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When the laser beam hits the primary mirror center spot and retraces its path all the way to the source, the primary mirror reflection is centered under the focuser -- however, not necessarily the secondary mirror appearance. Read the previous sentence few times before proceeding.
The secondary mirror acts as a “window” to the primary mirror. After centering the primary mirror reflection under the focuser using the laser collimator, now we can assess the position of the secondary mirror with respect to the primary mirror reflection. Should the secondary mirror moving lower? Should it move higher? Should it move to the left or right? Make the judgment and adjust. You can use the central bolt of the spider vanes to move the secondary mirror lower or higher with respect to the OTA. You can use the spider vanes thumb knobs to move the secondary mirror left with respect to the OTA. But before you make any of these assessments or movements make sure your focuser is reasonably squared with the OTA.
Important: Do not assess the position of the secondary mirror with respect to the primary mirror reflection unless you have completed the laser collimation steps. That is, when you start moving the secondary mirror ignore the primary mirror reflection. Make the movement then realign with the laser then assess the secondary position – keep re-iterating.
In the first attachment, the laser beam hits the primary mirror then retraces it path to the source in each frame. Note how the primary mirror reflection remains centered under the focuser.
The second attachment is an animation that explains how the secondary mirror can be viewed as a window to the primary mirror reflection. Again, each frame assumes the laser hits the center and retraces its path to the source.
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