Large Aperture Space Optical System Test Based On Scanning Hartmann

Optical systems is the core of space telescope.The aperture and focal length of space optical system has been increased for bigger field,higher resolution and wider coverage of space telescope.More requirements of optical processing and test technology are putting forward.Evaluation of the imaging quality of space optical systems is a key link in the telescope manufacturing process,including alignment of optical system in laboratory and the final test of telescope before launched into the space.First,the traditional method to align optical system is the auto-collimating test,which involves the use of a plane mirror and an interferometer.The wave emitted from the interferometer was reflected from the optical system and a plane mirror back to the interferometer.Then the wavefront of optical system could be test.This method have the advantages of simpleness,efficiency,and high precision.However,the auto-collimating test cannot be used for a finished optical system,because the imaging sensor of optical system is mounted in the focal plane already and cannot be replaced with an interferometer.And it is very difficult to fabricate a plane mirror with an aperture size in excess of two meters.Those limited the auto-collimating test's application.Second,the image quality of a finished space optical system is always evaluated using a large collimator before it is launched into space.After transport of a finished telescope from the laboratory to the launching site,its optical performance may change due to vibration.Thus,an outdoor test at the launching site is carried out as a final test procedure to ensure that the imaging quality of the telescope satisfies the needs of the project.In the collimator test,a collimated beam provided by a large collimator is passed through the optical system and focused onto the focal plane.The detected spot is analyzed to evaluate the imaging quality of the optical system based on the modulation transfer function(MTF).As the aperture of the optical system increases,evaluation of the imaging quality by this method requires a collimator with a large aperture and a long focal length.Thus,the cost increases dramatically.Therefore,confronted with the great challenge of testing large optics,we are eager to explore alternative test methods that afford simplicity and low cost.More than 110 years history passed since the Hartmann test method has been proposed.Scientists never stopped researching it.Hartmann test is based on the geometric property of light and measure slope information to retrieve surface or wavefront.Nowdays,wavefront detection teschnology based on hartmann principle is successfully used in many fields,including high-pricision optical test,adaptive optics,and biometrics.The article proposed a novel testing method for large aperture space optical system based on the scanning Hartmann testing(SHT)principle.This method uses a small collimator instead of a large and expensive collimator,and reduces the testing cost greatly.It had the advantages of simpleness and efficiency.Furthermore,we optimize the test plan and propose that a multi-beam array should be used to scan the full aperture of an optical system rather than a single collimated beam.The measurement information in the overlapping zones is used to determine the relative tilt between adjacent sampling positions.Then,the true wavefront is acquired by stitching together sub-apertures and the test accuracy is greatly imcreased.Furthermore,we develop a multi-beam array stitching test system(MASTS),use it to test an actual space optical system of ?800 mm and get the well test results.The multi-beam array stitching method based on scanning Hartmann is demonstrated to be valid for testing a large-aperture space optical system.This article conducts research on scanning Hartamnn test technology applied to large aperture space optical system which mainly includes the following aspects:1.The basic principle of traditional Hartmann test method is introduced.Based on Hartmann test,the scanning Hartmann test method is proposed and researched through the establishment of physical model and the deduction of mathematics.We also research the data processing method,including wavefront reconstruction algothrim and spot centroid extraction algothrim.The characteristic of scanning Hartmann is analyzed.The advantages for testing large aperture space telescope is discussed.2.In order to verify the scanning Hartamnn test principle,the whole testing process will be simulated on computer,including the accurate calculation of the center of mass for spots with different sub-apertures and reconstruction of low frequency wavefront aberration.The influence of some key parameters are simulated and analyzed,such as sampling density,beam aperture,and overlapping area between adjacent sub-aperutre.The influence of external error on testing results are simulated and analyzed.Furthermore,a measuring experiment was conducted to test the wavefront aberration of a 200 mm aperture optical system.The high-precision results verify the feasibility of the scanning Hartamnn test technology.3.Scanning Hartmann test method contains a crucial flaw in that pointing errors of the translation stage can severely affect the test accuracy.Therefore,a multi-beam stitching method is proposed to correct pointing errors by stitching together data obtained by successive sub-aperture acquisition.The test accuracy was significantly improved.The stitching algothrim based on slope Zernike fitting and a continuous slope condition was researched and optimized.Simulation results theoretically verify a high precision for the stitching algorithm.The influence of parameters of mutil-beam array and imaging sensor on accuracy and efficiency of the scanning Hartmann test.4.A multi-beam stitching test system was designed and developed.It included automatic scanning of translation stage,switch controllers of collimator source,capturing spot image,closed-loop pointing error compensation system,and data processing and showing.The errors of equipment were calibrated and analyzed.The multi-beam stitching test system was used to successfully test an actual space optical system of ?800 mm.It shows a deviation of 1/50 ?(? = 632.8 nm)root mean square(RMS)from the interferometric results and a repeatability of 1/80 ? RMS,which demonstrates high precision,high repeatability and low sensitivity to air turbulence compared to interferometric measurement.The applications value of multi-beam stitching test method was discussed.The article researches the large aperture space optical system test based on scanning Hartmann.The basic test principle was proposed firstly.Then the test feasibility was verified through computer simulation and principle experiment.Finally,the test system was developed and used successfully to test an actual space optical system of ?800 mm.In future engineering applications,the multi-beam stitching test system has great potential to solve the test problems of space optical systems using ultra-large apertures.