= Solution
\b[<Scanner bow>] is the relevant artefact. A piezoelectric tube scanner bends to produce lateral displacement; its moving end also follows a curved vertical trajectory. Cross-coupling of lateral and vertical motion therefore gives a roughly quadratic height background even over a flat reference. Over a wide scan it can mimic or conceal a genuine parabolic sample profile; its sign depends on scanner geometry and height convention.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2020/paper-1-scanner-bow.png]
{title=A curved scanner trajectory adds a background indistinguishable from a parabolic sample in one scan}
{source=paper-1-scanner-bow.py}
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