Our eyes have to dark-adapt before we can see astronomical
objects optimally. Usually, this requires fifteen to twenty minutes (or
more) in an environment as dark as the sky you will be viewing. First of
all, the pupil needs to dilate to its maximum aperture in order to collect
the most light. Another component of night vision is contained in the biochemistry
of the eye. A pigment known as rhodopsin is contained in our rod
cells and is sensitive to very low light levels. We need to use our rods,
as opposed to the color-vision cone cells, in order to see faint, extended
objects like nebulae and galaxies. Any source of bright light will saturate
the rod cells, destroy the sensitivity of the rhodopsin, and require twenty
minutes of further dark adaptation. Rhodopsin is less sensitive to red
light than to other wavelengths. This is why astronomers read star charts
and make log entries with the help of dim, red lights. Our red-absorbing
cone cells allow us to read, while we maintain most of our dim-light sensitivity.
Even red light sources, if too bright or aimed at the eye, can cause loss
of dark adaptation.
The failure of many novice astronomers to see faint objects
or pick out fine detail is due not only to their eyes not being fully dark
adapted, but to their eyes being "untrained". When our eyes have not seen
an object before, we are not sure what to look for. It takes time and patience
to get used to the appearance of an object; only when we have cleared this
obstacle can we perceive the finer detail. One must of course guard against
creative observing, in which the observer believes he/she sees details
which are not actually visible.
Deep-sky objects, for instance,
often appear as faint blobs or are invisible to first-time observers. Finding
these objects, most of which are far too faint to be visible with the naked
eye, can be more rewarding than viewing them once they have been located.
There is something satisfying in having "conquered" an object which one
has searched for over the course of many nights.
Averted Vision: When you look directly at an object, most
of its light is falling on cone cells in the fovea of your retina. This
provides the best color vision when looking at bright objects, but since
the cones are not sensitive to dim light, direct vision is not the best
way to detect a faint object. Instead, look to the side a bit; you have
more rod cells away from the fovea. This is called averted vision.
Extended Vision: When trying to see more detail in an object
you have found, try looking at it with either averted or direct vision
for 30 seconds to a minute or more. Extended vision is not well-understood,
but apparently the eye can build up an image over time, somewhat like a
time exposure on a camera. This seems to work best on faint objects.
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