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Old 09-02-2005, 11:19 AM
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rmcpb (Rob)
Compulsive Tinkerer

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Join Date: Jan 2005
Location: Blue Mountains, NSW
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Yep, its the rods and cones that let you see. They are the cells in your eye that detect light. The cones detect colour and are concentrated around the centre of the back of the eye. The rods detect lower light levels but produce a B&W image and are spread all over the back of the eye.

If you have a huge scope that concentrates heaps of light onto the centre of the back of your eye then it is possible that you will start seeing the colours in the nebulae BUT not many of us have access to these beasts.

This also explains why averted vision works. When you look directly at a nebula the image is focused on the centre of the back of the eye which is composed mainly of cones which are less sensitive to low levels of light so it is more difficult to see the image. By averting your vision a bit you project the image onto the area of your eye that is composed maily of rods which are more sensitive to low levels of light so you can see the image but it is in B&W.

Putting a red filter on an eyepiece to see the nebula in red would be counter productive. The light levels are already low (that is why you can't see the colour already) and the filter would just lower the light levels further by restricting the wvelenghts of the spectrum allowed through. Just enjoy them as B&W with occasional glimpses of colour in some of the brightest ones or when you scrounge a look through a large scope at a star party.

Cheers
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