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For large-aperture optical telescope, compared with the performance of azimuth in the control system, arc second-level
jitters exist in elevation under different speeds' working mode, especially low-speed working mode in the process of its
acquisition, tracking and pointing. The jitters are closely related to the working speed of the elevation, resulting in the
reduction of accuracy and low-speed stability of the telescope. By collecting a large number of measured data to the
elevation, we do analysis on jitters in the time domain, frequency domain and space domain respectively. And the
relation between jitter points and the leading speed of elevation and the corresponding space angle is concluded that the
jitters perform as periodic disturbance in space domain and the period of the corresponding space angle of the jitter
points is 79.1″ approximately. Then we did simulation, analysis and comparison to the influence of the
disturbance sources, like PWM power level output disturbance, torque (acceleration) disturbance, speed feedback
disturbance and position feedback disturbance on the elevation to find that the space periodic disturbance still exist in the
elevation performance. It leads us to infer that the problems maybe exist in angle measurement unit. The telescope
employs a 24-bit photoelectric encoder and we can calculate the encoder grating angular resolution as 79.1016'', which is
as the corresponding angle value in the whole encoder system of one period of the subdivision signal. The value is
approximately equal to the space frequency of the jitters. Therefore, the working elevation of the telescope is affected by
subdivision errors and the period of the subdivision error is identical to the period of encoder grating angular. Through
comprehensive consideration and mathematical analysis, that DC subdivision error of subdivision error sources causes
the jitters is determined, which is verified in the practical engineering. The method that analyze error sources from time
domain, frequency domain and space domain respectively has a very good role in guiding to find disturbance sources for
large-aperture optical telescope.
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