Abstract: An auxiliary mirror attachment unit for mounting on a primary mirror is provided having a support housing and an optical reflecting element assembled with the housing. The support housing includes an integrally formed base plate and a peripheral wall projecting laterally from the plate. The reflecting element is of plate form having a peripheral edge of a configuration to interfit with the housing's peripheral wall. Mechanical interlocking of the reflecting element with the housing in assembled relationship is effected by cooperatively interfitting tongue and groove conformations formed on the respective peripheral edge of the reflecting element and on an inwardly facing surface of the peripheral wall. The support housing is formed from a material having a minimum degree of resilience to enable mechanical assembly with the reflecting element. Attachment of the unit to a primary mirror is accomplished by adhesive.