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

A 2.5x Selsi achromatic Galilean focusable telescope was calibrated for ref raction at 6 m. In its calibration, minus power trial lens was placed at th e objective of the telescope to simulate vergence of a target at a finite d istance, before the back vertex power (BVP) of the telescope at each settin g (telescope length) was measured by a focimeter. By using a graphical pres entation 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 pra ctice, 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% con fidence limit of agreement of -0.3-+0.64 D. These results indicated that th e accuracy and precision of telescopic refraction were comparable to that o f retinoscopy. Therefore, the calibrated telescope could be considered as a reliable and inexpensive instrument for determining spherical refractive e rrors. Telescopic refraction is applicable in refracting economically disad vantaged population in underserved areas where modern equipment and electri city are not available. In addition, it provides an alternative subjective refraction method for low vision population because the magnification of th is calibrated telescope has the advantage of allowing low vision patients t o be refracted at the common 6 m testing distance in clinical practice. (C) 2000 The College of Optometrists. Published by Elsevier Science Ltd. All r ights reserved.