Each 2MASS camera consisted of a liquid nitrogen cryostat which contained three
256x256 NICMOS3 arrays. Each array viewed the same region of the sky via
beamsplitting dichroics. The light passed through both dichroics to the Ks-band
array. The first dichroic reflection fed the light to the J-band array. All three
optical paths shared the same first element located behind a cryogenic field stop.
Each optical path had six other lenses. This set of six lenses was identical for each
band. The lenses were composed of water-free fused silica (Infrasil) or calcium
fluoride. All lens surfaces were spherical. All optical elements were anti-reflection
coated and the integrated optical assembly transmitted ~70% of the incident light. A
band-limiting interference filter located near a pupil image established the system
bandpass. Table 1 summarizes the Survey's bandpasses:

Table 1:  2MASS 50% Transmission

Band    Optics and Camera       System + Atmosphere
J       1.11 - 1.40 um          1.12 - 1.36 um
H       1.51 - 1.79 um          1.51 - 1.78 um
Ks      2.00 - 2.31 um          2.02 - 2.30 um

This instrument catalog refers to the 2MASS camera used at the southern hemisphere
2MASS facility, at the Cerro Tololo International Observatory in Chile.