wherein 2y3 is given by the formula 22). --19--
The aperture of the spherical corrector-lens becomes
24)
wherein 2y3 is given by the formula 22).
The pre-adjustment is again the same as explained for the coma-free device. The corrector-lens has to be mounted turnable around its sagittal diameter and removable along its meridional diameter. Its center has to be installed in the computed distance a1m, respectively s. Its flat surface is turned against the incoming beam of rays. The bigger border of the wedge-shaped lens has to be installed in meridio- nal direction against the primary ( v. fig. 2 ).
By observing off-focus-disks of a star the lens will be continously inclined till the strongly elliptical shaped disks gets exactly cir- cular and homogeneously illuminated. If in the beginning of the operation the axis of the elliptical shaped disks doesn't lie exactly parallel to the meridional or sagittal diameters of the mirror's, the pre-adjust- ment isn't exactly done and cannot be reached by inclination of the lens. First we must eliminate this fault by inclining the primary with aid of its three adjustment-screws.
If the off-focus star-disks look circular but not homogeneously illu- minated, there will be a residual coma existent. If a concentration of light is visible in the border of the star-disks, which lies (in meridional direction) on the side of the primary, the coma is yet u n d e r corrected and the inclination φ3 of the corrector must be enlarged. If a concentration of light is conspicuous in the border opposite to the primary, the coma is overcorrected and φ3 must be diminished. Since each change of φ3 changes the astigmatism of the lens too, the shape of the out of focus disk will become slightly elliptical. By operating the meridional adjustment-Screw of the primary the elliptical shape will be brought back to its circular look. The focal image of the star must then show its ideal look. If in meri- dional direction a very faint spectrum is visible ( at a power of 30 times per inch aperture) the center of the corrector must be removed perpendicularly to the axis of the eyepiece in direction blueish to yellowish. This operation is to be controlled at a star with little zenith-distance, to avoid atmospherical spectrum. It is although recommended, to use for this trial exclusively achromatical eyepieces. Worthless to say again, that a little experience and a very good seeing is all.