Breakintheclouds

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 on the eye and how it works in the dark, in the hope it helps people understand all this stuff about red lights, averted gaze and telescope tapping. (I'm a psychologist with a background in neuroscience...)

THE RETINA

As you probably know, the back of the eye is coated with a layer of cells called the retina. Many neuroscientists see the retina as an extension of the brain because it isn't just a passive light receiver, like the CCD or CMOS sensor in your digital camera. Rather, the retina does quite a lot of active image processing - in particular, your ability to see the edges of objects is largely thanks to processing that takes place on the retina rather than in the brain proper.

The retina is coated with light-sensitive nerve cells which, when they receive enough light (usually it takes several photons arriving at more-or-less the same time, even with the most sensitive cells), send signals to other nerve cells on the retina, which send signals to other nerve cells on the retina, which then send signals to the brain proper.

There are several types of light-receptor cells on the retina, but the most important are cones and rods. Cones are good at seeing colour and shapes but don't really process movement too well; rods are really good at seeing movement, but are pretty much colour-blind. Cones need a lot of light to function; rods don't need anywhere near as much, and indeed often get washed-out completely and don't work in daylight.

(Incidentally, the light that falls on the retina, having been focused by the cornea and lens, is upside-down: the only reason we don't notice this is because we're used to it, never having seen the world the right way up. So when you look through a telescope, and everything feels upside-down, the light on your retina is actually the right way up for once!)

(Also incidentally, the various links in the chain - from the retinal photoreceptors to the the cells in the brain - are 'noisy': they fire spontaneously from time to time and give the illusion of seeing light. Ever been somewhere completely dark, like down a cave? You might notice that in the absence of all light you don't perceive blackness, but rather a mid-grey. It's known as 'cortical grey' and is the product of this spontaneous activity in the visual system. Cortical grey is the real background against which you see everything, and is another reason why it's difficult to see faint grey objects - by which I mean half the things we want to view in astronomy! We see them against a grey background of our own making, which reduces their contrast just like light pollution does.)

THE FOVEA

Stare at an object - any object. Done that? Good. As you stared at it, most of the light that came from that object fell on the central, most sensitive part of your retina, called the fovea. The fovea is packed with cones, and so is really good at seeing colours and shapes. However, it contains next to no rods. These are much more common towards the edge of the retina, and so contribute more to your peripheral vision. The number of rods grows as you move outwards and the number of cones falls correspondingly, until right at the very edge of the retina you have only rods, and no cones at all.

Here's something interesting you can try, to see this for yourself. Get two or three coloured pens or pencils, and mix them up behind your back. Pick one at random and hold it up, at arm's length, behind yourself, so that you can't see it as you stare straight ahead.

Now - continuing to stare straight ahead - begin to wiggle the pen with your fingers and then move your arm round until the pen enters your peripheral vision. You should notice two things. First, you will probably have no idea what colour the pen is! You can see it moving, and might even be able to tell more-or-less what shape it is, but you can't see any colour. That's because you're only seeing the pen using rods, and they are colour-blind. Now, keeping your arm in the same place, stop wiggling the pen. You should find that it becomes invisible. That's because rods are much better at seeing movement than they are at seeing shapes.

RED LIGHT AT NIGHT

During the day, when it is bright, your rods are often completely saturated by light and don't really contribute too much to your overall vision. They come into their own when light levels are low.

One important thing to know about rods is that if you removed one from an eye and looked at it under a microscope you would see that it is, more-or-less, red in colour. That's because it is filled with a reddish pigment called rhodopsin. Think for a moment what it means if an object - like an apple - is red. A red object is red because it bounces back any red light which falls on it and absorbs all other light - particularly green, which is red's opposite. So that is why we use red lights in astronomy: our rods, which we rely on when we look at things in low light levels, just bounce it straight back. Your cones certainly pick up red light, and so you can use them to look at star charts and so on, but it doesn't matter that these cones then get washed out from the exposure: the all-important rods are pretty much unaffected because they just don't see red light (a person who had only rods in their eyes wouldn't notice if you turned a red light on).

The second consequence of rods being red is that they are most sensitive to green light - the opposite pattern to during the day when you are most sensitive to red. So on the one hand avoid any exposure to green light like the plague when observing, and on the other hand you should notice a phenomenon called the Purkinje Shift: notice how grass almost seems to glow when you're out at night? And notice how red things look black? That's the Purkinje Shift: your eyes' sensitivity has shifted from red to green as you've moved from using mostly cones to using mostly rods.

AVERTED GAZE AND SCOPE TAPPING

Averted gaze and telescope tapping are two tricks used to see faint objects, such as nebulae, in a telescope. You might already have worked out, from what I've said about the eye's physiology, how they work.

Let's say you're looking at the Ring Nebula. If you stare right at it then it might well be almost impossible to see its form. That's because the faint light from the nebula is falling on your fovea, which is packed almost exlusively with cones. Cones are intended for daytime use, and the light from the nebula just isn't bright enough to make them fire. By averting your gaze -- that is, by looking a couple of degrees to one side -- you make sure that the light from the nebula falls outside your fovea, onto a part of the retina which has more rods in it. Rods are better at seeing faint light, and so the nebula seems to become brighter and more visible. Incidentally, the fovea is only about 1-2 degrees across, so you don't need to avert your eyes that much before your rods come into play. But as the number of rods gets higher and higher as you move closer to the edge of the retina, the more you can avert your gaze the better in many cases. (But remember to look to the right with your right eye and to the left with your left eye, so that the object doesn't fall into the eye's blind spot.)

The downside with averted gaze is that the photoreceptors are much less tightly packed outside the fovea, and so you have less acuity when averted than when looking straight at something. If you want proof, try to read text whilst staring at a dot in the page's margin...

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.

So there you go: red lights, averted gaze and telescope tapping - they all work for pretty basic biological reasons.

WANT TO LEARN MORE?

Semir Zeki's book A Vision of the Brain is the classic text. Also any textbook on human perception (they're all called 'Sensation and Perception' or something very similar) will have loads of useful information. Sekuler & Blake's 'Perception' is particularly good.