Last updated on
4/11/2006

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Viewing Conditions
To see dim deep-sky objects, atmospheric conditions at your location need to be good for that night (i.e. minimal haze and turbulence). An easy way to check viewing conditions is by observing the stars in Ursa Minor (the Little Dipper). Click on the image on the right for directions on how to do this.

There is an outstanding Web site most experienced observers use to predict the viewing conditions for the current and next night at many different locations in North America: Attilla Danko�s Clear Sky Clock site.  He is a software developer and amateur astronomer who uses forecast data (generated from a numerical weather model specifically run for amateur astronomers by the Canadian Meteorological Centre), and generates a graphical representation of predicted viewing conditions that are generated by the CMC model, hourly for cloud cover and transparency and in three-hour time blocks for seeing. He also includes a link that shows you the light pollution level at each site, taken from data provided jointly by the Light Pollution Science and Technology Institute at the University of Padova in Italy and the NOAA National Geophysical Data Center in Boulder, Colorado.  Attilla Danko�s work (and the data behind it) is a terrific aid for North American amateur astronomers - highly recommended!  Note also that the Arkansas Sky Observatory Web site includes a good explanation of the meaning of the Clear Sky Clock graphic representation - on that site go to the �ASO Sky Clock� link on the left, and then click on the �Instructions� link on that page.

Learning to See
Assuming good viewing conditions, there are two techniques you need to develop to see dim (i.e. most deep-sky) objects through your scope: 1) night-vision adaptation, and 2) averted vision.

Night-Vision Adaptation
You are probably already aware that your night vision takes a while to establish.  But you may not realize how long that actually takes: typically 30 minutes for most people.  After you leave a well-lit house to go out to your scope for an observing session, you should plan on waiting that long before your eyes have their maximum ability to see deep-sky objects through your scope.  Note that it may appear to you that you have good night vision in 10 or 15 minutes, but you actually gain most of your ability to really see dim objects after 30 minutes or so - it�s worth developing patience and waiting that long (unless you only plan to observe bright objects like the moon or Jupiter for the evening).

To retain your dark adaptation it is important during your observing session to avoid looking at bright lights, which is why all experienced observers use a dim red flashlight specifically designed for astronomical observing sessions. Note that there is some controversy about the red color - a few experts claim that green is a better color for preserving night vision but in any event red is the only color available in astronomical observing flashlights. Experienced observers also assert that alcohol or nicotine use prior to, or during an observing session reduces night vision capability.

Averted Vision
Most deep-sky objects are fairly dim even in an 8� scope and at first you may be disappointed when you start observing the Messier objects, even though they tend to be the least dim deep-sky objects. There is a technique for observing deep-sky objects that can help you a lot, called averted vision.

The retina of your eye has two sets of light-sensitive cells, called rods and cones. The cones are used for normal (well-lit) vision, and the rods are far more light-sensitive (and motion-sensitive) and are used for night vision. (Although rods do not detect color - only cones do.  Note also that the rods are stimulated by secretion of a hormone - rhodopsin - and this takes a while to occur which is why I noted above that night vision takes a while to establish.) You need to know that the cones are concentrated at the center of your retina, and the rods are distributed outward of the center. This means that when you concentrate on the part of your vision that sees straight ahead you are concentrating on your cone cells, while your rod cells are providing your peripheral vision. In a normally lit situation the cone cells transmit so much information to your brain that you don�t even notice the peripheral vision information your rod cells are transmitting, unless there is motion there.

Now, at night your cone cells can�t see much while your rod cells are actually detecting a lot.  You�ve probably never been asked to pay attention to your peripheral vision but at night that�s where your vision is strongest.  What this means is when you want to detect a dim deep-sky object through a telescope eyepiece you will see it best if you direct your eye towards the edge of the eyepiece but concentrate your mind on the center.  (Note also that you want the object between your eye and your nose - i.e. avert your vision in whichever eye you are using, towards your ear on that side of your head.  This is because you have a blind spot where your optic nerve enters your retina, and this orientation will prevent you from placing the celestial object into that blind spot.)

This averted vision feels very unnatural or even unsatisfying at first, because you can�t detect as much detail with your rod cells.  But practice this technique - with practice you will find that you can absorb the view in the center of the eyepiece while using this averted vision, and you will find that you can detect dim objects much better this way. You can also get a better feel for the complexity of a dim object with averted vision; for example, you can detect the very faint stars in a star cluster even if you can�t resolve them directly.

The fact that the rods detect motion well, can also be used to your advantage when seeking faint objects with a GoTo scope. If you watch through the eyepiece while the scope closes in on the final position of an object you�ve selected, you will detect it more readily because it is moving.

One of the most interesting and in-depth discussions of observing faint objects is contained in Dr. Roger N. Clark�s book Visual Astronomy of the Deep Sky, published by Cambridge University Press in 1990. Like many excellent astronomy books, it is now out of print but it�s worth getting Amazon.com to try to find you a used copy - it is a classic and well worth having. Alternatively, the experienced observer Stephen James O�Meara in his The Messier Objects book, describes other observing techniques including jiggling a scope and heavy breathing, and his discussion of these techniques is worth reading.

Sky & Telescope Magazine has a discussion of deep-sky observing techniques on their web site, titled Secrets of Deep-Sky Observing, which is also worth reading.
 

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