= Off-axis pinhole test for coma
The institution-issued <AY 105 Lab Experiment 3: Optical aberrations by California Institute of Technology> (Spring 2011 offering, Part A, pp. 2–3, 7–8) specifies a concrete <optical bench>: a lamp, opal diffuser and condensing <optical lens> illuminate a $25$ or $30\,\mu\mathrm m$ <optical pinhole>; a photographic telephoto <optical lens> acts as a <collimator>, producing an approximately $75\,\mathrm{mm}$ beam. A test <optical lens> sits on a rotation stage, followed by an adjustable <optical microscope>. The procedure establishes the on-axis image, then rotates the test <optical lens> a few degrees and examines the off-axis image through <optical focus>. It compares this readout with through-focus <ray tracing> at the corresponding field <angles> and offers a <Ronchi ruling> to calibrate the measuring eyepiece.
Part B (pp. 9–10) substitutes a spherical mirror of $1\,\mathrm m$ radius of <curvature>, with the <optical microscope> beside the incoming beam so the image is necessarily off-axis. It calls for measuring the field <angle>, calibrating the eyepiece and recording the image's length and width at minimum size. These are specified apparatus dimensions and instructions, not supplied measurement results or dimensions of the normalized figures.
A controlled extension would record images at zero and equal positive and negative field <angles>, repeat zero afterwards, and scan through <optical focus> at every setting. Keep the illuminated <optical pupil> and illumination fixed, recenter the <optical microscope> without clipping the bundle, and use its calibrated scale to separate a mere image displacement from a change in blur shape. The pure <Seidel coma pupil mapping> predicts reversal and linear growth; a stationary asymmetric central image instead calls for checking alignment. A focus-dependent change between perpendicular elongations suggests an <astigmatism> contribution. These comparisons test the model rather than assuming every off-axis flare is pure <coma>.
The <optical pinhole> has finite angular size, approximately its <diameter> divided by the <collimator>'s <focal length>; the <collimator> and observing <optical microscope> also contribute their own <optical aberrations>. A broadband lamp can mix <chromatic aberration> with the geometric blur, and <diffraction> limits the smallest visible structure. A narrow spectral band, repeated field settings and a reference image help separate these effects. Consequently a measured length and width are useful checks on off-axis <image formation>, but they cannot by themselves determine a unique <coma> coefficient or recover the full <point spread function>.
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