Dark Adaptation and VSO

When exposed to dim light, the pupil of your eye can open within a couple of seconds to about 7 or 8 millimeters. As you have no doubt noticed, you cannot see very well in dim light. When your iris opens in response to dim light, the amount of light entering your eye increases by no more than a factor approaching sixteen. However, you have also noticed your eye's sensitivity increases, as times goes by, by a factor of many thousands. This process is called dark adaptation.

Dark adaptation is the result of a chemical process. In dim light, a chemical called rhodopsin is produced within your eye and it concentrates within the rod and cone cells. The quantity of rhodopsin determines the sensitivity of your eyes. Dark adaptation that is adequate for variable-star observing is usually complete after about ten or fifteen minutes. Longer time is needed for observing very faint stars. You can begin this dark adaptation process early in the evening by wearing an eye patch an hour or so before you begin to observe.

Another characteristic of your eye that is important to you as a variable star observer is sensitivity. The sensitivity of your eye will ultimately determine how accurately you will be able to compare the brightness of stars. As you know, comparing the brightness of stars is the critical element of variable-star observing.

You have probably noticed that when the apparent size of an object increases you are able to better see it, regardless of dark adaptation. In other words, the bigger something is, the less need you have for dark adaptation. If you live out in the country and step out your back door, you will be able to see a barn, since it's so large, but you'll have difficulty in locating a door from a distance (unless you move closer and the door appears larger). As your eyes adapt, fine details, such as doors and fences become visible. This is a critical concept for you to understand. You will find that not only the brightness but the size of an object as seen in the eyepiece will affect its visibility. The importance of this will become apparent in a moment.

Variable-star observation depends not just on detecting a faint star but also on contrast discrimination. For stars that are just detectable, you will notice that as the background sky becomes darker, the size of the star appears larger. Your eye's ability to see a point source, such as a star, increases as the background glow decreases, in other words, as the sky darkens. Dark country skies are better than city skies for seeing dim stars because the background is less bright, not entirely because the country skies are significantly more transparent.

If an object is at your eye's threshold of detection and smaller than the optimum size for your eye and light conditions, increased magnification will usually make it easier to see. By increasing the magnification you decrease the brightness of the background sky and improve your ability to see a faint star. This phenomenon is the reason that you will need more than one or two eyepieces for observing variable stars. A selection of eyepieces will allow you to configure an optical path best suited for existing light conditions, your eyes and the brightness of the stars that you are observing. Of course, you can go too far and over-magnify an object. It's advisable to slowly increase the magnification until the star that you are observing is bright enough to estimate. If you over-magnify, the star will become fuzzy and start to fade. If this happens, just go back to the last eyepiece that provided the best view. Remember, as you increase magnification you decrease field-of-view (FOV). That means that your comparison star may not be visible within your FOV. It's a good idea to record in your log or record book, the eyepiece that you determine to be best for a particular star so that you can quickly select the appropriate one during observing. It saves time.

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