculars, point your gaze above the object instead of looking to the side. Looking only sideways makes one eye more sensitive at the expense of the other.
Don’t give up if it doesn’t work immediately. Using averted vision on purpose is a lot more complicated than it looks.
How to observe faint objects even better?
Here are some additional tips that will help you to better view dim objects:
- Give your eyes about 30 minutes to adapt to the darkness. Don’t look at anything bright, and don’t use your phone unless it has a night mode. By the way, minimizing your exposure to the sunlight during the day also helps — use sunglasses to protect your eyes.
- Use a red flashlight — it doesn’t ruin your night vision.
- Look for the faint objects at or near the zenith (directly overhead). There will be less atmosphere to look through and, therefore, less dust that light has to penetrate to reach the eyepiece.
- If you use a telescope, try to move it back and forth slightly to shift the object around in the field of view. The eyes are more sensitive to moving objects than to static ones.
- Wait for nights with good sky transparency; haze and humidity in the air can block objects’ details. Also, ensure there are no clouds or the bright Moon in the sky. You can check the stargazing index in Sky Tonight — the higher it is, the better viewing conditions are.
- Averted vision technique has been known since ancient times; Aristotle used it while observing the night sky. Nowadays, apart from astronomy, averted vision is used in military training.
- Averted vision has also been used by sailors whose duties included standing lookout watches. It helped them to spot dim lights from other ships or shore locations at night.
- Averted vision works differently for each person because there are large variations in how our eyes are made up. In fact, our retinas are even more distinctive than our fingerprints.
Averted vision scale
Ron Morales from the Sonoran Desert Observatory developed an averted vision scale to allow some distinction in the visibility of deep-sky objects. This scale is probably for those interested in the more advanced night sky viewing, but we’ll leave it here to complete our averted vision guide.
An object can be seen with averted vision but once found, the object can occasionally be seen with direct vision.
An object can be seen only with averted vision but is held steady. Here the sweep of one’s vision makes the object detectable.
An object can only occasionally be seen with averted vision as it “comes and goes” with the seeing conditions. In this case, the object is seen more than 50% of the time.
An object can only occasionally be seen with averted vision as it “comes and goes” with the seeing conditions. In this case, the object is seen less than 50% of the time.
An object can only be glimpsed with averted vision after continuously viewing the field for a few minutes or more. This level of averted vision usually occurs when one carefully observes a field for a lengthy period of time. This might occur within the first 3 to 5 minutes of viewing the field.
Bottom line: Averted vision is an essential technique for stargazers, especially those who enjoy observing faint objects such as galaxies, star clusters, and nebulae. It reveals objects about 40 times fainter than direct vision does! To use it, look slightly to the side of the object you’re trying to see.
We wish you clear skies and happy observations!