Optical images of astronomical objects viewed through ground-based telescopes are blurred by the atmosphere[
Chinese Optics Letters, Volume. 14, Issue 10, 100101(2016)
First light on an adaptive optics system using a non-modulation pyramid wavefront sensor for a 1.8 m telescope
Our adaptive optics system based on a non-modulation pyramid wavefront sensor is integrated into a 1.8 m astronomical telescope installed at the Yunnan Observatory in LiJiang, and the first light with high-resolution imaging of an astronomical star is successfully achieved. In this Letter, the structure and performance of this system are introduced briefly, and then the observation results of star imaging are reported to show that the angular resolution of an adaptive optics system using a non-modulation pyramid wavefront sensor can approach the diffraction limit quality of a 1.8 m telescope.
Optical images of astronomical objects viewed through ground-based telescopes are blurred by the atmosphere[
The 1.8 m telescope at the Yunnan Observatory is a Cassegrain optical structure, and light from a star is reflected by the primary mirror, the secondary mirror, the third mirror, and the other five reflective mirrors in the Coude room. The 127-element AO system installed on the 1.8 m telescope works based on the a Shack–Hartmann wavefront sensor with a
In 2014, the new AO system, based on the PWFS and installed on the telescope, provided an alternate wavefront sensor for the existing AO system with a Shack–Hartmann wavefront sensor. The AO system based on the PWFS was installed in the Coude room (Fig.
Sign up for Chinese Optics Letters TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
Figure 1.Optical layout of the AO system based on the PWFS for 1.8 m telescope. M: reflective mirror, PR: parabolic mirror, TM: tilt mirror, BS: beam splitter mirror.
The spectral bandwidth of the imaging system is 700 to 900 nm, and the telescope diameter is 1760 mm. If the center wavelength 800 nm is used as the calculation parameter, the diffraction-limited angular resolution can be calculated according to the following formula:
The optical layout of the PWFS is illustrated in Fig.
Figure 2.Schematic diagram of the PWFS.
The intensity distributions of these four sub-pupil images on the detector camera are expressed, respectively, as
When the PWFS is used with modulation by oscillating the optical component, the linear measurement range is increased. Let us consider that the beam is modulated with a circular path with an angular amplitude
This linear modulation scheme is used for the calibration of the AO loop closing. In an astronomical AO system like the one presented here, atmospheric turbulence provides for the modulation of the beam around the PWFS[
In Eq. (
In order to analyze the characteristics of the non-modulation PWFS using in the 1.8 m telescope, a closed-loop AO simulation module is established. The simulation parameters are as follows (consistent with the actual 1.8 m telescope):
First, the theoretical limit fitting ability of the 127-element DM to the Zernike aberration is analyzed and simulated. The relative error variance (defined as parameter
In Eq. (
Figure 3.Relative error variance of the top 35 orders of Zernike aberrations.
Second, the closed-loop AO simulation module based on the non-modulation PWFS and the 127-element DM is used to correct the top 35 orders of Zernike aberrations, and the blue histograms in Fig.
Third, the correction ability for the Kolmogorov phase screen composed of 3 to 104 Zernike modes (according to Fried parameter
Figure 4.Simulation results of the initial and corrected far-field images for the turbulence phase screen (
Figure 5.Simulation results of the initial and corrected PWFS images for the turbulence phase screen (
The full width at half-maximum (FWHM) for the two-dimensional section of the closed-loop spot is illustrated in Fig.
Figure 6.FWHM of the diffraction-limited spot, and the two-dimensional FWHM of the simulated closed-loop spot.
The above simulation results verify the feasibility of the non-modulation mode for the PWFS and also lay a good theoretical foundation for the development of the field experiment. The detection CCD of the PWFS for the 1.8 m telescope is an OCAM2 camera with a
Figure 7.(a) Original image (each sub-pupil has
Figure 8.Open-loop and closed-loop experiment PWFS images.
The optical path length aberrations of a beam are corrected by the mechanical displacement of a DM’s surface. There are three main DM families: discrete actuator mirrors using a continuous faceplate; bimorph mirrors, in which the actuator is combined with the faceplate; and segmented mirrors[
Figure 9.Space arrangement of the 127 actuators (the outer circle represents the mirror edge of the DM, and the gray area represents 1.8 m telescope pupil plane).
In late October 2014, we performed the final integration, alignment, and calibration of the AO system based on the PWFS, and high-resolution imaging for the first light of the natural stars was obtained. The open-loop and closed-loop star images of the AO system are shown in Fig.
Figure 10.Open-loop and closed-loop star images (Name: HIP113881, Magnitude: 2.44).
The FWHM of the imaging spot is a criterion method of the actual angular resolution. In order to make a quantitative study on the imaging performance of the AO system, the FWHM for the two-dimensional section of the closed-loop star image is illustrated in Fig.
Figure 11.Two-dimensional section of the closed-loop image.
Compared with the above simulation results, the
In conclusion, the first light of an AO system based on a non-modulation PWFS on a 1.8 m telescope is reported. The results show that the AO system using the non-modulation PWFS can work satisfactorily and the diffraction limit performance can be obtained. A significant advantage of the PWFS is that the number of sub-apertures can be adjusted according to the change of the turbulence, so that the PWFS has better adaptability compared with a Shack–Hartmann wavefront sensor. The corresponding experiment has been carried out and will be reported in the future.
[2] J. W. Hardy. Adaptive Optics for Astronomical Telescopes(1998).
[6] R. Ragazzoni, J. Farinato. Astron Astrophys., 350, 23(1999).
[18] H. Duan, E. Li, H. Wang, Z. Yang, Y. Zhang. Acta Opt. Sin., 23, 1143(2003).
Get Citation
Copy Citation Text
Shengqian Wang, Kai Wei, Wenjia Zheng, Changhui Rao, "First light on an adaptive optics system using a non-modulation pyramid wavefront sensor for a 1.8 m telescope," Chin. Opt. Lett. 14, 100101 (2016)
Category: Atmospheric and oceanic optics
Received: Apr. 13, 2016
Accepted: Aug. 19, 2016
Published Online: Aug. 2, 2018
The Author Email: Shengqian Wang (wsq_ioe@126.com), Changhui Rao (chrao@ioe.ac.cn)