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James Webb Space Telescope
NIRSpec Instrument Design

NIRSpec is an all-reflective system with a total of 14 mirrors along with 7 interchangeable dispersive elements and 8 interchangeable filters. The instrument measures 190 cm across and weighs close to 200 kg.

Figure 1. Computer Aided Design layout of NIRSpec (credit: EADS Astrium)

Figure 1 shows the Computer Aided Design layout of the instrument and Figure 2 shows the schematic light path.

Figure 2. Schematic light path

Apertures/Modes

NIRSpec Apertures

NIRSpec is optimized for observations of objects that are distant, faint, compact, and numerous. NIRSpec also offers observing modes that can accommodate studies of sources that are nearby, bright, and can be extended. This is achieved by the apertures listed below (also see figure 3) and the modes described in the table.

Figure 3. NIRSpec apertures as projected onto the detector plane. The location of the fixed slits and the IFU are illustrative only.
  1. Micro-Shutter Assembly (MSA)
    • 4 separate quadrants
    • 365 (dispersion) x 171 (spatial) shutters per quadrant
    • Observer specifies which shutters to open and close
    • There are stuck-open (few) and stuck-closed (many) doors
  2. Fixed slits (FS)
    • Always open, no overlap with MSA on detectors
    • One 0.4” x 3.8” slit
    • Three 0.2” x 3.3” slits (offset along dispersion axis)
    • One 1.6” x 1.6” large aperture
  3. Integral Field Unit (IFU)
    • 3” x 3” field of view (covered when not in use)
    • 30 image slices, each 0.1” (dispersion) x 3” (spatial)

To avoid the confusion that would be caused by overlapping grating orders, the full spectral range of NIRSpec is divided into three separate wavelength intervals, each requiring a separate exposure with a different order-sorting long-pass filter.

NIRSpec Instrument Modes

Mode Target Type Wavelength Range Aperture Mask Spectral Resolution
MSA Spectroscopy rich fields or very extended objects 0.6 - 5 µm any configuration of 0.2" x 0.46" micro-shutters
R ~ 100, 1000, 2700
Fixed Slit Spectroscopy single compact object 0.6 - 5 µm 0.1" x 1.9" or 0.2" x 3.3" or 0.4" x 3.8" FS
R ~ 100, 1000, 2700
Integral-field Spectroscopy moderately extended object 0.6 - 5 µm 3.0" x 3.0" IFU
R ~ 100, 1000, 2700
Target Acquisition reference stars (n~10-20) 0.6 - 5 µm shutters open except around bright targets Undispered imaging (MIRROR)
Internal Calibrations none 0.6 - 5 µm any configuration of micro-shutters and FS/IFU undispered or
R ~ 100, 1000, 2700
 

Filters/Gratings

Filter Wheel Assembly (FWA)

The NIRSpec filter wheel contains two broadband filters, four long-pass filters, one clear aperture and an opaque position that doubles as an instrument shutter to the telescope side and a coupling mirror for the Calibration Assembly (CAA) on the NIRSpec side.

The main structural components of the NIRSpec filter wheel assembly

The eight filter wheel elements and their main applications are summarized in table 1. All filters have a clear aperture diameter of 66 mm. All the filters except F140X and F110W are made of CaF2 and are 10.15 mm thick. To facilitate cross-calibration between NIRSpec and NIRCam, the transmission curve of F110W is substantially similar to the corresponding NIRCam filter. Both F110W and F140X are made of BK7G18 and are 8.9 mm thick. In order to avoid ghost reflections, the filters are tilted by 5.3° with respect to the optical axis in the spatial direction. The OPAQUE position prevents light from the JWST telescope from reaching the MSA. It will be used for dark current measurements, and whenever NIRSpec is not being used. In addition, it has a flat mirror mounted on the back side (i.e. facing the MSA) which is aligned such that the beam from the CAA will be directed exactly as if it had originated from the telescope.

Table 1. Summary of NIRSpec Filters
Name BandpassAverage Transmission Application
F140X0.8 µm < λ < 2.0 µm >85%target acquisition (BB-B)
F110W1.0 µm < λ < 1.2 µm >90%target acquisition (BB-A)
F070LPλ > 0.7 µm >80%
MSA/IFU/SLIT
over 0.7 µm ≤ λ ≤ 1.2 µm
F100LPλ > 1.0 µm >80%
MSA/IFU/SLIT
over 1.0 µm ≤ λ ≤ 1.8 µm
F170LPλ > 1.7 µm >80%
MSA/IFU/SLIT
over 1.7 µm ≤ λ ≤ 3.0 µm
F290LPλ > 2.9 µm >80%
MSA/IFU/SLIT
over 2.9 µm ≤ λ ≤ 5.0 µm
CLEARλ > 0.6 µm >80%alignment tests at optical wavelengths also MSA/IFU/SLIT with PRISM on GWA
OPAQUEn/a n/ashutter, used for calibrations with the CAA

Grating Wheel Assembly (GWA)

The GWA contains eight optical elements: one flat mirror, one double-pass prism, and six blazed reflection gratings. The gratings and the mirror are made from gold-coated Zerodur-0, a glass ceramic with a low coefficient of thermal expansion (CTE). The prism is made from CaF2, also with a gold-coated reflective surface. The spectral resolution, central wavelengths, and main scientific applications of the optical elements in the GWA are summarized in table 2.

