The Multi-channel Photometric Survey Telescope (Mephisto) is a wide-field ground based telescope with a 1.6m primary mirror and 2° field of view, proposed by Yunnan University. The telescope will be capable of imaging the northern sky in three colors simultaneously and deliver a colored movie of the universe. The R-C system with lens corrector was adopted as the optical system considering of the image quality requirement, light obscuration and camera arrangement, in which three cubic splitters were adopted for the three channel beams in order to get satisfied image quality over the whole field of view. Dichroic coating on the cubic will lead to nonuniform efficiency on the focal plane due to the polarization problem and wide incident angle, which need calibration. The image quality represented in 80% encircled light energy is around 0.6arcsec. In order to keep the optimal image quality in any operational conditions, a 5-DOF mechanism was designed to actively adjust M2 mirror positions according to the wavefront sensors or by direct star psf. Now the telescope are under developing in Nanjing, expected to be installed at Lijiang observatory before the end of 2021.
The Multi-channel Photometric Survey Telescope (Mephisto) have a 1.6m primary mirror with 1.3 focal ratio, a 2degree field of view. Mephisto is capable of imaging the northern sky in three colors simultaneously. Its 0.7m secondary mirror is quite sensitive for alignment. The field dependent image quality of the misaligned Mephisto optics would be characterized as asymmetric, continuously varying and nonlinear with the misalignments. When the real-time field-PSF is measured from the astronomical images, the residual field-PSF of nominal optical design is defined as the goal of the merit function, the misaligned values of Mephisto secondary mirror can be calculated, by using the PSO (Particle swarm optimization) algorithm. The truth is field-PSF can be numerically expressed with ellipticity of the imaging stars or distribution of the ellipticity. However, for resolving the misalignment values, the stable field-PSF modes are needed, instead of random modes. PCA (Principal component analysis) algorithm is used for eliminating the random perturbations, such as turbulence.
The Zwicky Transient Facility (ZTF) is a new time-domain survey project that will use a new camera with a 47 square degree field of view mounted on the Samuel Oschin 48-inch Schmidt telescope. To achieve good image quality over all sixteen 6K*6K CCDs (386mm＊395mm corner to corner), a trim plate was added in front of the existing cemented achromatic doublet to form an air-spaced triplet corrector. The trim plate is a Schmidt corrector using fused silica with 1348mm in diameter but only 15mm in thickness. The plate has been completed at NIAOT and already been mounted to the ZTF, the PSF seems very good.
The detailed fabrication process and testing to the trim plate from fine grinding to finish are presented in this paper, which includes CMM test, NULL test design, and Computer Controlled Polishing process developed at NIAOT.
Due to their excellent ability to improve the performance of optical systems, free-form optics have attracted extensive interest in many fields, e.g. optical design of astronomical telescopes, laser beam expanders, spectral imagers, etc. However, compared with traditional simple ones, testing for such kind of optics is usually more complex and difficult which has been being a big barrier for the manufacture and the application of these optics. Fortunately, owing to the rapid development of electronic devices and computer vision technology, fringe reflection technique (FRT) with advantages of simple system structure, high measurement accuracy and large dynamic range is becoming a powerful tool for specular free-form surface testing. In order to obtain absolute surface shape distributions of test objects, two or more cameras are often required in the conventional FRT which makes the system structure more complex and the measurement cost much higher. Furthermore, high precision synchronization between each camera is also a troublesome issue. To overcome the aforementioned drawback, a virtual-stereo FRT for specular free-form surface testing is put forward in this paper. It is able to achieve absolute profiles with the help of only one single biprism and a camera meanwhile avoiding the problems of stereo FRT based on binocular or multi-ocular cameras. Preliminary experimental results demonstrate the feasibility of the proposed technique.
