Does a camera offset from the hinge axis cause tracking errors on a barn-door tracker?
Asked 1/3/2020
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I'm planning to build a simple barn-door tracker for basic astrophotography. In most designs, the camera is mounted a few inches away from the tracker’s rotation axis rather than directly on it. If the tracker is aligned to the pole, does that offset make the lens move in a small circle and introduce tracking error, even if the camera keeps pointing in the same direction?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
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There is no such problem because you are shooting at objects very, very far away. If you take a picture of a mountain range, you can move a few steps sideways and it won't change anything to the picture if the camera has the same direction.
More scientifically, the pixels in the picture represent the amount of light with a given incidence angle on the lens. The tracker compensates the earth rotation so the lens axis keeps the same absolute direction in space. Stars are so far away that as long at the lens axis is the same, the change in angle of incidence due to a position shift of the lens in space (arc sine of the lens shift over the distance of the star) is very, very small.
In fact the biggest shift doesn't come from the rotation of the camera around the mount axis, but from the movement of the earth on its orbit (30km/s). You would start noticing a difference for close stars only if you took pictures several days apart (because the camera would have moved enough, this is how the distance to close stars is measured by astronomers)(*).
(*) This is where the "parsec" astronomical unit comes from.The closest stars are more than one parsec away.
Originally by user75947. Source · Licensed CC BY-SA 4.0
user75947
6y ago
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In practice, no. For astrophotography, stars are so far away that a small sideways shift of the camera by a few inches is negligible. What matters is that the tracker keeps the camera’s optical axis pointed in the correct direction as the sky appears to rotate.
On a barn-door tracker, you align the hinge/pivot axis with the celestial pole. As the tracker turns, the camera rotates around that polar axis along with the sky. Although the camera body is physically offset from the hinge and traces a tiny circle in space, the resulting change in viewing angle to celestial objects is effectively zero because of their enormous distance.
It’s similar to taking a photo of a distant mountain from a few steps to the side: if the camera is aimed the same way, the framing barely changes. For stars, the effect is vastly smaller.
So the offset itself is not a meaningful source of error in a barn-door tracker. Accurate polar alignment and a correct tracking rate matter far more.
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