一直以来，河外星系的距离主要采用Cepheid变星的周光关系（PLR）来估计。由于Cepheid变星PLR零点的误差较大，正确估计河外星系的距离比较困难。寻找能更精确估计河外星系距离的指示器是长期未解决的难题。Mira变星在红外有很亮的辐射且有很好的周光关系，其数量比Cepheid变星多一个量级。在红外空间望远镜JWST时代，Mira变星可成为重要的距离指示器。Mira变星中有脉泽，VLBI脉泽天体测量位置精度已达十微角秒，视差精度可好于1%，结合精确的红外光变曲线，能高精度测定Mira变星PLR的零点。本项目将依托天马65米射电望远镜（TM65）扩大适合VLBI观测的Mira变星样本，利用包含TM65的高灵敏度VLBI网测量约15颗Mira变星的精确距离，结合以往的观测结果，使得Mira变星PLR零点精度比目前提高约3倍。这对于建立宇宙距离阶梯的标尺以及测定哈勃常数都具有重要意义。

Period Luminosity Relation (PLR) for Cepheid variables is one the most important methods to estimate distances of extra-galaxies. However，due to large uncertainty of the PLR zero point, it is hard to accurately estimate extra-galaxies distances. Thus, seeking for a better distance indicator is a very important research topic. Mira variables show well-defined Period Luminosity Relation (PLR) at infrared. Compared to Cepheid variables, Miras are brighter at infrared and have greater number. They are thus likely become important extragalactic distance indicators in the space infrared telescope (i.e., JWST) era. There are masers in the circumstellar envelope of Miras. VLBI maser astrometry have achieved accuracies about ten micro-arcseconds, leading to parallaxes uncertainties better than 1%. and then can be used to accurately calibrate the PLR zero point of PLR for Miras. In this project, we increase the number of Miras which is suitable for VLBI parallax measurement based on monitoring observation using the Tianma 65m radio telescope (TM65), and then measure accurate parallaxes for about 15 Miras using the high sensitivity VLBI network with TM65 as a key element. This will improve the accuracy of the current PLR zero point by about 3 times, which is very important for the cosmic distance ladder and the Hubble constant measurement.