Two-lens telecentric model eyes for image distortion measurement in adaptive optics ophthalmoscopes

Here we present the design of telecentric model eyes for measuring image distortion in adaptive optics ophthalmoscopes, using pairs of achromatic or near-achromatic commercial off-the-shelf (COTS) and/or custom doublet lenses. The proposed model eyes can operate over visible or visible plus near infrared wavelength ranges, across a 15-diopter focus range, with an 8 mm entrance pupil diameter. COTS lens selection was implemented as a systematic evaluation of catalog lenses after optimization of axial distances. Custom lens optimization was initially guided by wavefront aberration constraints derived from object-shift 3rd order aberration theory, before traditional variation of surfaces and distances to minimize a performance metric. When a wavefront corrector compensates for field-constant aberrations, the model eyes achieve nominal wavefront RMS below /20 across an ~6.9° circular full field of view for COTS and hybrid lens pairs, limited by vignetting from a self-imposed 27.9 mm clear aperture. For custom lens pairs, the field of view is about 10.2°, limited by aberrations. A tolerance study indicates that a COTS lens pair for 450-1100 nm can achieve as-built diffraction-limited wavefront error of 0.044  @450nm at 95% production yield. Model eyes with this lens pair yielded telecentricity as image size change per diopter of 0.1%/D when assembled with a camera and 0.01%/D when assembled with a power meter. The model eye was tested with an optical setup that emulates a fundus camera with Badal focus correction.