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.
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.
A large ion beam figuring plant which can process up to 1.5m mirror in diameter has been developed. The plant adopts
five axes scanning mode. The mirror with which the five axes scanning system is set face to face is placed vertically
when working that can reduce the adverse effect of the flour dust to the mirror and the ion source. The mirror and the
five axes scanning system are installed on two doors respectively. The open-close type of the door is a hanging bridge
type. And the door will be horizontal when completely opening that will be convenient for installing the mirror and
maintaining the system. Two software which are used for calculating the dwell-time and controlling the five axes system
are programmed. The five axes system will always keep normal direction following when working.
Optical plates (OP) play more and more important role in modern ground-based telescopes. They can be as segments
composing primary mirror, deformable mirror for correcting air turbulence or active stressed lap used in polishing large
aspherical optics. When control the deformation of these plates, we always confronts with common situations: high
shape precision requirement, rapid deformation frequency with real time demand, intrinsic multi-channel coupling
characteristic. So how to improve OP deformation performance becomes a critical task in practical design. In this paper,
the control principle of OP is first introduced. Then a three-layer control architecture is presented. They are application
layer, real time control layer and motion execution layer. After that we designed a prototype system following this
framework, targeting active stressed polishing lap which has twelve motion channels. Both the hardware and software
development are discussed thereafter. OP surface deformation experiments are carried out and surface shape obtained
using LVDT array. Results verify the effectiveness of the design. And we are looking forward to use this control design
in more channel and time demanding applications.
We have designed an active stressed lap and its control mode is that an computer sends control orders to the control
system of the active stress lap, under windows operating system. The control mode has a shortage, which is a limit exists
when we use the lap to complete an aspheric surface deformation task with a fast focal ratio, because windows system is
not real-time. Therefore, we have designed a real-time deformation control system for the active stressed lap.
The paper makes an introduction to the structure of the deformation control system of the active stressed lap.
Secondly, gives a detailed presentation on the design of the test system from software and hardware two
aspects .Finally, the paper makes a dynamic response test on the deformation control system of stressed lap, a stability
test on the control system and an overall performance test.
Nanjing Institute of Astronomical Optics & Technology (NIAOT) is investigating two types of sub-aperture polish
technique for manufacturing aspheric components in large astronomical telescopes. One technique is computer
controlled optical surfacing (CCOS). It removes material by a small polish tool through traditional mechanical and
chemical process. The other is ion beam figuring (IBF) technique. It employs a neutralized ion beam to physically
sputter material form optical surface. Although the basic mechanism of the two techniques is different, they true share
the same mathematical model and fabrication diagram which will be put forward firstly in this paper. Then tool design
and material removal function in CCOS will be studied following by a fabrication instance using CCOS. After that some
recent progresses achieved in IBF is presented. The last part will focus on the complementary relationship of CCOS and
IBF. Using them alternatively optimal combination of surface precision, efficiency and edge control could be obtained.
Simulation is provided to support this view and experiment will be done in near future.
Mirrors with large aperture, high asphericity and tight shape tolerances are indispensable for modern astronomical
telescope. This pressing demand provides new challenges on optical fabrication. Paper here presents a 2.5m CNC
machine which has been designed and constructed in Nanjing Institute of Astronomical Optics & Technology (NIAOT).
Mechanical layout of 2.5m OFM is first studied basing on analyzing fabrication requirements. Equipped with six motion
axes, it can process parts up to 2500mm in diameter. Rotary symmetrical component can be finished under cylindrical
coordinates and Cartesian coordinates are applied for off-axis workpiece. Then control system using CAN field bus is
discussed. This control architecture offers many characteristics, such as high openness, expansibility and flexibility.
Since 2.5m OFM is developed to combine faculties of grinding, lapping and polishing, we arrange a series of tests for
each function. Now a 1.1m hexagonal mirror is carried for grinding. Polishing test using sub-aperture tool and active
stressed lap will be programmed later.
NIAOT has made a stressed lap polishing machine and finished a φ910mm, F/2.0 paraboloid. In the process we found shape control strategy is an important technology for stressed lap polishing tool and this kind of content has not been discussed systematically before. So this paper mainly dedicates to the method of lap shape control. Firstly a mathematical model of stressed lap is introduced. Then three shape control methods are put forward one by one concerning aspects as shape accuracy and deformation hysteresis. The fundamental method is least square algorithm. On the base of it we put forward its reformation form: least square algorithm with damping factor. To get more satisfied performance a new algorithm using optimization under constrains of linear inequalities is proposed. Through theoretical analysis and computer simulation some comparisons are made among three methods. Finally we have done experiments using stressed lap polishing machine in NIAOT and the results obtained substantiate the feasibility and efficiency of our method.
An extremely large telescope named Chinese Future Giant Telescope (CFGT) has been presented. The primary mirror of CFGT is a 30-meter diameter hyperboloid with a focal ratio F/1.2 and it consists of over one thousand of sector-shaped segments with the size about 1.1-meter in diagonal. Based on the optical design concept and the experience of existing large segmented primary mirror, we explore the segment fabrication and testing issues in this paper. The relationship between external contour, the size and the asphericity of sub-mirror is studied. Two potential segment fabrication approaches for mass-production-scale are discussed. One is the optical replication. The other is stressed-mirror polishing. Both of two processes are tightly combined with several key techniques and devices, the ion-beam figuring, large annular polisher, and the stressed lap. Some preliminary concepts for testing of l-meter class convex/concave off-axis aspheric surface are discussed.