Astronomy Principles and Practice Fourth Edition.pdf

256 The optics of telescope collectors
Figure 16.20. Points T and S represent the foci of rays passing through the tangential and sagittal planes of a
lens; the spread of the astigmatic image lies between the points T and S.
Between these points there is a position where the smallest image can be found and again this image is
known as the circle of least confusion.
By examining the images produced by objects with a range of distances from the optic axis, it is
possible to record the surfaces on which the focus, T , of the rays of the tangential plane and the focus,
S, of the rays of the sagittal plane lie. They are found to approximate to paraboloids of revolution.
A single achromatic combination is likely to exhibit appreciable astigmatism. However, the
combination of two achromats at the correct spacing can provide a paraboloidal surface containing
both the T and S images. This surface is known as the Petzval surface, after the investigator of this
property.
16.5.6 Curvature of field
A system which is designed to remove completely astigmatism suffers from curvature of field, asthe
points of sharp focus lie on a curve rather than on a plane. Flattening of the field can be achieved at
the expense of having incomplete removal of astigmatism and usually some compromise is made. The
judicial placement of stops in the combination of lenses can also be used to reduce the effect of field
curvature.