Large aperture telescope mirrors require reflective films with higher reflectivity to improve their optical efficiency and better stability to maintain their reflective performance. In this paper, an enhanced aluminum reflective film was deposited on a 2.5-meter mirror using ion beam-assisted deposition technology in a 3.2-meter coating equipment. Through the enhancement of the reflective performance of the aluminum reflective film by the dielectric film, the average reflectivity of the aluminum reflective film was improved to 91.3% within the 350-1700 nm wavelength range, and the reflectivity of the three working wavelength bands was increased to 95%. A full-dielectric reflective film was deposited on a 1.7-meter mirror, and the average reflectivity in the full aperture range of 1010-1090 nm was greater than 99.8%. After testing, the reflective mirror coated with the outermost dielectric film exhibited good friction resistance, and the dielectric film itself possessed excellent environmental stability. Applying it to the surface of metal films can effectively reduce the contact between metal films and the external environment, thus slowing down the degradation of the reflective film's performance. Dielectric films can improve the reflectivity of metal films and enhance the overall environmental stability of the reflective film, making them highly suitable for use in large aperture telescope mirrors.
Due to the large number of sub-mirrors for very large telescope, the recoating is heavy workload, so the reflector needs to take into account the film efficiency and life. Based on the requirements of China's great telescope, we carried out the experimental research on UV-enhanced silver reflective film system. The contents include the design of UV-enhanced silver reflective film system, the UV- enhanced silver reflective film system plating process, and the film life and environmental stability verification tests. The experimental results show that the method is successful in broadening the silver film to the ultraviolet band. It can greatly improve the lifetime and environmental stability of the film, and is suitable for application to extremely large splicing astronomical telescope.
The Multi-Channel Photometric Survey Telescope (Mephisto) is a wide-field ground-based telescope with a 1.6 m primary mirror and 2◦ field of view (FOV). The telescope is capable of synchronous photometric imaging in blue, yellow, and red channels covering a wavelength range from 320 nm to 1000 nm by using large cubic splitters with dichroic coatings. A high-accuracy photometric survey requires higher requirements for the transmittance and reflectance of dichroic coatings. However, the light ray incident angle on the cubic splitter surface is 45°±6°, and the refractive index difference between the p-component and s-component of incident light causes an obvious p/s efficiency difference of both the reflection and transmission through the cubic, which will introduce extra error on the photometric accuracy. This paper proposes an effective method to calculate the uniformity of the imaging efficiency of the Mephisto. By sampling on the pupil plane with each FOV, the efficiency of each light ray can be calculated by the curve fitting method. The efficiency difference over the full focal plane is less than 2.5%, 1.5%, and 0.5% for the blue channel, yellow channel, and red channel respectively, introduced by the cube dichroic coatings and the filters, which can be used for the calibration. The method paves the way to solve coating uniformity for large aperture telescopes.
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.
Along with development and progress of modern optics manufacturing technology and optical test technology, there are
a series of modern polishing technologies based on the application of computer software and hardware, such as
CCOS(Computer Controlled Optical Surfacing), Stress Lap Polishing, Magnetorheological Finishing, Ion Beam Milling,
etc. The emergence of these new technologies promotes aspherical mirrors widely used in optical system. Vacuum
evaporation technique is applied widely on making optical coating. In this paper, we discuss the feasibility on applying
vacuum evaporation technique to making large-size aspherical mirrors. The technology is to take original sphere as
substrate, deposit film of certain thickness distribution on the surface, then get aspherical mirror. This technology has a
special advantage and tremendous development space in the off-axis aspheric processing, silicon carbide aspheric surface
processing and modification, super-glossy aspheric processing and great telescope mirror batch processing.
