  Co-phase and co-focus detection is one of the key technologies for
large-aperture segmented mirror telescopes. In this paper, a new edge sensor
based on fringes of equal thickness is developed, which can detect each
segment's relative piston, tilt, and tip errors from the interferograms. Based
on the co-focus demand for many ground-based seeing limited segmented mirror
telescopes, an edge sensor prototype based on such a principle is built and
applied in the indoor segmented mirror experiment system in the lab. According
to the co-focus requirement of the Large Sky Area Multi-Object Fiber
Spectroscopic Telescope, many simulations and experiments are carried out for
co-focus error detection of the segmented mirror system. Experiment results
show that the co-focus accuracy is better than 0."02 rms, which can meet the
co-focus requirements of most large or extremely large segmented mirror
astronomical telescopes.
