February 7, 2017

NISP Instrument


Structural and Thermal Model of NISP instrument

The Near IR Spectrometer Photometer (NISP) instrument's role is to measure galaxy 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 (Grating + Prism), or in "Photometry" mode using several filters. A grating wheel with 4 grisms enables spectral channels to be selected for observation in "spectrometry" mode as well as the orientation of the spectra, and a filter wheel with three filters enables the spectral channel to be selected for photometric observation. In spectrometry mode the filter wheel is in open position. In photometry mode the grism wheel is in the 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 different orientations (roll angles) to disentangle confused spectra. Each spectroscopic image is then the association of four frames collected for two spectral bands covering [0.9-2.0] µm and two spectrum orientations (0 deg or 90 deg).

 J. Zoubian
Illustration of slitless spectrometry.
The light of each galaxy on the image on the left
is spread across the focal plane of the spectrometer
(image on the right). Credit: J. Zoubian

NISP is composed of:

  • NI-SA: Structure Assembly: a structure in SiC
  • NI-OA: Optical Assembly: consists of corrective optics (CoLA) and camera optics (CaLA)


Demonstration model of the optical assembly

  • NI-FWA: Filter Wheel Assembly: carries three Filters (Germany + Spain)
  • NI-GWA: Grism Wheel Assembly: carries four grisms (France)
    Both wheels are equipped with a cryomechanism
  • NI-CU: Calibration Unit
  • NI-TC: Thermal Control, desined to control the optics at a temperature of 140K ±0.3 K throughout the mission.


Instrument cold parts


Structural and Thermal Model of the instrument in final assembly at LAM

The instrument focal plane NI-DS (NISP Detection System) is mounted on the Ni-SA structure. The focal plane is made up of a structure made of molibdenum accommodating 16 H2RG detectors (supplied by NASA and produced by Teledyne). This assembly is mounted on a SiC structure cooled to under 100 K and linked by flexible connectors to the read-out and digitizing electronics (named Side-Car). The following pictures show the thermo-structural model of NI-DS on its test bench:


Structure carrying the ASICs

Electronics: ensures the interface with the satellite, control of the instrument, acquisition and the on-board processing of images (deglitching, compression, etc.). The instrument functional diagram is illustrated in the following graphics.

The instrument functional scheme

The observation sequence of the instrument is shared between the two modes as shown in the graph below:

The observation sequence of the instrument