The Near IR Spectrometer Photometer (NISP) role is to measure the galaxies' redshift. It observes adjacent non-overlapping areas of sky in each pointing. The baseline for NISP is an Infrared instrument allowing observation in slitless "spectrometry" mode thanks to several Grisms (Gratting + Prism), or in "Photometry" mode using several filters. A grating wheel with 4 Grisms enables to select the spectral channels for the observation in "spectrometry" mode as well as the orientation of the spectra and a filter wheel with 3 filters enables to select the spectral channel for the photometric observation. In spectrometry mode the filter wheel is in open position. In photometry mode the Grism wheel is in open position.
The spectrometer mode is characterized by one single main observing mode: the acquisition of a slitless spectroscopic image of the monitored field. However, slitless data reduction techniques require that each field is observed at a different orientations (roll angles) to disentangle confused spectra. Each spectroscopic image is then the association of 4 frames collected for 2 spectral bands covering [0.9-2.0] µm and 2 spectrum orientations (0 deg or 90 deg).
Illustration of the slitless spectrometry.
The light of each galaxy on the image on the left
is spread on the focal plane of the spectrometer
(image on the right). Credit: J. Zoubian
The NISP is composed of:
NI-SA: Structure Assembly: a structure in SiC
NI-OA: Optical Assembly: consists in a corrective optic (CoLA) and a camera optic (CaLA)
NI-FWA: Filter Wheel Assembly: carries 3 Filters (Germany + Spain)
NI-GWA: Grism Wheel Assembly: carries 4 Grisms (France)
Both wheels are equipped with a cryomecanism
NI-CU: Calibration Unit
NI-TC: Thermal Control which objective is to control the optics at the temperature of 140K ±0.3 K during all the lifetime.
NISP instrument under different viewing angles
The instrument focal plane NI-DS (NISP Detection System) is mounted on Ni-SA structure (see above). The focal plane is made up of a structure in SiC on which are 16 H2RG detectors (supplied by Teledyne industry) cooled under 100K and linked by flexible connectors to the reading and digitizing electronics (named Side-Car). The following graphics illustrates the NI-DS and shows its thermostructural concept:
Electronics: ensures the interface with the satellite, the control of the instrument, the acquisition and the on board processing of the images (deglitching, compression, ...). The instrument functional scheme is illustrated in the following graphics.
The observation sequence of the instrument is shared between the two modes as shown in the graph below: