Note 30 — Chapter IX · THE CARE AND TESTING OF TELESCOPES
Reading edition
Sometimes with ever so careful centering the ring system in the middle of the field is still eccentric with respect to the small mirror, showing that the axis of the parabola is not perpendicular to the general face of the mirror. This can usually be remedied by the adjusting screws of the main mirror as described, but now and then it is necessary actually to move over the small mirror into the real optical axis. Draper (loc. cit.) gives some experiences of this sort.