--15--
To avoid the unduely long r.o.c. of the plano-convex-cylinder, we
can construct the corrector-lens as a meniscus-cylinder of a coma-
free shape. Its power f3 and its focal-distance a1m will be the same.
Its two radii of cylindrical curvature will become
21) 
The minus-sign concerns the r.o.c. r3, the plus-sign the r.o.c. r4;
n is the medium refraction-index of the lens' material.
The difference between the two r.o.c. resulting only about 1,8 %,
the accuracy must be within close limits.The diameter of the corrector
(although of the plano-convex-cylinder) will be
22) 
that is, supposing our above mentioned fundamental system, about
3/4 of the secondary' s diameter ( vide appendix to page 15).
The pre-adjustment of this type of Schiefspiegler is the same as ex-
plained for the coma-free device. It is convenient to do this step
without the corrector-lens. The minute adjustment will again be done
on a real star. The corrector-1ens is turnable and removable mounted
in a sliding tube to adjust the exact position of the cylinder's
axis and distance a1m. Since the exact location, of the meridional
focus in the uncorrected second focus of the system is difficult to
determine, it is recommended to compute the distance s of the corrector
from the center of the secondary
23) 
By looking through the eyepiece the observer removes axially and turns radially the correctors sliding tube till the extra-and intra- focal star-disks are equally circular. The very exact adjustment is a matter of little experience but of very good seeing.
The second catadioptrical device, which was designed to avoid the pro- blems concerning the cylindrical surfaces, uses a spherical corrector- lens. This device, which theoretically seems to be the most compli- cated, but proves most simple in realizing, starts from a mirror- system, which lies between the coma-free and the anastigmatic type.