By looking at the eyepiece of a telescope with the head held 

 back a few inches a small round disc of light will be seen. This is 

 an image of the objective formed by the eyepiece. It is called the 

 exit pupil for the reason that all the light which emerges from the 

 telescope must pass through this circle. If we use d and m in the 



same sense as above, the diameter of the exit pupil is equal to ' 



m 



Since the brightness of the image was found to be proportional to 

 - it is proportional to the square of the diameter of the exit 



IT12 



pupil. 



The apparent illumination of the image seen through the tele- 

 scope is dependent upon the relation between the diameter of the 

 exit pupil and the diameter of the pupil of the eye. Maximum 

 brightness is obtained only when they are equal. If the exit pupil 

 is smaller in diameter than the pupil of the eye, the brightness is 

 less than the maximum; if larger, the brightness remains at the 

 maximum value, for not all the aperture of the objective will be 

 utilized. The diameter of the effective aperture, in the latter case, 

 will be equal to the product of the magnification and the diameter 

 of the pupil of the eye. 



Quality of Lenses. The performance of a telescope can n< 

 come up to the measure of the theoretical value in respect to resolv- 

 ing power or illumination if the lenses are defective in either desij 

 or technique. Surfaces which are not perfectly polished cause tl 

 image to appear grey, surfaces which are not spherical result ii 

 images which appear drawn or strained when the objective is rack* 

 in and out of focus. If the lenses, especially the objective, are m 

 centered the image will come into and go out of focus with a 

 sided lack of symmetry. Technical skill of highest order and tl 

 best of mechanical equipment are necessary for the production 

 high grade telescope optics but these will be of no avail if the design 

 of the lenses is faulty. 



Fig. 39 



A single lens used as a telescope objective has two principal 

 defects: chromatic and spherical aberrations. The focal length of 

 any single lens is different for each different color of light so a beam 

 of white light, composed as it is of a mixture of all the colors of the 

 spectrum, instead of uniting in a single focal point is distributed 



