Selecting an Observing Site, Seeing and Transparency

A. Selecting an Observing Site

Since most astronomical objects are faint, observing them from dark skies will give you the best views. While some objects, such as the planets and Moon, are bright enough to see clearly even from light-polluted city skies, for nebulas, galaxies, and most star clusters, the less ambient light there is to reduce contrast, the better.

When it isn’t possible or convenient to get out of town to a pitch-dark observing location, try to set up in a spot that is removed from street and building lights and that has a clear view of a large portion of the sky. For observing faint deep- sky objects, choose a Moonless night. Use of a light-pollution filter can mitigate the effects of background sky brightness, enhancing the view of faint objects.

B. Seeing and Transparency

Atmospheric conditions play a huge part in quality of viewing. Light from stars and other celestial objects must travel through miles of Earth’s atmosphere to reach our eyes. The air in the atmosphere will refract and bend the light. Atmospheric tur- bulence will worsen the effects of refraction, which can cause the image you see in your telescope to be unstable. The steadiness of the atmosphere is called “seeing.”

In conditions of good “seeing,” star twinkling is minimal and objects appear steady in the eyepiece. Seeing is best over- head, worst at the horizon. Also, seeing generally gets better later in the evening as much of the heat absorbed by the Earth during the day has radiated off into space. In condi- tions of bad seeing, stars will twinkle and objects will appear unsteady and blurry in the telescope.

“Transparency” is the clarity of the atmosphere, which can be adversely affected by the presence of moisture, smoke, and dust. All tend to scatter light, which reduces an object’s brightness. Good transparency is desirable for astronomical observing, especially for viewing faint objects.

One good measure of transparency is by how many stars you can see with your unaided eyes. If you cannot see stars of magnitude 3.5 or dimmer then transparency is poor. Magnitude is a measure of how bright a star is. The brighter a star, the lower its magnitude. A good star to remember for this is Megrez (magnitude 3.4), which is the star in the Big Dipper that connects the handle to the “dipper.” If you cannot see Megrez, then you have fog, haze, clouds, smog, light pol- lution or other conditions that are hindering your viewing (see

outdoors is more than 40°, allow an hour or more. In the win- ter, storing the telescope outdoors in a shed or garage greatly reduces the amount of time needed for the optics to stabilize. It also is a good idea to keep the scope covered until the Sun sets so the tube does not heat greatly above the temperature of the outside air.

SkyQuest XT8 and XT10 models are designed to accept an optional Orion Cooling Accelerator Fan (#7814). When attached to the rear cell of the telescope, the fan hastens the equilibration of the primary mirror to ambient temperature.

D. Let Your Eyes Dark-Adapt

Do not expect to go from a lighted house into the darkness of the outdoors at night and immediately see faint nebulas, galaxies, and star clusters — or even very many stars, for that matter.Your eyes take about 30 minutes to reach perhaps 80% of their full dark-adapted sensitivity. Many observers notice improvements after several hours of total darkness. As your eyes become dark-adapted, more stars will glimmer into view and you will be able to see fainter details in objects you view in your telescope. Exposing your eyes to very bright day- light for extended periods of time can adversely affect your night vision for days. So give yourself at least a little while to get used to the dark before you begin observing.

To see what you are doing in the darkness, use a red-filtered flashlight rather than a white light. Red light does not spoil your eyes’ dark adaptation like white light does. A flashlight with a red LED light is ideal, or you can cover the front of a regular incandescent flashlight with red cellophane or paper. Dim light is preferable to bright light. Beware, too, that nearby porch and streetlights and automobile headlights will spoil your night vision.

Eyepiece Selection

By using eyepieces of different focal lengths, it is possible to attain many different magnifications with your telescope. The telescope comes with two high-quality Sirius Plössl eyepiec- es: a 25mm focal length, which provides a magnification of 48x, and a 10mm focal length, which provides a magnification of 120x. Other eyepieces can be used to achieve higher or

Figure 25).

C. Cooling the Telescope

All optical instruments need time to reach “thermal equilibrium” to achieve maximum stability of the lenses and mirrors, which is essential for peak performance. Images will be unstable if the optics are not in equilibrium with the outdoor temperature. When moved from a warm indoor location outside to cooler air (or vice-versa), a telescope needs time to cool to the out- door temperature. The bigger the instrument and the larger

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the temperature change, the more time will be needed.

Allow at least 30 minutes for your SkyQuest IntelliScope to equilibrate. If the temperature difference between indoors and

Figure 25. Megrez connects the Big Dipper’s handle to it's “pan”. It is a good guide to judging the “seeing” conditions. If you cannot see Megrez (a 3.4-magnitude star) then seeing is poor.