by Ernie Rossi
Originally appeared in TPO Volume 10, Issue 2.
©1999 Typographica Publishing.
May not be reproduced for commercial purposes.
Many times when I am viewing deep sky objects or faint planetary details and someone
from the public or a novice amateur astronomer peaks through the telescope, they
can never see the detail I can. Why? Are my eyes better? Am I imagining things?
If an experienced observer comes over, he or she may be able to identify what I am
seeing and maybe even more. Why the difference? The answer is that experienced
observers have special techniques for seeing. What I am going to explain here are
the basics of preparing your eyes to see faint and difficult objects.
Our eyes have to dark adapt before we can see astronomical objects optimally.
Usually, this requires 15 to 20 minutes (or more) in an environment as dark as the
sky you will be viewing. Dark adaptation consists of two stages. The first is the one
most people are familiar with. The pupil dilates to its maximum aperture in order to
allow the most light into the eye. The second part of night vision takes place in the
eyeís biochemistry. The retina has two kinds of cells, known as "rods" and "cones".
Each are used for different kinds of sight depending on light levels. A pigment
known as rhodopsin is contained in our rod cells and is very sensitive to very low
light levels. We need to use our rods, as opposed to the color-vision cone cells, 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
another 20 minutes of dark adaptation! Rhodopsin is less sensitive to red light than
to other colors. 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 directly at the
eye, can cause loss of dark adaptation. [see "Red Revisited" in this issue of TPO for
more on the use of red light.]
The failure of many a novice astronomer to see faint objects or pick out fine
detail is due not only to their eyes not being fully dark adapted, but to the way
they are using their eyes. When a novice looks through my scope and canít see the
detail I can see, I explain to them in detail what to look for and, more importantly,
how to look. When our eyes have not seen astronomical objects before, we are not
sure what to look for. It takes time and patience to get used to the appearance of
an object and only when we have cleared this mental obstacle can we perceive the
finer detail. Even experienced observers must guard against "creative observing"
in which the observer believes he or 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. Just finding these objects at the limits of visibility can
be more rewarding than viewing them once they have been located. There is something
satisfying in having finally "conquered" an object which one has searched for over
the course of many nights!
There are a few basic visual techniques which can be used to locate such
When you look directly at an object, most of its light is falling on cone cells in
the fovea of your retina. The human eye obviously evolved primarily for use during
the day, so our regular direct vision optimizes use of the cone cells. 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 a bit to the side, above or below; you have more rod cells away from
the fovea. This is called averted vision.
When trying to see more detail in an object you have found, try looking at it with
either direct or averted 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,
something like a time exposure in a
camera. This seems to work best on faint objects. Stephen James OíMeara, author and
keen eye observer, uses this method and usually stares at the objects for much
longer periods of time to catch every photon on light.
Another method used to betray an elusive object is to move the suspected object in
and out of view, or jiggle the telescope slightly so that the light of the
object "smears" such as to make the contrast between the object and the sky background
stand out more. [I once detected the California Nebula under fairly light polluted
skies with a 10" Newtonian and an H-beta filter by scanning across the area in broad
sweeps at low power. I "detected it" - I canít really say "observed" - as a sudden
increase in background brightness. Iíve used field shifting many times since and
found it does indeed work very well. - GB]
Some observers use a black patch over the eye they view through to make sure no
light has a chance to affect its night vision, removing it only to look through
the eyepiece. Another method of guarding against possible distractions and unwanted
light is to pull a black cloth over your head and eyepiece while observing. The use
of nebula and light pollution filters also play an important role in putting the
odds in your favor when hunting faint nebulae or galaxies.
If you have any comments or questions, the author may be contacted via
email at EROSSI40@aol.com or by writing care of TPO.