The main structural components of the NIRSpec grating wheel assembly
Table 2. Optical Elements of the Grating Wheel Assembly(GWA)
Name ResolutionPeak Efficiency Application
G140MR~1000 ~1.3µm
For MSA/IFU/SLIT
over 0.6 µm ≤ λ ≤ 1.8 µm
G235MR~1000 ~2.2µm
For MSA/IFU/SLIT
over 1.7 µm ≤ λ ≤ 3.0 µm
G395MR~1000 ~3.7µm
For MSA/IFU/SLIT
over 2.9 µm ≤ λ ≤ 5.0 µm
G140HR~2700 ~1.3µm
For MSA/IFU/SLIT
over 0.6 µm ≤ λ ≤ 1.8 µm
G235HR~2700 ~2.2µm
For MSA/IFU/SLIT
over 1.7 µm ≤ λ ≤ 3.0 µm
G395HR~2700 ~3.7µm
For MSA/IFU/SLIT
over 2.9 µm ≤ λ ≤ 5.0 µm
PrismR~100 >~2.5µm
For MSA/IFU/SLIT
over 0.6 µm ≤ λ ≤ 5.0 µm
Mirrorn/a n/aMirror for undispered images

The table below shows the optical elements that can be used with different filters.

CLEAR F140X F110W F070LP F100LP F170LP F290LP
Prism
G140M
G140H
G235M
G235H
G396M
G395H
mirror
 

NIRSpec Detectors

Design

  • Two HAWAII-2RG sensor chip arrays (SCAs) manufactured by Teledyne Imaging Systems (TIS).
  • The light-sensitive portions of the two SCAs are separated by a physical gap of 3.144 mm which corresponds to 17.8" on the sky. In imaging mode, the gap region will be obscured behind the gap between MSA quadrants. In dispersed mode, the detector gap will cause loss of spectral information over a range in wavelength that depends on the location of the target and the dispersive element used. The lost information can be recovered by dithering the targets.
  • Each SCA has 2048 x 2048 pixels that can be addressed individually and read out in a non-destructive way.
  • The FPA is mounted to a thermal strap that connects to a dedicated radiator. In this way, the NIRSpec SCAs can be coldloaded, and maintained at a stable operating temperature using heaters controlled by a thermal control circuit. This is to ensure thermal stability which is crucial for good performance of NIR detectors. The specified operating temperature is in the range 30 - 40 K.
Figure 1.Design principle of the NIRSpec focal plane assembly (FPA)

The relative orientation of the two SCAs with respect to the optical bench and their pixel numbering scheme is illustrated in the figure 2. SCA-2 is located closest to the optical bench. The dispersion direction runs along FPA rows (i.e. constant j coordinate), while FPA columns (constant i coordinate) follow the cross-dispersion direction.

Figure 2. Physical orientation and pixel numbering convention for the NIRSpec FPA

Detector System Performance

The characteristics of the NIRSpec SCAs are a crucial element for the performance of the whole instrument. Parameters such as dark current, read noise, and quantum efficiency are strongly tied to the overall NIRSpec sensitivity. Therefore, the requirements for these parameters as summarized in table 1 are at the limit of what is technologically feasible.

ParameterRequirement
Array size Two SCAs with 2048x2048 pixels each
Pixel size 18 µm x 18 µm
Wavelength range 0.6 µm - 5 µm
Quantum efficiency
0.6 µm - 1.0 µm
> 70%
Quantum efficiency
1.0 µm- 5.0 µm
>80%
Total noise
(incl. electronics)
< 6 e-(in MULTIACCUM 22x4)
Dark current 0.01 e-/s/pixel
Full well capacity 60,000 e-
Operational temperature range 30 - 40 k

Readout Pattern

The System for Image Digitalization, Enhancement, Control, and Retrieval (SIDECAR) ASIC is used for detector readout and control. It is a special-purpose electronic device individually matched to its corresponding FPA.

In order to increase the readout speed and maintain low power consumption, each SCA is read out via four output channels. Each channel comprises a region of 512x2048 pixels as indicated in Figure 2.

  • channel I
      reading from j = 1 to 512 and from i = 1 to 2048
  • channel II
      reading from j = 513 to 1024, and from i = 1 to 2048
  • channel III
      reading from j =1025 to 1536, and from i = 1 to 2048
  • channel IV
      reading from j = 1537 to 2048, and from i = 1 to 2048.

Pixels located in the first and last four columns and rows (i.e. i,j < 5 and i,j > 2044) are insensitive to light and can only be used as reference pixels to track the behavior of the readout electronics. Due to design constraints, channels II and III do not have reference pixels along the slow scan direction.

The Multiaccum readout scheme.

Before and after each integration, the ASIC resets each pixel in sequence. During an integration the ASIC will nondestructively sample at regular intervals the charge that accumulates in each pixel. The ability to sample pixels multiple times reduces the impact of read noise, improving signal-to-noise for faint sources. Regular sampling also facilitates cosmic ray correction and measurement of detector non-linearity.

Subarray Mode

The NIRSpec detectors have a subarray mode in which only a contiguous rectangular subset of pixels is read from each SCA. In this case, a single output channel is used. The following constraints apply to the size and location of the subarrays:

  • only one rectangular subarray is allowed per SCA.
  • subarray size must be identical for both SCAs.
  • subarray dimension must be a power of two along each dimension, e.g. 64   x 256 pixels.
  • a subarray must contain at least 1024 pixels.
  • neither dimension may be smaller than 8 pixels.

Depending on their size and location subarrays may or may not have reference pixels. Note that the subarray location in the two SCAs may be set independently.

Subarray mode is advantageous for bright sources that would cause pixel saturation within the minimum full-frame readout time. Fixed slit spectroscopy with NIRSpec will use subarray mode by default because most of the detector area will not be illuminated, and many of the high-contrast measurements require maximum dynamic range.

Several default subarrays are defined for the various fixed slits to sample those areas of the detector where the light from the slit will fall. For bright object science, several smaller subarrays will also be available to improve the dynamic range and prevent saturation. The best sizes and detector locations for these are now being determined from analysis of test data.