Analysis of Spectra Annular Spot in Large Core Fiber

The introduction of optical fiber into astronomical telescope observation combined with big data technology has improved the development of astronomy. Multi-target optical fiber measurement technology is mainly affected by the optical fiber transmittance and focal radio degradation (FRD) characteristics. During the rotation of the telescope, the fiber will be flexural deformed between the focal plane and the spectrograph, meanwhile, if the convergence center of the beam from the telescope is not aligned with the main axis of optical fiber, the FDR will be changing with it, which greatly affect the reception of the spectrograph. One of the main sources of fiber spectral efficiency loss is caused by FRD of the fiber, which often results in ring-shaped emergent image. In this paper, we obtained the ring-shaped emergent images produced by FRD of large diameter fibers in Guo Shou-Jing (LAMOST) telescope during the commissioning. According to the character CCD image, we applied spectral extraction method based on profile fitting to convert the two-dimensional CCD image into one-dimensional spectrum. We selected LAMOST spectra in wavelength range around 500-600nm, in which there were Fe emission lines. Then we studied the effect of the FRD on the Fe emission line peaks through convoluting with the obtained ring-shaped emergent profile. Finally, we discussed using flat diffuse screen before the main mirror Mb to eliminate annular plaque-induced deviation caused by the effect of ring-shaped emergent profile.