10. Astronomical Observing
The SkyQuest XTg GoTo Dobsonian provides prodigious capability 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 astronomical observing tips and briefly summarize what you can expect to see.
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. Avoid pointing the telescope above buildings, if possible, since they radiate heat which degrades images. For observing faint deep sky objects, choose a moon- less night. Using the optional light shroud will be a necessity (see below). Also, use of a light-pollution filter, like the Orion SkyGlow Broadband filter, can mitigate the effects of back- ground sky brightness, enhancing the view of faint objects.
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 bet- ter 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.
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 or so to equilibrate. If the temperature dif- ference between indoors and outdoors is more than 40°, it will likely take longer. In the winter, storing the telescope outdoors in a shed or garage greatly reduces the amount of time need- ed for the optics to stabilize. Also, after setting up outdoors, it 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 air.
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 per- haps 80% of their full dark-adapted sensitivity. Many observ- ers 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 daylight 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. Dim light is preferable to bright light.
Be aware, too, that nearby porch and streetlights and auto- mobile headlights will spoil your night vision. Close your eyes when you hear an automobile approaching your observing site!
Eyepiece Selection
By using eyepieces of different focal lengths, it is possible to attain many different magnifications with your telescope. Different eyepieces can be used to achieve higher or lower powers. It is quite common for an observer to own five or more eyepieces to access a wide range of magnifications. This allows the observer to choose the best eyepiece to use depending on the object being viewed. At least to begin with, the two supplied eyepieces will suffice nicely.
Whatever you choose to view, always start by inserting your lowest-power (longest focal length) eyepiece to locate and center the object. Low magnification yields a wide field of view, which shows a larger area of sky in the eyepiece. This makes acquiring and centering an object much easier. If you try to find and center objects with high power (narrow field of view), it’s like trying to find a needle in a haystack! Once
