Why use red lights to preserve night vision?

by Phil Beastall

Dim red lights are usually used in observatories for reading maps and data, displays for astronomical software designed to be used at night are often in shades of red or a red filter is put over the computer monitor. Red torches (flashlights) are usually used to avoid trip hazards or read maps when using portable instruments.

This should come as no surprise; after all we are used to being shown to our cinema seats by someone wielding a torch with a red surround. The aim is to avoid spoiling the dark adaption of our eyes by shining bright white light. A rudimentary knowledge of these things is important in helping us to use light without it interfering with our observations.

As we leave a brightly-lit room, or as dusk falls, so our eyes grow used to the lower light levels and the receptor cells in the retina become more sensitive. There are two kinds of cells, known as rods and cones. The cones see in colour but are less sensitive than the rods, which see in black and white.

When we need to illuminate a chart or a book by the telescope, we can do so in such a way that only the dark adaption of the cones is reduced. The peak response of the rods is in the blue and green areas of the spectrum - this is why dim blue lights are used on some submarines, for efficient sight in reduced light conditions. The opportunities for looking at stars out of the window are somewhat limited in a submarine. The cones respond strongly in red light to which the rods are quite insensitive. Hence, we can use red light to see our charts via our cone cells without badly affecting the dark adaption of the rods, and we can keep our rods well dark adapted for seeing diffuse objects through the telescope. The cones are of little use for the dimmer objects anyway so there is very little sensitivity lost.

This is a good reason for using red light for illuminating a reticule in an eyepiece or a zero power finder (such as a Telrad). The cones respond to the red light and the more sensitive rods do not lose their dark sensitivity when viewing the objects in the field of view.

Another trick that is useful with a straight-through finder with an illuminated reticule is to simultaneously view the sky with one eye whilst viewing through the finder with the other. It is then possible to move the telescope in such a way that the two views come together from opposite directions and their centres coalesce in the mind. A variation on this is to have a red-illuminated drawing pad viewed by one eye whilst the other eye views the object through the telescope. It is possible to arrange this so that the two are co-located in visual perception. It is then possible to trace the view from the telescope onto the paper - rather like using a camera lucida.

The rods and cones are not evenly distributed on the retina. Our primate origins may be responsible for the colour cones being denser towards the centre of the field of view, always handy for spotting ripe fruit. The more sensitive rods predominate towards the edge of the field of view, very useful to avoid being surprised by a predator. This is why you experience the advantage of averted vision when looking at something that is dimly illuminated. Centre a faint nebula in the eyepiece and look straight at it, then move the centre of your vision a little to one side and see it visibly brighten. This is a familiar technique to those who observe faint and diffuse objects, and it can be very useful. Please note that it requires some practice not to look straight at the thing you are trying to view. You should also note that the density of receptor cells is less away from the centre of the retina, this means that resolution will be lower - this is not so important with nebulae.