Free-form optics have a wide range of applications since they can simplify the structure of an optical system, meanwhile significantly improving the system performance. Compared to optics with traditional profiles, optical testing for freeform surface is more difficult. Although a laser interferometer can reach the surface measurement precision in nanometer scale, it has problems of a limited measurement range, a complex system configuration and relatively high requirements of working conditions. Fringe reflection technique (FRT) is gradually becoming a powerful tool for free-form surface testing owing to its advantages of simple system structure, high measurement accuracy, large dynamic range, etc. However, multiple groups of fringe images are required to display in two orthogonal directions respectively during the FRT measurement to obtain the corresponding surface gradient information. It hinders the fast detection of free-form surface to a certain extent. In order to overcome the above shortcoming, a phase retrieval algorithm based on color-frequency encoding for FRT is proposed in this paper, which can achieve the absolute phase meanwhile reducing the required number of fringe images for measurement. Experimental results demonstrate the effectiveness of the proposed method.
A new type Stressed Mirror Polishing method using annular polishing machine is developed in NIAOT. It provides good efficiency for the massive production of off-axis segments for the extremely large telescope because 3 or more pieces of segment can be polished simultaneously on a AP machine. With an annular polishing machine with 3.6m diameter, two scale-down TMT segments have been polished. Both 2 segments are Φ1100mm in diameter, with the vertex radius of curvature of 60m and aspheric constant K=-1.000953. The off-axis distances (OAD) are 8m and 12m respectively. After SMAP process, the acceptable surface accuracy can be reached, which is 1.12μm/0.23μm of PV/RMS value for the segment with 8m OAD, and 1.22 μm/0.26 μm for another one.
The research works are summarized for Φ1.1m off-axis aspheric segments which are scaled-down TMT segments polished by NIAOT using SMAP (Stressed Mirror Annular Polishing) method in the previous phase and testing preparations for 1.45m mirrors. The detailed introduction is given on result of errors analysis in contact detection testing of segments. In the second part, the selected basis of sampling number for contact-type detector arrays and the terms number of Zernike polynomials we need are studied. And the situations on orthogonality destruction of Zernike polynomials in discrete points sampling case and spectral analysis for each order Spherical aberrations in continuous sampling are introduced.
In recent years, high frequency errors of mirror surface are taken seriously gradually. In manufacturing process of advanced telescope, there is clear indicator about high frequency errors. However, the sub-mirror off-axis aspheric telescope used is large. If uses the full aperture interferometers shape measurement, you need to use complex optical compensation device. Therefore, we propose a method to detect non-spherical lens based on the high-frequency stitching errors. This method does not use compensation components, only to measure Aperture sub-surface shape. By analyzing Zernike polynomial coefficients corresponding to the frequency errors, removing the previous 15 Zernike polynomials, then joining the surface shape, you can get full bore inside tested mirror high-frequency errors. 330mm caliber off-axis aspherical hexagon are measured with this method, obtain a complete face type of high-frequency surface errors and the feasibility of the approach.
When polishing and modifying the large aperture flat by the traditional polishing tools, people usually test it by
a spherical mirror as a standard surface which been called the Ritchey-Common method, that not only can
break through the limitation of the aperture, but also can achieve high precision wave front if the surface of the
standard mirror is perfect. However, when doing ultrahigh precision modifying by the modern polishing
equipment such as: ion beam polishing, this testing method cannot meet the need of the high precision
location, because of the error caused by the nonlinear transformation of the coordinate, the testing result
usually cannot been very exactly developed point to point, that restrained the polishing accuracy. Here the
error has been studied in order to exactly developing the testing result. At first, in principle, the Ritchey-
Common testing path has been analyzed in detail. Secondly, the point to point transfer equation has been
deduced, and some feature points have been chosen to help analyze the relationship between the object
surface and the image result. Then a program has been written according to the deduced equation, by which
the image can be well developed. Finally the error has been compared by using different developing methods
in the experiment. The study can solve the nonlinear point to point transfer and location problem caused by
the Ritchey-Common testing method, so when manufacturing the large aperture flat, the Ritchey-Common
testing method can be used in the ultrahigh precision polishing.