Averted Vision and VSO

When observing a faint light source and looking off to the side you are using averted vision. Most amateur astronomers quickly learn about averted vision and use it to observe faint objects that are not visible when viewed straight-on.

The eye seems to have a limited integration capability similar to photographic film. For the detection of the faintest objects, it seems that light must collect on the retina for several seconds. If you have ever strained at an eyepiece, attempting to view a star just at the visual limit of your equipment until you where able to finally catch a fleeting glimpse of it, then you know this to be true.

When using averted vision it's important to take your time. When searching for faint objects within your FOV, you will miss many faint stars if you scan the area too fast. Concentrating on a point within your FOV while using averted vision will allow you to see many stars that are too faint to observe when viewed directly.

It takes practice to concentrate on a point, usually without a star present, while using averted vision because the eye tends to jerk around slightly (remember the blind spot). You will find that fatigue also compounds the problem. This is why it's a good idea to take frequent, short breaks and to eat something while observing for more than an hour or so. Observing really is a physical effort.

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