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What the heck is going on in my finder scope and why? |
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Virtual Cone of the Equatorial Platform |
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The description below details the motion seen in an upright correct image finder. A finder can be upright and reversed L/R. It can also be both upside down and reversed. Check by looking at a land object. Trying to perform this test without a correct upright finder will likely drive you crazy. |
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The goal is to get the Pole Star to be in the center of the platform's rotation. The picture to the right shows the finder scope pointing at the Pole Star and the platform center pointing at the Pole Star. If the platform rotates 15 degrees, the finder will rotate around the center of the platform's center and the Pole Star will not move in the finder. |
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In this example of a misaligned platform, the platform is pointing east of the Pole Star. The platform is perfect in the north / south alignment. Without moving the platform, the scope is pointed at the Pole Star. We can never really see where the platform center is pointing, so we have to watch what happens to the Pole Star when the platform is rotated.
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Zero degrees rotation of the platform.
Pole Star is centered in the finder. |
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Rotation of the platform will cause the finder to rotate while the pole star does not move. We humans tend to see the change in the Pole Star position in the finder as the star moving up in the finder. We do this because out there in the dark, there is no frame of reference. |
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15-degrees rotation of the platform and the finder. |
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If the platform moves 15-degrees, then the finder rotates 15-degrees and the pole star appears to move in a 15-degree arc. The star appears to move to the top of the finder. So a platform that is east of the pole star, the pole star appears to rise in the finder during rotation. Remember, it is the finder that moved, not the star or the platform's center axis (hopefully the platform's axis just rotates):
1) platform east of pole star, star appears to rise
2) platform west of the pole star, start appears to sink
3) platform north of pole star, star appears to move left
4) platform south of pole star, star appears to move right
Visualize the finder scope with the star centered in all 4 positions and rotate the platform. It will help you save time during alignment. Once you have the finder scope rotating in your mind, picture that "the star always gets left behind". |
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