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by Ernie Rossi Originally appeared in TPO Volume 10, Issue 2. ©1999 Typographica Publishing. May not be reproduced for commercial purposes. |
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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 eye’s 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 can’t 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 challenging objects. Averted Vision 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. Extended 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 O’Meara, 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. Field Shifting 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 can’t really say "observed" - as a sudden increase in background brightness. I’ve used field shifting many times since and found it does indeed work very well. - GB] Additional Tips 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. |