Orion 9966 instruction manual Astronomical Observing, Star-Testing the Telescope, Site Selection

Figure 22. The three thumb screws that lock the primary mirror in place must first be loosened before any adjustments can be made.

Now, try tightening or loosening one of the larger collima- tion thumbscrews with your fingers (Figure 23). Look into the focuser and see if the secondary mirror reflection has moved closer to the center of the primary. You can tell this easily with the collimation cap and mirror center mark by simply watch- ing to see if the “dot” of the collimation cap is moving closer or further away from the “ring” on the center of the primary mirror mark. When you have the dot centered as much as is possible in the ring, your primary mirror is collimated. The view through the collimation cap should resemble Figure 19e. Re-tighten the locking thumbscrews.

A simple star test will tell you whether the optics are accu- rately collimated.

Note About the Collimatable 2" Focuser

The 2" focuser of the SkyQuest XT12 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 not 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.

Star-Testing the Telescope

When it is dark, point the telescope at a bright star and accu- rately center it in the eyepiece’s field-of-view. Slowly defocus the image with the focusing knob. If the telescope is correct- ly collimated, the expanding disk should be a perfect circle (Figure 24). 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 doughnut. If the “hole” appears off-center, the telescope is out of collimation.

If you try the star test and the bright star you have selected is not accurately centered in the eyepiece, then 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.

Figure 23. The tilt of the primary mirror is adjusted by turning one or more of the three spring-loaded collimation thumb screws.

5. Astronomical Observing

For many users, the SkyQuest XT12 telescope will be a major leap into the world of amateur astronomy. This section is intended to get you ready for your voyages through the night sky.

Site Selection

Pick a location away from street lights and bright yard light- ing. 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

asite 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.

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

Out of collimation Collimated

Figure 24. 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 right if optics are perfectly collimated. If circle is unsymmetrical, as in illustration on left, scope needs collimation.

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