Modulation transfer functions of low power telescopes.

The modulation transfer functions (MTF's) of 131 low power Galilean and Pechan roof prism Keplerian telescopes comprising 20 models from 7 vendors were measured. MTF results are compared according to model, by type (Galilean or Keplerian), and magnification. Measurements were made on-axis and at the +/- 0.7 field angles. In addition to measuring tangential and radial MTF's some devices were tested with target grating azimuths of 45 degrees and 135 degrees. We also measured the effect of 3 and 6.4 mm exit pupil diameters on the MTF, and compared a color-corrected with a non-color-corrected design in monochromatic and white light. Galilean telescopes exhibited superior MTF's compared to Keplerian designs. The MTF's of Galilean telescopes tested on-axis with vertical and horizontal gratings were equivalent, as expected of rotationally symmetrical devices. However, similarly tested Keplerian telescopes exhibited significantly higher MTF's with vertical gratings. Tests at +/- 0.7 field angles showed that the tangential MTF's of Galilean telescopes were consistently poorer than radial MTF's, but the opposite was true for Keplerian telescopes. The comparatively poorer results obtained with the Keplerian telescopes are due to image doubling and deviation errors of the roof prisms that are dependent on the azimuthal orientation of the prism roof edge. Failure to adopt and maintain the same orientation of hand-held prism monoculars may result in experiencing a variable sharpness of image each time that they are used. Prism deviations of binocular devices must be controlled to avoid vertical disparities.