Measuring fiber position errors from spectral data

Precise fiber positioning is crucial to a wide field, multi-fiber spectroscopic survey such as the Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, LAMOST). Nowadays, most position error measurements are based on CCD photographic and image processing techniques. These methods only work for measuring errors orthogonal to the telescope optical axis, but there are also errors that lie parallel to the optical axis of the telescope, such as defocusing, and errors caused by the existing deviation angle between the optical axes of a fiber and the telescope. Directly measuring the two latter types of position errors is difficult for an individual fiber, especially during observations. Possible sources of fiber position errors are discussed in brief for LAMOST. By constructing a model of magnitude loss due to the fiber position error for a point source, we propose an indirect method to calculate both the total and systematic position errors for each individual fiber from spectral data. Restrictions and applications of this method are also discussed.