On Telescopic Star Images

Diffraction theory is applied to discuss the properties of the polychromatic, three-dimensional star images formed by refracting doublet objectives. The effects of the secondary spectrum on the light distribution in the diffraction image and on the position of best focus are examined quantitatively in two special cases: (A) an f/15 refractor of $$24\frac{7}{8}$$ inches aperture, (B) a similar refractor of one-third the linear dimensions. The image formed by an f/15 reflector working without central obstruction (Case R) is analysed for comparison purposes.