Page:A Study of Lunar Research Flights.pdf/19

 threshold value in moderately hard to find. To convert threshold illumination (vertical axis) of Fig. I to source intensity necessary for detection, one has only to multiply E by the square of the distance, which in this cane is 240,000 miles, expressed in centimeters. The intensity valued shown in Fig. 2 were obtained in this way

Fig. 2(a) presents average brightness values of light and dark moon and of night sky 3000 miles from the moon's surface at full moon, The source intensities necessary for detection with the naked eye against these back. grounds are also indicated.

B. Telescopes and Improvement of Visual Thresholds by Magnification

A telescope increases illumination from a source relative to the brightness of a uniform background and therefore makes it possible to see some sources which are below the limit of the naked eye, Tousey and Hulbert3 derive an expression for the gain by the use of telescopes which can be written aa

$$

in which D = diameter of objective lens,

P = diameter of eye pupil,

M = magnification,

t = transmission factor of telescope,

i = illumination on objective lens when point source appears at threshold,

i' = unaided eye threshold,

(i'/i) = improvement in threshold due to telescope 128