Out of collimation | Collimated |
Figure 18. A star test will determine if a telescope’s optics are properly collimated. An unfocused view of a bright star through the eyepiece should appear as illustrated on the right if the optics are perfectly collimated. If the circle is unsymmetrical, as in the illustration on the left, the scope needs collimation.
the image with the focusing knob. If the telescope is correctly collimated, the expanding disk should be a perfect circle (Figure 18). If the image is unsymmetrical, the scope is out of collimation. The dark shadow cast by the secondary mirror should appear in the very center of the out-of-focus circle, like the hole in a donut. If the “hole” appears off-center, the tele- scope is out of collimation.
If you try the star test and the bright star you have selected is not accurately centered in the eyepiece, the optics will always appear out of collimation, even though they may be perfectly aligned. It is critical to keep the star centered, so over time you will need to make slight corrections to the telescope’s position in order to account for the sky’s apparent motion.
Note About the Collimatable 2" Focuser (XT8 and XT10)
The 2" focuser of the SkyQuest XT8 can be collimated using three pairs of push-pull screws located at the base of the focuser. The focuser was collimated at the factory however, and should never need to be adjusted. Focuser collimation is only required under very rare circumstances, but has been made available for this telescope should such a need arise.
5. Astronomical Observing
For many users, the SkyQuest XT telescope will be a major leap into the world of amateur astronomy. This section is intended to get you ready for your first voyage through the night sky.
Site Selection
Pick a location away from streetlights and bright yard lighting. Avoid viewing over rooftops and chimneys, as they often have warm air currents rising from them, which distort the image seen in the eyepiece.
Similarly, you should not observe through an open window from indoors. Better yet, choose a site out-of-town, away from any “light pollution”. You’ll be stunned at how many more stars you’ll see! Most importantly, make sure that any chosen site has a clear view of a large portion of the sky.
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. When moved from a warm indoor location to cooler air outside(or vice-versa), a telescope needs time to cool (or warm) 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 XT to equilibrate. If the scope experiences more than a 40° temperature change, allow an hour or more. In the winter, storing the telescope 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 XT8 and XT10 have the ability to mount a small fan to make cooling the tube faster. On the bottom of the mirror cell there are four holes where a fan can be screwed on.
Seeing and Transparency
Atmospheric conditions play a huge part in quality of viewing. 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 after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space. Typically, seeing conditions will be better at sites that have an altitude over about 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 observing 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.
One good way to tell if conditions are good is by how many stars you can see with your naked eye. If you cannot see stars of magnitude 3.5 or dimmer then conditions are poor. Magnitude is a measure of how bright a star is, the brighter a star is, the lower its magnitude will be. A good star to remem- ber for this is Megrez (mag. 3.4), which is the star in the Big Dipper connecting the handle to the “dipper.” If you cannot see Megrez, then you have fog, haze, clouds, smog, light pollu- tion, or other conditions that are hindering your viewing. (See Figure 19)
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