KEYWORDS: Mirrors, Control systems, Domes, Telescopes, Control systems design, Astronomy, Temperature metrology, Optical instrument design, Observatories, Astronomical telescopes
Preliminary site testing shows that Antarctic inland Dome A is likely to be the best astronomical observatory site on the
ground, Chinese first Antarctic astronomical equipment CSTAR has been successfully run Dome A. Three Antarctic
Schmidt Telescopes (AST3) is the next important Antarctic astronomical equipment, one of which will be mounted
Dome A. In the year of 2010, and the three will be installed Dome A finally. Because of the very low temperature and
saturation vapor pressure, and the temperature gradient changes fast near the ground layer at Dome A, the mirror is easy
to be frosted, which is one of difficulties to AST3.Indium Tin Oxide (ITO)is an N-type semiconductor material, because
it has few resistors, good light transmission, good weather resistance, small environmental impact, low cost, and it is
easy for large area coating, so it is widely used in many fields. The mirror is heated by ITO that is coated on the surface
of the mirror, the voltage on the ITO will be tuned by changing the output pulse width, and then the system achieves a
closed-loop control. The difference between the mirror temperature and ambient temperature will be maintained in an
ideal range, and this will not only ensure that the mirror surface will not get frosting, but to minimize the impact of
mirror seeing to guarantee the image quality of the telescope. The experimental results show that the temperature control
system can control the different temperature between the mirror surface and the ambient less than 2 degree in real time,
which can improve the mirror's working environment, and the overall effectiveness of the telescope's observations.
We have produced a series of reflective mirrors using the newly installed 1.6m evaporation chamber at the Nanjing
Institute of Astronomical Optics and Technology (NIAOT) of the National Astronomical Observatories of China. The
main task of this equipment is to coat the mirrors of the LAMOST. The chamber have thermal evaporation system,
electron beam source, ion beam source, quartz crystal deposition controller and optical monitoring system, so can
evaporate all kinds of metal and oxide film and effectively control film thickness. Now, we have utilized this chamber to
aluminize the mirrors of LAMOST primary mirror, the average reflectivity is above 89% in the wavelength range from
370nm to 900nm. Recently, we have completed the enhanced silver reflector experiment, by controlling the dielectric
layers optical thickness, the reflectivity is increased from 370nm to 400nm. The average reflectivity of enhanced silver
reflector is above 97% in the wavelength range from 370nm to 900nm.
Fabry-Perot interferometer has an important effect on near-infrared high spectral resolution spectrograph. In 1896, Ch. Fabry and Alfred Perot designed and used the Fabry-Perot interferometer for the first time. Since then the instruments using Fabry-Perot interference phenomena have been applied broadly to multi-field, such as astronomy, laser, and fiber-optic transmission. Fabry-Perot interferometer has many advantages such as narrow passband, high spectral resolution, high throughput, easy wave-length adjustment, simple structure and large aperture. Comparing with traditional visible light, the solar observation in near-infrared has many advantages: for example, weaker magnetic field strength can be more precisely measured with near-infrared spectrum .So developing the key technology of near-infrared high spectral resolution spectrograph--Fabry-Perot interferometer has become urgent.
For developing near-infrared Fabry-Perot interferometer, there are four difficulties: producing high quality optical plane: peak-to-valley surface flatness better than λ/100; coating Fabry-Perot interferometer plates with broadband multilayer dielectric films(including spectrum performance, thickness uniformity and stress effects); controlling the distance of interference cavity; keeping constant temperature.
In this paper, the process of designing broadband dielectric reflective and antireflective coatings applied in near-infrared Fabry-Perot is described and some problems of designing Fabry-Perot interferometer are discussed: the design of broadband dielectric mirror is described with reflectivity of 93.9±1.0% over spectral ranges from 1.0μm to 1.7μm; by reflective phase shifts in the design of mirror coating, computing the required film thickness uniformity atλ/100 of peak-to-valley surface flatness; degradation of surface figure is perhaps more thanλ/100 even if the soft coating materials-zinc sulfide and cryolite are used, and in order to reduce the degradation of surface figure brought by the stress of dielectric mirror coating, antireflective coating adopts same materials of dielectric mirror coating, ZnS and Na3AlF6, and similar film total thickness.
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