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| 16) equivalent saggital focal-length of the system | |
| 17) astigmatical difference of equivalent focal - lengths | |
The astigmatical difference of focal-lengths will especially engage
us in designing the catadioptrical designs of the Schiefspiegler.
described later.
Concerning the coma-free catoptric design, the overcorrected astig-
matism, i.e. the difference k between the cutting lengths of meridio-
nal and sagittal rays, will be eliminated by a cylindrical deforma-
tion of the spherical secondary along its sagittal diameter ( in fig.2
perpendicularly to the center of the secondary). By this step the path
of meridional rays will be shortened to the sagittal focus and the
astigmatical cutting-lengths-difference is eliminated herewith. An
astigmatical difference between the meridional and sagittal diameters
of the cone of light, coming from the primary, existing on the
secondary
there will theoretically remain in the corrected second focus a very
little residual of astigmatic difference of focal-lengths (about 0,2 %),
which cannot be eliminated by this device, but which is really negli-
gible.
The cylindrical deformation of the spherical secondary is a marvellous hobby for experienced hands and is done empirically till the extra- and intrafocal star-disks proves to be exactly circular. A minute re- sidual of under-or overcorrected astigmatism, visible as an elliptical shape of the off-focus star disks, can be eliminated by adjusting the inclination φ1 of the primary.
Promising attempts the author made in cylindrical warping the secondary with help of a specially designed cell. By turning two fine threaded screws, attached to the sagittal borders of the cell, pressur was transmitted by a system of pressur-points to the sagittal edges of the secondary. Difficulties only turned up through uncontrollable elastical and thermical reactions of the cell's material.
The instrument shown in fig.5 is an eight inch ( 200 mm ) Schiefspieg- ler in coma-free design with cylidrically deformated secondary.
The adjustment of this type is very simple. After pre-adjusting in full daylight according to the anastigmatic device,the minute adjust- ment is limited to eliminate the last deviations of the exactly cir- cular look of an off-focus star-disk with aid of the primary's three adjustment-screws.