There are two ways the eye regulates the amount of light the sensors in the retina.
One is mechanical and is controlled by the opening and closing of the pupil from 2 to about 8mm, a factor of 16x, and as we age, the ability of the pupil to enlarge lessens. The other is chemical and is controlled by the pigment rhodopsin in the rods of the retina. The Rods are sensitive to faint monochrome light and are generally arranged around the more outer parts of the retina, whereas the Cones are sensitive to bright coloured light and more concentrated in the fovea, the centre of vision.
The rods are 1000 times more sensitive than the Cones, and to protect them from damage, they are made insensitive if hit by an initial exposure to bright light, by the bleaching of rhodopsin. As long as the light is above a certain threshold, the pigment stays bleached and the rods do not work. After the light has fallen below the cut-off threshold, the rhodopsin begins to come back and our vision gradually becomes more and more sensitive to faint light and we are better able to see under low light conditions. It can take up to an hour.
Techniques
for Improving Viewing of Faint
Fuzzies
To
see the very faintest
objects that we can, we must be completely dark
adapted.
Exposure to very bright light should be limited before an observing session, otherwise, it can take a long time, hours, to reach complete dark adaption.
Wear dark Sunglasses in the daytime, Blues Brothers style.
Make the journey to a dark site if possible. The rewards will be great, particularly if that site is also on high ground, above the thick atmosphere in lowland areas.
Observe on a night of good transparency, when the air is
clean, perhaps after washed by rain.
Seeing is
when the air is steady, usually associated with still hazy nights. So
they rarely go together.
At the telescope.
To preserve dark adaption, a faint red light can be used to read star charts. But a bright red light will affect dark adaption.
Use
averted vision. Look slightly to the side of the object to be observed.
The
rod cells, which see faint light, are located off to the side, with the
maximum
concentration about 15 - 20 degrees from the centre of your vision.
Tap the scope lightly while observing, as the rods are extremely
sensitive to
motion. Hence the saying, to catch sight of something, ”out of the
corner of
your eye”.
Don’t hold your breath at the eyepiece, the eyes and brain need oxygen to function.
Use a dark cloth over the head while observing to shield out all extraneous light, even at a dark sky location.
For
naked eye viewing, concentrate on the area being viewed by cupping the
hands around the eyes as if binoculars were being held, looking through
two
black painted toilet rolls is not going too far.
Increase the magnification. There is a minimum amount of magnification
needed
by the eye to see faint objects because the rods are not as densely
packed as
the cones in the fovea, and they do not have as high a resolving power.
Magnification
also darkens the contrast between the object and the sky background.
The amount
needed is more than you’d imagine.
A light snack and a cup of tea can be helpful while observing, both for
vision
and for keeping warm on cold nights, but not in a lighted room or dark
adaption
will be lost.
Don't smoke, carbon monoxide reduces the amount of oxygen in
the blood.
As we grow older, the ability of the pupil to expand will be reduced, from a maximum when young of 8mm, to perhaps 6 when 45, reducing the amount of light allowed to enter the eye by almost half. Increased magnification helps by reducing the pencil of light from the eyepiece into the smaller pupil. The eye's lens also yellows with age and may form cataracts, damaging vision.