When testing aspheric surface by computer generated hologram (CGH), there are some engineering problems in experiments. Within a lot of experiments, some unknown conditions appeared, and we analyzed two of them in detail and simulated them in Zemax. The unwanted diffraction orders maybe bring ghost image, and ring source is a simple method to eliminate it. The parallelism error of CGH substrate will bring a non-rotational symmetric error in test result, which is shown as coma. The size of the coma is related by parallelism error and the degree of the coma is related by the angle compared with the origin position. And a rotation method was presented to know the influence of parallelism error. After that, the projection distortion is expounded in detail and distortion calibration is successful to guide to enhance the precision of aspheric surface. These analysis results are very useful in practical engineering applications.
To test an off-axis aspheric surface in high precision, a multiple combined computer generated hologram (CGH) is designed and fabrication. Multiple combined CGH which is a hologram including different areas with different purposes
can not only measure off-axis aspheric surface, but also align every element in test system. The design methods of test CGH and alignment CGH were deduced in detail. Ray trace and B-spline were used to devise test CGH, reflection CGH
was used to align interferometer and CGH, and hologram alignment mark was used to align CGH and off-axis aspheric surface. A design example was given to test an off-axis paraboloid with 50mm aperture and 35mm off-axis distance.
Meanwhile, this CGH was fabricated and this paraboloid was measured by it. The test result (PV=0.305λ，RMS=0.042λ)
is matched well with the outcome verified by autocollimation (PV=0.320λ，RMS=0.043λ). At last, we analyzed and
solved the noise in the CGH test result. It proves that these designs are correct and this method can test off-axis aspheric surface in high precision.
When testing an off-axis aspheric surface using Computer Generated Hologram (CGH), multiple combined CGH can not
only test the off-axis aspheric surface but also align every element in test system. The alignment transmission CGH with
a rectangle aperture, which produces an alignment mark at the designated point, can be used to align the off-axis aspheric
surface and CGH. Hologram alignment mark is introduced firstly, then the design of hologram alignment mark is
deduced in detail and a formula of calculating the position coordinate of every point on fringes is got. After that its
diffraction characteristics are analyzed based on Fresnel diffraction theory, which is in line with Fraunhofer diffraction,
and the effects of different length-width ratio of rectangle aperture are simulated. Finally an example of hologram
alignment mark and its corresponding actual mark picture are given.