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Primer: Understanding night vision, averted gaze and telescope tapping


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As a relative newbie to astronomy I can't really contribute any tutorials on nebulae and planets, but in the spirit of giving back to this forum, from where I've learnt so much, here is a short primer

I've been reading stuff on this site for a while now but I finally decided to register after reading this just to say thanks.  I learned a LOT from this article and it really helped me understand why

Hi well I have just signed up to this forum with all intentions to ask about equipment and the like. I didn't think that my first post on the forum would be this but after reading this post (one

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I've never heard of scope tapping until now. I'll give it a go! Might be a good idea to try out my wobbly bino tripod :-)

Edited by MikeWilson
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@Ian, I remember the experiment of the man wearing googles with invert lenses. I recall he even rode a motorcycle while wearing them. I think it was in Popular Mechanics or Science a few decades ago.

Loved the informative article. Beginner here and appreciate the insight. Thanks

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That is very helpful, thank you.

For a complete beginner like me that is food for thought and good advice I look forward to testing when the skies clear again!

Having just ordered some cheap bits and pieces to help me a long I now realise I should have done something about a red lense for my head torch.... doh!

Martin

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You can actualy train your eyes to work better in low light condiotions. It is something the british army has done since i joined back in 1999. Just go out for a walk or even just sit down in a dark area, away from street lights etc etc and try to focus on making objects out. I used to use the figure 8 technique which is to imagine i was drawing a figure 8 with my eyes over the object i wanted to focus on.

Over time you will find that your eyes work alot better than what you might expect them to in low light enviroments :p

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Another trick for seeing faint objects (the Veil Nebula comes to mind here) is gently to tap the telescope, to make the view shake. This is because, as you saw with the pens, your rods are much better at seeing things that are in motion than they are at seeing unmoving objects. The extra little bit of motion really helps your rods detect the object.

I'd never heard of this before. Would i be correct in assuming that tapping the scope to make the image shake, increases the apparent size of the object and allows your eyes to detect it better and once you have found it then you can stop tapping the scope and zero in on it.

Thats a very good tip for locating faint fuzzies such as galaxies. Galaxies etc, once located look better with averted vision.

"Telescope Tapping"....................i've learned something new today.

Edited by LukeSkywatcher
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Paul, you can tap the scope or you can simply sweep the scope left/right or up/down across the field of view. The idea is simply to add motion (the size of the object won't change). Probably the reason this works is that you're adding extra information by moving the image. This information comes in the form of correlations. Your brain identifies objects in the visual scene by looking at correlations in neural activity (so brain cells firing simultaneously, basically). With a static image you only have contrast to go on. If you add motion then you can also look for correlated activity in this domain.

EDIT:

I think that was probably a bit cryptic, so let me explain this another way. Imagine you're a single photoreceptor and are detecting light from a tiny area out in the world. You see your little patch of world become brighter than darker again. There are two things that could have happened:

1. Something that was already there became brighter and darker again.

2. A brighter thing passed transiently through your little bit of the world.

So the first is scenario where something flashed on and off. The second is a scenario where there was motion. You can't disambiguate these two things just by looking at the activity from a single photoreceptor. This is because, as you've probably guessed by now, single photoreceptors can only provide information about luminence not motion. To extract motion information you have look over many photorecptors and compare what they're all telling you. This allows you to diambiguate brightening from motion.

So the brain is wired up to pool activity from very many photoreceptors and what you see (particularly at night) is the average activity of vast numbers of photorecptors. You take advantage of this averaging whether there's motion or not, but motion activates extra pathways in the brain. It's rather interesting why this is the case. Some pathways coming out of the eye are, loosely speaking, specialised to detect shape and others specialsed to detect motion. A single photoreceptor (rod or cone can feed its signals into both pathways. This is a form of parallel processing, allowing the brain to process information rapidly by splitting it up. So if you're looking at a static image, you're activating the shape pathway only but if you move the image you are activating both. So possibly the improvement you see when you're moving the telescope is because you're providing the brain with extra information by activating a neural pathway that otherwise wasn't being used.

Edited by umadog
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