Abstract
The purpose of this paper is to investigate a large radio telescope support point number effect on its pointing accuracy and provide a useful guideline for the large radio telescope design engineer. In a large radio telescope system, the azimuth track is used to support the whole telescope structure and the mounting error as well as the telescope wheel-track contact in a long term can cause unevenness on the azimuth track, which can further deteriorate the telescope pointing accuracy. Even though various compensation methods have been proposed to compensate for this pointing error, it remains as one of the challenges for the telescope pointing error reduction. In this paper, a general telescope pointing error estimation formula has been proposed to investigate different telescope support-point number designs on its pointing accuracy. In this approach, the azimuth track unevenness has been modelled as the Fourier function using the least square method after the raw track profile has been measured. Next, the elevation position matrix, azimuth position matrix, and the azimuth profile matrix can be constructed for different telescope support point numbers, and the telescope pointing error can then be obtained based on the proposed general formula. The telescope pointing error root mean square (RMS) value is used to quantify the effect of the telescope support point number on the pointing accuracy. Two interesting results can be observed in the numerical example. The first one is that the telescope pointing error curves have different dominant peaks during one azimuth track rotation, which is corresponding to the support point number. Another interesting finding is that the RMS value experienced a complex trend with the support point number change, and they are not a simple monotonous increasing or decreasing relationship with the support number. All the results in this paper can provide a useful guideline for reducing the telescope pointing error in the initial design stage.
Highlights
In 2014, China has proposed to build the Qi Tai Telescope (QTT), which will be a fully steerable telescope with a 110 m aperture. e whole telescope structure will be around 6000 t in weight and 120 m in height
It is interesting to notice that the whole pointing error curve has six similar peaks during one azimuth rotation, which is corresponding to six support points. is phenomenon can be explained by the analysis presented in the four-support-point case, where the support point number 1 will take the positions of A, B, . . ., F successively, and the telescope pointing error curve will repeat itself in the domain AB and DE, domain BC and EF, and domain CD and FA, respectively
This general formula provides a quick estimation tool for the telescope pointing accuracy with different support point numbers, and the most difficult part in this formula is the construction of the matrix PiA, which is different for different support point designs
Summary
In 2014, China has proposed to build the Qi Tai Telescope (QTT), which will be a fully steerable telescope with a 110 m aperture. e whole telescope structure will be around 6000 t in weight and 120 m in height. In 2000, Gawronski et al derived the algorithms for the telescope pointing error estimation using the inclinometer data because the azimuth track unevenness is a type of repeatable disturbances for the telescope. Later, he developed a look-up table method to minimize the pointing error [17]. With the increasing pointing accuracy requirement, two representative telescope track designs have been proposed with new welding technologies. Even though some work has been done to improve the contact between the wheel and track [23], the little result has been reported on how to design the support points in order to reduce the telescope pointing error.
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