--20--
Not to discourage ATM's, which are not fond of mathematics but only as a control for computists the three conditions for this simplest device of medium and big sized Schiefspiegler are given in the following.
1) condition of focal lengths
2) condition of coma
wherin 
and 
3) condition of astigmatism
Finally two peculiarities of the Shiefspiegler have to be mentioned. The axis of the incoming beam of light being not parallel to the axis of the beam, reflected from the secondary, a finder is necessary, which must be inclined against the incoming beam of rays under the variation- angle
25)
The second peculiarity: the extended field of view doesn't lie per- pendicularly to the axis of the eyepiece, but is inclined under a certain angle Γ against it. This effect is common to all off-axis telescopes. For visual observation this peculiarity will do less harm, because the accommodation of the human eye equals the caused differences in focusing. For photographic work the plateholder will be inclined under the angle Γ, which can be determined empirically or by a simple computation.