The calibration of a 2.5x Galilean focusable telescope as an optometer for refraction

A 2.5x Selsi achromatic Galilean focusable telescope was calibrated for refraction at 6 m. In its calibration, minus power trial lens was placed at the objective of the telescope to simulate vergence of a target at a finite distance, before the back vertex power (BVP) of the telescope at each setting (telescope length) was measured by a focimeter. By using a graphical presentation of the results, the BVP of the telescope at each setting could be determined at different selected target distances. For a target vergence of -0.167 D or a distance of 6 m, the common testing distance in clinical practice, this calibrated telescope had a BVP or refraction measuring range of -7.27(-)+7.52 D. When this telescope was used to measure simulated manifest refractive errors at 6 m, it yielded a mean error of +0.13 D with a 95% confidence limit of agreement of -0.38(-)+0.64 D. These results indicated that the accuracy and precision of telescopic refraction were comparable to that of retinoscopy. Therefore, the calibrated telescope could be considered as a reliable and inexpensive instrument for determining spherical refractive errors. Telescopic refraction is applicable in refracting economically disadvantaged population in underserved areas where modern equipment and electricity are not available. In addition, it provides an alternative subjective refraction method for low vision population because the magnification of this calibrated telescope has the advantage of allowing low vision patients to be refracted at the common 6 m testing distance in clinical practice.