Deep Sky Observing

Planet watching is fun, but there is no denying the siren call of deep space. After looking at the Moon and a planet or two, most new CAT owners are eager to see all the stuff in the Great Out There: majestic spiral galaxies, great glowing nebulas, blazing globular clusters, and gas-clogged nests of newborn stars. An 8-inch SCT is capable of showing thousands of distant and lovely objects and showing considerable detail in the brighter ones.

Deep Sky Hints and Tips Beyond the obvious, "Get the CAT to the darkest site possible," what can the new observer do to maximize deep sky "returns"? We've already discussed light pollution reduction filters; they work, on nebulas anyway, and can make the difference between seeing and not seeing elusive objects. Shielding the observing position from ambient light is also very important. But is there anything else that can make dim objects easier to see? Yep, averted vision.

To use averted vision, look away from a deep sky object in the eyepiece rather than directly at it. That will bring the eye's dim light sensors into play, and objects at the edge of the visual field will become surprisingly brighter. There's another peculiarity of the human eye the deep sky observer can capitalize on: moving objects are easier to see than stationary ones (maybe an evolutionary adaptation that helped our ancestors detect stalking predators). Gently rap the tube of the scope so that it vibrates a little, and "not seen" objects may suddenly pop into view. Using these two techniques can make a so-so observing session better and can make a good site great. One thing's sure, as in planetary observing, experience helps more than anything else. The more you look, the more you'll see.

Deep Sky Object Brightness (Magnitude) This is probably a good time to talk about "magnitude," the system that describes the brightness of sky objects. We've thrown the term around a little previously, but it really becomes important when describing deep sky objects. How does it work? The human eye can see stars as dim as about magnitude 6 unaided, and each whole number magnitude jump makes an object 2.5 times dimmer (or brighter). A magnitude 7 star cluster is 2.5 times dimmer than a magnitude 6 one, and a magnitude 5 cluster is 2.5 times brighter than the magnitude 6 group. Brightness goes down as positive numbers become larger and goes up as they get smaller. Objects brighter than magnitude 0 are assigned negative magnitudes. A magnitude -1 object is 2.5 times brighter than a magnitude 0 one, which is 2.5 times brighter than magnitude 1.

The magnitude system works well for stars but can be deceptive with other deep sky objects. A galaxy may be said to have a magnitude of "3.5" but appear far, far dimmer than a magnitude 3.5 star. That's because the galaxy's given "visual" magnitude expresses what its brightness would be if it were squeezed down to the size of a star. Obviously, that makes a big galaxy like M31 very dim. Defocusing a magnitude 3.5 star until it is several degrees across would make it nearly invisible. For a better idea of a deep sky object's true brightness, some deep sky observers suggest using a magnitude system that reflects "surface brightness" not "visual magnitude." Many books and lists will give both types of magnitude. The values given in the section below are in visual magnitude, but with a bit of experience it's easy to get an idea how bright objects are from this figure and their given sizes. Surface brightness is better in some ways, but you might find it easier to remember how an object with a visual magnitude value of 3.5 will appear in your telescope than with a "mean surface brightness of 23.1 per square centimeter." Use whichever magnitude "system" you prefer.

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