Reflector or Refractor ?

In the matter of resolving power, the 6-inch mirror will excel the 5½ -inch lens, although diffraction caused by the obstructive secondary mirror and supporting vanes of the reflector has a more-or-less deleterious effect on the image.

The open ends of the reflector tube admit air currents which may further contribute to the impairment of image formation. Thus it may not be possible to realize the full theoretical limit of resolution of the mirror.

A considerable amount of correction of off-axis aberrations is effected in a suitably designed doublet lens; this cannot be accomplished in a mirror alone.10 If the instrument is confined to visual use, however, these aberrations do not seriously handicap a telescope, as the observer naturally brings the object under observation to the center of the field, where definition should be limited only by diffraction.

On the other hand, the mirror is perfectly achromatic, while the doublet cannot be entirely freed of color. In order to reduce the residual color to a tolerable minimum, the ordinary refractor is usually designed in ratios from about f/15 to f/20, although, in the smaller diameters, it is sometimes made as low as f/10. The more versatile reflector may range from f/3 to about f/12, limited in the higher ratios by the accessibility of the eyepiece. In the short instruments, provision can be made for amplifying the ratio, when desired, permitting of observation at either the Newtonian or Cassegrainian focus.

Reflector or Refractor continued