At that time, this was not possible. A detailed explanation of this necessity would lead into the narrower area of practical optics, and the basics are easily comprehensible, so the discussion will be brief. Deviations of the axis are obvious, even with an assembled instrument. However, in these prism designs, there is no mechanically unified optical axis, except perhaps in the simplest and least effective types shown in figures 6 and 7. The essence of most of these designs is the frequent shifting and rotating of the axis back and forth.
Collimation therefore requires an incomparably more difficult procedure.
Otherwise, the ocular views an off-center part of the image given by the objective, or views it from a deviated direction, to ruin the quality of the image. The achievement of a satisfactory quality of image in the monocular Porro telescope also requires precise execution of the prisms in the telescope. The required precision increases when the prisms are closer to the objective of the telescope and with stronger magnification. Those who know practical optics are aware that there is no more sensitive test for the quality of a mirror, and especially for its flatness, than the test that is involuntarily applied here, the observation through a telescope of a reflected image, encompassing a large incident angle of the rays. This critical test occurs no less than 4 times in each tube, and requires a high level of technical ability from the maker.