This chapter contains the description of the NICMOS filters and optical elements. The NICMOS focus history, image quality, encircled energy, Point Spread Functions, and the NIC3 performance are also presented. The photometric performance of the instrument, together with issues regarding both photon and cosmic ray persistence, are discussed. Finally, the background seen by NICMOS is described. Additional considerations regarding the special modes: coronagraphy, polarimetry and grism capabilities, are reported in Chapter 5.
