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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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