A new method for fine co-focus adjustment, based on cross-calibration, has been proposed in a study published in Optics Express. This method, developed by PhD candidate WANG Kunyan and senior engineer DAI Yichun from the Astronomical Technology Laboratory of Yunnan Observatories, addresses the fine co-focus issues of the Chinese Giant Solar Telescope (CGST).

The method has achieved co-focusing with an accuracy better than ±15μm on the segmented prototype of CGST. This advancement simplifies the subsequent implementation of co-phase adjustment, which is crucial for high-resolution observation and research of the solar surface’s delicate structures.

The segmented primary mirror of the CGST must achieve stable co-phase within the 1 μm wavelength range to observe the intricate 20-km structures on the sun. Fine co-focus adjustment of the segment is an essential prerequisite for achieving phasing and maintenance of phasing, involving high-precision alignment of the segments and adjusting the millimeter-scale piston between the mechanically adjusted segments to fall within the phasing measurement system’s capture range.

To enhance the co-focus performance of the CGST, a detection device with higher accuracy was employed to calibrate and fit the measurements from the Shack-Hartmann Wavefront Sensor (SHWFS), mitigating the impact of atmospheric turbulence and systematic errors on piston measurement accuracy during co-focus adjustment.

Experiments on the 800mm ring segmented mirror system to validate the method’s feasibility. SHWFS measurements were cross-calibrated using the system's built-in Linear Variable Differential Transformer (LVDT) with a measurement accuracy better than 3μm RMS. The experimental results showed that the fine co-focus adjustment accuracy through cross-calibration was approximately 26μm PTV, significantly better than the 91μm PTV accuracy achieved using SHWFS measurements alone. Additionally, the LVDT can record the segment’s state during the co-focusing, allowing for quick return to the co-focus state.

This fine co-focus adjustment method utilizing LVDT and SHWFS cross-calibration can serve as a reference scheme for co-focus adjustment in CGST and provide new insights for fine co-focus adjustment in other segmented telescopes. It simplifies the implementation of co-phase of segments.

Figure 1, the primary mirror and actuator and LVDT of the 800mm ring segmented mirror system. Image by WANG.

Figure 2, co-focus detection optical path of the 800mm ring segmented mirror system. Image by WANG.

Contact:
WANG Kunyan
Yunnan Observatories, CAS
E-mail:wangkunyan@ynao.ac.cn