THE CORRECTOR SYSTEM proposed for the FOC, HRS and FOS consists of two mirrors, M1 and M2 (Figure 4). M1 forms an image of the OTA pupil at M2 and an image of the OTA 
field between the mirrors. The latter image is relayed by M2 to the SI aperture. M1 has the function of a field mirror and is a simple sphere. The correction of spherical aberra­tion is done by M2 and is fully equivalent to correction at the OTA primary mirror itself. This feature is unique among the SI-external optical corrector systems considered in this report. It has the advantage that the corrected field is free of coma.
R.A Brown and H. C. Ford
1990-10-01
NASA technical report CR-187826 - Report of the HST Strategy Panel: A Strategy for Recovery