Orion XT12 Astronomical Observing, Selecting an Observing Site, Seeing and Transparency

the retaining and tensioning knobs until they are free of the base and tube (make certain the spacer does not fall off the retaining knob). Then, using both hands, carefully lift the tube off the base. The tube is somewhat heavy, so don’t hesitate to have a friend help lift the tube, if necessary. Do not use the navigation knob as a carry handle! It is not designed to sup- port the weight of the telescope tube and could break off or damage the tube.

When putting the SkyQuest XT12 IntelliScope into a vehicle, common sense prevails. It is especially important that the optical tube does not get knocked around; this can cause the optics to become misaligned, and could dent the tube. We recommend transporting and storing the tube assembly in an

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optional padded case for proper protection.

5. Astronomical Observing

The SkyQuest XT12 IntelliScope provides prodigious capa- bility for observing the many wonders of the heavens, from the major planets to deep-space nebulas and galaxies. In this section we give you some observing tips and briefly summa- rize what you can expect to see.

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-black 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

Figure 31. 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 transparency is poor.

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 Figure 31).

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 the temperature change, the more time will be needed.

Allow at least 30 minutes for your telescope to equilibrate. If the temperature difference between indoors and outdoors is more than 40°, allow an hour or more. In the winter, 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 telescope covered until the Sun sets so the tube does not heat greatly above the temperature of the outside air.

The SkyQuest XT12 IntelliScope is designed to accept an optional Cooling Accelerator Fan. 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

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