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Averted Vision


ZiHao

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Yes you are correct. Basically you are utilising your monochrome rods which surround the central colour detecting cones. On faint objects and assuming dark adaptation, only the rods are stimulated. If you look at something directly very few rods are stimulated and the light is not bright enough to activate the cones so you see less. 

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No idea how accurate this graph is, I found it on google.

ret_dens.gif

It shows the low concentration of rods in direct vision (center of the eye) and the highest concentration of rods a little further out. Note, there's also a blind spot where the optic nerve exits the eye, just outside the center too - I've read, therefore, that you might prefer putting the object "towards your nose" rather than "towards your ear".

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I use averted vision quite a lot, it takes a little practise but it does work. Another great little trick to see those elusive objects is to just tap the telescope tube to induce a little wobble and sometimes the object appears very briefly.

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It is a very useful technique to help spot those "on the limit" targets and also seeing a little more extent to already visible objects. There is no prescription of to what extent or in which direction you need to avert your vision - it's something that you need to experiment with. Moving the suspected target to one side of the field of view is one way, keeping the target in the centre and then looking to one side or above or below is another. The so called "1000 mile stare" is another form I reckon where you relax your eye and stare "past" your target, rather like trying to view those "magic eye" 3D pictures. This latter approach works quite well for spotting faint moons close to bright planets I've found. It's suprising how well this can work - I reckon it can add a magnitude or so to a really hard to see point source.

 

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Our club does a lot of public events. I used to get a lot of people who would tell me they couldn't see anything or weren't sure what they were supposed to look for. I would always have to tell them not to stare directly at the object and use averted vision then go through the explanation of why and how that works. That's when I got smart. I no longer center the object in the field of view. I always put it just off to the side. When people look through an eyepiece, their natural tendency is to immediately look in the center, assuming that's where it is. Now rather than the question of "what am I supposed to see?" I get asked, "is it that faint smudge just to the left/right side?" They see it almost immediately and they're less disappointed.

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