The pupil mask/filter wheel is of a similar design as the two filter wheels, is 530mm in diameter and contains apertures for up to 12 masks and filters. The nominal pupil image has a diameter of 36mm. In fact, this is where the 5 NIRI grisms (transmission grating prisms) are located to facilitate observations in its spectroscopic mode.
The NIRI collimator lens is a two element lens made of LiF and BaF2. The first lens are both spherical with the first being a meniscus while the second is plano-convex. They both have a diameter of ~55mm. It is located between the two filter wheels on the science path optical table. Below are two photos of the collimator unit
without the filters wheels mounted on the table.
The NIRI filter wheels are 530mm in diameter and each contains 12 apertures for mounting near-IR filter. Filter wheel #1 is located immediately after the two fold mirrors and is in the uncollimated telescope beam. This filter wheel will hold broad band near-IR filters i.e. those not as sensitive to the collimated nature of the incident radiation. Filter wheel #2 is located after the collimator lens in the collimated beam and will hold narrow band near-IR filters
The field lens transfers the telescope beam onto the fold mirrors which in turn pass the beam to the filters wheel and collimator lens. Below will be a photo of the field lens installed in the science instrument (once one has been taken). The field lens is a single component spherical lens made of ZnSe It is 138mm in diameter. The field lens reduces (?) the telescope beam f-ratio to Y.
The NIRI beam-splitter wheel contains three beam-splitters to direct the science beam and OIWFS (On-instrument wavefront sensor) beam to the appropriate on-board instruments. The beam-splitters are flat mirrors of different sizes to allow the OIWFS beam for each camera, f/6, f/14 and f/32, to be picked off without modifying the science
On Gemini, science instruments are mounted at the cassegrain focus below the 8.2 meter primary mirror. The picture below shows a schematic of the telescope and specifically the bottom end where instruments are mounted. The f/16 telescope beam passes through the 1.18 meter diameter central primary hole, through the Acquisition &