Fraunhofer’s objective, of which Fig. 54a is an example worked by modern formulæ for the sine condition, gives very exact corrections over a field of 2°-3° when the glasses are suitably chosen and hence is invaluable for any work requiring a wide angle of view.
With certain combinations of glasses the coma-free condition may be combined successfully with Clairault’s, although ordinarily the coma-free form falls between the two forms clear of spherical aberration, as in Fig. 52, b, which has its oblique rays well compensated but retains serious axial faults.
Fig. 54.—The Fraunhofer Types.
Fraunhofer’s objective has for all advantageous combinations of glasses the front radius of the flint longer than the rear radius of the crown hence the two must be separated by spacers at the edge, which in small lenses in simple cells is slightly inconvenient. However, the common attempt to simplify mounting by making the front flint radius the shorter almost invariably violates the sine condition and reduces the sharp field, fortunately not a very serious matter for most astronomical work.
The only material objection to the Fraunhofer type is the strong curvature of the rear radius of the crown which gives a form somewhat susceptible to flexure in large objectives. This is met in the flint-ahead form, developed essentially by Steinheil, and used in most of the objectives of his famous firm. Fig. 54b shows the flint-ahead objective corresponding to Fig. 54a. Obviously its curves are mechanically rather resistant to flexure.[11]
Fig. 55.—Clark Objective.
Mechanical considerations are not unimportant in large objectives, and Fig. 55, a highly useful form introduced by the Clarks and used in recent years for all their big lenses, is a case in point. Here there is an interval of about the proportion shown between the crown and flint components.
This secures effective ventilation allowing the lenses to come quickly to their steady temperature, and enables the inner surfaces to be cleaned readily and freed of moisture. Optically it lessens the deviation from the sine condition otherwise practically inseparable from the equiconvex crown, reduces the chromatic difference of spherical aberration, and gives an easy way of controlling the color correction by slightly varying the separation of the lenses.