A form of equatorial mounting that provides a fixed, downwards observing position along the polar axis. The instrument is a Newtonian telescope, with its diagonal reflecting the beam through the declination axis; a tertiary flat then reflects the light up along the polar axis. The observing position is particularly comfortable and convenient. However, a large counterbalance is needed to ensure that the declination axis is at the centre of gravity of the tube, and the three reflections result in a dimmed, laterally reversed image. It is named after the town of Springfield, Vermont, where it was invented by American telescope-maker Russell Williams Porter (1871–1949).
Subjects: Astronomy and Astrophysics.