When I first became interested in the night sky, I was sometimes disappointed when I looked for faint objects.  My experiences seemed to indicate that my eyesight was nowhere near as sensitive as the eyesight of the authors that I read.  Many times I would fail completely to see the object they described.  I have since learned that it was not my eyesight that was flawed.  I simply had no knowledge of the viewing skills necessary to see faint objects, and I didn’t understand the importance of certain environmental conditions. 

The eye is incredibly adaptable.  The brightest star that we sometimes encounter in our field of view, the sun, (no, don’t look at it!) differs from the brightness of the faintest star we can detect by a factor of around 10 trillion.  Understanding a little bit about how the eye functions over such an incredible range of intensities can really help your ability to see faint objects.  Most of this discussion is based on an excellent review of the matter in the book, Visual Astronomy of the Deep Sky, by Roger N. Clark.  

First of all we’ll talk about environmental conditions.  When I was young I had no idea how much my view was affected by the light pollution of my suburban night skies.  Doing nothing more than getting far away (at least 20 to 30 miles) from the glow of city lights can result in a gain of 2 -1/2 stellar magnitudes or more!  That means you are seeing objects 10 times fainter than from the city suburbs.  This is equivalent to tripling the size of the lens or mirror in your telescope!  So if it’s at all possible, get away from the city. 

If you are looking for faint objects in the sky, even a thin crescent moon will diminish your view considerably.  Pick times when the moon is below the horizon.

Use the weather to your advantage.  You are much better off conducting your search on a night after a cold front has passed through your area.  The skies are usually much clearer in these conditions.

The position of the object in the sky is also very important.  Objects are best seen when they are at the directly overhead at the zenith.  The closer to the horizon an object is, the more its light is dimmed by the atmosphere.  Be patient and wait till the object transits (crosses the north-south line in the sky), when it’s at its maximum elevation from the horizon.

Now for the skills.  First of all, make sure that your eyes are completely dark-adapted before you begin your search.  When you are in a very dark environment, your eyes adapt in a couple of ways.  First, the pupils of your eyes widen to a maximum diameter of around 7.5mm (the actual maximum diameter varies from person to person, and decreases with age).  The second adaptation is a chemical process.  In dark conditions, your eyes produce a chemical called rhodopsin, or visual purple.  It takes about 30 to 40 minutes for your eyes to become completely dark-adapted. This time must be spent in near complete darkness.  If I’m getting up in the early morning hours to look for a faint object, I will set out my clothes and plan my route through the house so I don’t have to turn on any lights (or walk past any night lights) on my way out of the house.  I’ve been rewarded with a few stunning sights by doing this, as well as a few stunning blows from various objects I’ve stumbled into on my way out.  Be careful!  Once outside, avoid looking toward any outdoor lights or streetlights.

A second very useful skill is to know how to use averted vision. There are two types of light-detecting cells in the retina of the human eye, cone cells and rod cells.  Cone cells are concentrated in a small area at the visual center of the eye called the fovea.  Cone cells are responsible for our color vision.  Rod cells are incapable of detecting color, but are more sensitive to faint light.  The rod cells increase in density from zero at the center of the fovea to a maximum density about 18 degrees off-axis from the center of vision.  Since the highest concentration of rod cells in the eye is away from the center of vision, you must look slightly to one side of a very dim object in order to see it best.  For faint objects, this can make the difference between seeing the object or not seeing it. 

It’s best to experiment to find where the most light-sensitive areas of your own eyes are.  I usually have the best luck by looking slightly to the right and upward of the faint object I’m trying to see.  The faintest stars will disappear when you look directly at them.  Be sure to breathe normally – sometimes people will slow their breathing when concentrating – you really need that oxygen for your eyes to be effective!   Every now and then, take a break and relax for a moment before continuing your search.

Finally, once you get an impression that a faint object is there, try to move your head as little as possible.  For the faintest objects, the light must accumulate on the retina for at least six seconds.  You must be patient.  If you wear glasses and cannot see the object, try taking them off.  In my own case, I can see fainter when I’m not wearing my glasses, even though my vision is not quite as acute.  I will sometimes spend 30 minutes or more trying to see a faint object.  Unless I’m sure I’ve seen it several times, I don’t count it.

Don’t be discouraged if you do not see the object.  Instead, congratulate yourself on being honest!  Sometimes people trying too hard to see faint objects succumb to what has been called “averted imagination.”  Use poor quality or moonlit nights to memorize the star fields around your target object so that you will be prepared when that perfect night comes along. 

Binoculars can be a tremendous help in spotting faint objects.  It used to be thought that binoculars had to have a large exit pupil to be effective for astronomical subjects.  This is not true, and the trusty 7x50 binoculars are actually not the best choice for all night time objects.  I routinely use my Zeiss 8x30 binoculars to spot galaxies in dark sky situations.  Some small, faint objects are more easily detected in binoculars that have a little more power.  The reason for this is that even though the image may not be quite as bright as with 7x50 binoculars, a larger image is more easily detected by the retina.  Allen Adler, a very skilled and experienced observer, advises rating binoculars for astronomical use by multiplying the binocular power times the square root of the aperture.  The higher the number, the better the binocular will be for astronomical purposes.  Don’t forget to use averted vision when you’re using your binoculars.

If you are looking with a telescope, in addition to the other techniques mentioned above, try gently rocking the telescope back and forth slightly.  Sometimes this will make a faint object in the field more noticeable.  Many times your first impression of a very faint object will simply be an odd feeling that “something’s there.”

As with all skills, it is important that you practice.  It’s also important to learn your constellations, so you know where to look.  And even poor nights can be used to memorize the location of objects.  A few nights spent contemplating the heavens in a reclining lounge chair will rapidly improve your skills.  Start out with relatively easy objects, like the Andromeda Galaxy, then work your way up to fainter objects.  See how many stars you can count within the “Square of Pegusus.”  If you can count 13 stars within the square, you’ve reached the 6^th magnitude, an excellent goal, but one that may take a number of tries to accomplish.  Can you go fainter?  Be sure to keep the environmental considerations mentioned above in mind and wait till the “square” is overhead.

Finally, remember that if you fail seeing a faint object, it just makes it more special when you do eventually succeed.  Just keep persevering.  The most satisfying objects I’ve seen are the ones that took many years of attempts before I was successful.  Although your views may not be as sensational as the images appearing on the 6:00 o’clock news, you will have the quiet satisfaction of really connecting with an object.  After all, the photons from the Andromeda Galaxy striking the retina of your eyes are ending a journey that began more than two million years ago!  They are for your eyes only.  Good luck!

  M-33 © 2006 Leaps

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