Orion 9966 Cooling the Telescope, Let Your Eyes Dark-Adapt, Tracking Celestial Objects

3000 feet. Altitude helps because it decreases the amount of distortion causing atmosphere you are looking through.

A good way to judge if the seeing is good or not is to look at bright stars about 40° above the horizon. If the stars appear to “twinkle”, the atmosphere is significantly distorting the incom- ing light, and views at high magnifications will not appear sharp. If the stars appear steady and do not twinkle, seeing conditions are probably good, and higher magnifications will be possible.

Also, seeing conditions are typically poor during the day. This is because the heat from the Sun warms the air and causes turbulence.

Good “transparency” is especially important for observ- ing faint objects. It simply means the air is free of moisture, smoke, and dust. All tend to scatter light, which reduces an object’s brightness.

Cooling the Telescope

All optical instruments need time to reach “thermal equilib- rium” to achieve maximum stability of the lenses and mirrors, which is essential for peak performance. When moved from a warm indoor location outside to cooler air (or vice-versa), a telescope needs time to cool to the outdoor temperature. The bigger the instrument and the larger the temperature change, the more time will be needed.

Allow at least 30 minutes for your SkyQuest XT12 to equilibrate. If the scope has more than a 40° temperature adjustment, 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 scope covered until the Sun sets so the tube does not heat greatly above the temperature of the outside air.

The SkyQuest XT12 has the ability to mount a small fan to make cooling the tube faster. On the bottom of the mirror cell there are four threaded holes (M4-.7) where a fan can be attached. Orion sells a cooling fan for the SkyQuest XT12; consult the Orion catalog or OrionTelescopes.com for details.

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, gal- axies, 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.

Beware, too, that nearby porch and streetlights and automo- bile headlights will spoil your night vision.

Tracking Celestial Objects

The Earth is constantly rotating about its polar axis, complet- ing one full rotation every 24 hours; this is what defines a “day”. We do not feel the Earth rotating, but we can tell that it is at night by seeing the apparent movement of stars from east to west. This movement translates into a rate of approxi- mately .25° per minute, or 15 arc-seconds per second. (There are 60 arc-minutes in 1°, and 60 arc-seconds in one arc-min- ute.) This is called the sidereal rate.

When you observe any astronomical object, you are watching a moving target. This means the telescope’s position must be slowly updated over time to keep an object in the field of view. To keep the object in the telescope’s field of view (to “track” it), the telescope must be moved by small increments every now and then, in the direction the object is moving. This is easy to do with a SkyQuest XT12 Dobsonian because of its buttery smooth motion on both axes. As the object moves off toward the edge of the field of view, you just lightly nudge the telescope to bring it back to the center.

You will notice that it is more difficult to track objects when the telescope tube is aimed nearly straight up. This is inherent to the basic design of the Dobsonian, and stems from the fact that there is very little leverage to move in azimuth when the tube is in a near-vertical position. To gain more leverage, try grasping the tube close to the altitude side bearings with both hands. Also, when looking overhead, if the telescope cannot be moved any more in altitude, rotate the telescope 180° in azimuth to continue motion.

Remember that objects appear to move across the field of view faster at higher magnifications. This is because the field of view becomes narrower.

Eyepiece Selection

By using eyepieces of varying focal lengths, it is possible to attain many magnifications with the SkyQuest XT12 Dobsonian. The telescopes come with two high-quality Sirius Plössl eyepieces: a 25mm, which gives a magnification of 60x, and a 10mm, which gives a magnification of 150x. Other eyepieces can be used to achieve higher or lower powers. It is quite common for an observer to own five or more eye- pieces 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 you’ve centered the object in the eyepiece, you can switch to higher magnification (shorter focal length eyepiece), if you wish. This is especially recommended for small and

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