The influence of surface defects on high power laser optical elements will cause some harm to the performances of imaging system, including the energy consumption and the damage of film layer. To further increase surface defects on high power laser optical element, on-machine defects measuring system was investigated. Firstly, the selection and design are completed by the working condition analysis of the on-machine defects detection system. By designing on processing algorithms to realize the classification recognition and evaluation of surface defects. The calibration experiment of the scratch was done by using the self-made standard alignment plate. Finally, the detection and evaluation of surface defects of large diameter semi-cylindrical silicon mirror are realized. The calibration results show that the size deviation is less than 4% that meet the precision requirement of the detection of the defects. Through the detection of images the on-machine defects detection system can realize the accurate identification of surface defects.
Spinel (MgAl2O4) is an ideal material for infrared window and dome,which plays an important role in infrared imaging terminal guidance. Due to the uneven grain size, the flexible polishing surface of the spinel has grain morphology, which significantly affects the surface quality. In this paper, uniform surface modification of spinel by high-current pulsed electron beam(HCPEB)is investigated. On that basis, flexible polishing is carried out. A new way is provided for reducing and eliminating the grain morphology of the flexible polished surface.
Firstly, the mathematical model and numerical simulation for temperature field of spinel modified by HCPEB is carried out. The optimized modification parameters for spinel remelting are obtained: the irradiation energy density is 2J/cm2 and the irradiation time is 5μs. Then, the spinel is modified on the high-current pulsed electron beam equipment by using those parameters. The change of infrared transmittance is examined on a high resolution spectrometer before and after modification. The modified spinel is polished on magnetorheological finishing machine. The surface morphology and roughness of modified layer is observed on the Zygo white light interferometer after polishing. It is found that the grain is refined and the surface roughness is reduced from Ra 13.28nm to Ra 8.86nm.
Spinel has outstanding optical performances and mechanical performances, and it is suitable material for infrared window and dome. But it is extremely difficult to machine spinel with high accuracy, especially in the machining process with compliant polishing tools, grain profiles will appear on the machined surface, which will result in the deterioration for machining accuracy, surface quality and optical performance. In this paper, analyzing the microscopic figure of spinel in the process of complaint polishing and founding the gradient change obviously on the edge of the grain profile, which has significant influence on the surface roughness. The spinel substrates are separately polished by using pitch tool, polyurethane tool and magnetorheological fluid, the values of surface roughness are respectively Ra 2.3nm, Ra 8.5nm and Ra 64.6nm, and the corresponding characteristic scales of grain profile are 180μm~200μm, 160μm~200μm and 200μm~250μm. Furthermore, the peak value of grain profile is proportional to the polishing tool flexibility, inverse proportional to the rigidity and the size of polishing powder, and the flexibility expression for polishing tool is given on the conditions of different machining parameters. On this basis, the high accuracy and ultra smooth surface of the spinel are obtained by the optimum polishing conditions.
In order to meet the requirement of aerodynamics, the infrared domes or windows with conformal and thin-wall structure becomes the development trend of high-speed aircrafts in the future. But these parts usually have low stiffness, the cutting force will change along with the axial position, and it is very difficult to meet the requirement of shape accuracy by single machining. Therefore, on-machine measurement and compensating turning are used to control the shape errors caused by the fluctuation of cutting force and the change of stiffness. In this paper, on the basis of ultra precision diamond lathe, a contact measuring system with five DOFs is developed to achieve on-machine measurement of conformal thin-wall parts with high accuracy. According to high gradient surface, the optimizing algorithm is designed on the distribution of measuring points by using the data screening method. The influence rule of sampling frequency is analyzed on measuring errors, the best sampling frequency is found out based on planning algorithm, the effect of environmental factors and the fitting errors are controlled within lower range, and the measuring accuracy of conformal dome is greatly improved in the process of on-machine measurement. According to MgF2 conformal dome with high gradient, the compensating turning is implemented by using the designed on-machine measuring algorithm. The shape error is less than PV 0.8μm, greatly superior compared with PV 3μm before compensating turning, which verifies the correctness of measuring algorithm.
KEYWORDS: Wavefronts, Telescopes, Monochromatic aberrations, Device simulation, Space telescopes, Calibration, James Webb Space Telescope, Error analysis, Large telescopes, Interferometers
The wavefront error of large telescopes requires to be measured to check the system quality and also estimate the
misalignment of the telescope optics including the primary, the secondary and so on. It is usually realized by a focal
plane interferometer and an autocollimator flat (ACF) of the same aperture with the telescope. However, it is challenging
for meter class telescopes due to high cost and technological challenges in producing the large ACF. Subaperture test
with a smaller ACF is hence proposed in combination with advanced stitching algorithms. Major error sources include
the surface error of the ACF, misalignment of the ACF and measurement noises. Different error sources have different
impacts on the wavefront error. Basically the surface error of the ACF behaves like systematic error and the astigmatism
will be cumulated and enlarged if the azimuth of subapertures remains fixed. It is difficult to accurately calibrate the
ACF because it suffers considerable deformation induced by gravity or mechanical clamping force. Therefore a selfcalibrated
stitching algorithm is employed to separate the ACF surface error from the subaperture wavefront error. We
suggest the ACF be rotated around the optical axis of the telescope for subaperture test. The algorithm is also able to
correct the subaperture tip-tilt based on the overlapping consistency. Since all subaperture measurements are obtained in
the same imaging plane, lateral shift of the subapertures is always known and the real overlapping points can be
recognized in this plane. Therefore lateral positioning error of subapertures has no impact on the stitched wavefront. In
contrast, the angular positioning error changes the azimuth of the ACF and finally changes the systematic error. We
propose an angularly uneven layout of subapertures to minimize the stitching error, which is very different from our
knowledge. At last, measurement noises could never be corrected but be suppressed by means of averaging and
environmental control. We simulate the performance of the stitching algorithm dealing with surface error and
misalignment of the ACF, and noise suppression, which provides guidelines to optomechanical design of the stitching
test system.
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