by Dr. Arthur Babcock |
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| Many of the brighter
nebulae, star clusters, and galaxies have popular names originating from visual
observations, often made with telescopes of modest size. Some of these names make just as
good sense for observations in large telescopes, such as the Ring Nebula, M57 in Lyra,
which looks pretty much like a ring whether you observe it in a 3-inch refractor or a
200-inch research instrument. Others, however, have names that fit only when observed
through very small instruments. M11, a beautiful open cluster in Scutum, is sometimes
called the Wild Duck Cluster because the brighter stars are arranged somewhat like a
flight of migrating ducks, but anything bigger than a 4-inch telescope shows so many stars
that the vee shape is lost. An interesting example of this phenomenon is NGC 6826, the "Blinking" Planetary in Cygnus. The nebula does not actually change in brightness, but only appears to do so, when the right instrument and the right technique are used. One of the tricks that visual observers learn is that of averted vision: to see the faintest objects possible, it is necessary to look not directly at the object, but slightly to one side. This is because of the nature of the eye’s light receptors, the rods and cones that line the retina. These cells have specialized functions: cones are color-sensitive and give great acuity (sharpness) but are not very sensitive to low light levels; rods are not good at detecting color or detail, but are good low-light receptors. Rods and cones are distributed fairly equally in most of the eye, but the fovea centralis, directly opposite the pupil, has a preponderance of cones. If we look straight at something, then, cones are doing most of the work, and we gain color and detail at the expense of faint light. If, on the other hand, we want to perceive the faintest object, it is best to look slightly to one side of it, to let the rods pick up the faint light. This is how the Blinking Planetary "blinks:" if you look straight at it, you see the bright central star, but not much of the nebula. Look slightly away, and the nebulosity pops into view. Move back to the central position, and the nebula winks out again. All bets are off if you use too large a telescope; with the MIRA 36-inch, if you want NGC 6826 to blink, you’ll have to close your eyes. |
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 Go back to Winter 2000 Newsletter index | MIRA home Last updated 1/10/01 et |
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