51吃瓜Pro

Integral Field Spectrograph

Wavelength: 0.84-2.4 μm

Spectral resolution: R = 4,000 for all modes with broadband filters and select modes offering R = 8,000 and 10,000.

Spatial Scales: Lenslet: 0.004", 0.009".      
Slicer: 0.025", 0.05"

Field Size:
Lenslet channel
@ 4mas: 0.512" x 0.512" (full lenslet array 128x128) & 0.064" x 0.512" (masked array 16x128)
@ 9mas: 1.152" x 1.152" (full lenslet array 128x128) & 0.144" x 0.512" (masked array 16x128)
* Note: All requirements are being met over a slightly smaller number of lenslets.

Slicer channel
@ 25mas: 1.125" x 2.200" (full Slicer IFU array 88 slices each 1x45 resolution elements) & 1.125" x 1.100" (half masked IFU)
@ 50mas: 2.250" x 4.400" (full Slicer IFU array 88 slices each 1x45 resolution elements) & 2.250" x 2.200" (half masked IFU)

Filters: Broadband filters (Y, Z, J, H, K), Narrowband (5 % bandpass) filters, and specialized filters (< 1% bandpass)

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Imager

Wavelength: 0.84-2.4 μm

Spatial Scale: 0.004"

Field Size: 34" x 34"
* Note: the imager field is comprised of a 2x2 mosaic of 4kx4k H4RG-10 detectors. As such, the field is not contiguous.

Astrometry Accuracy: absolute astrometry 2 mas, differential astrometry 50 µas in a 100 s exposure with 10 µas systematics

Filters: Broadband filters (Y, Z, J, H, K), Narrowband (5 % bandpass) filters, and specialized filters (< 1% bandpass)

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Status: Entered Final Design Stage November 2017.

Interim FDRs (April to June 2021) covered most subsystems - Defined the path toward completing technical design work.

Principal Investigator


Project Scientist


Project Manager

Focal Plane Layout

IRIS will house low-order wavefront sensors (WFS) that will be used by NFIRAOS to monitor tip-tilt, astigmatism, and focus. IRIS will have three WFSs, which will sample stars as faint as J = 22 mag over a 2 arcminute field-of-view (FOV). The WFSs must be deployable over the entire FOV to maximize sky coverage, and allow for optimum AO correction (the best correction is obtained when the WFSs are deployed symmetrically about the science target). The figure below shows the geometric configuration of the three tip-tilt guide arms and the locations of the IRIS imager and spectrograph.

Integral Field Spectrograph

IRIS’s Integral Field Spectrograph provides users with two means of spatially sampling the field. The Lenslet channel of the spectrograph provides the finer sampling scales (4 & 9 mas) and probes the diffraction limit of the telescope. The Lenslet channel features an oversized array and meets full requirements over a sampling size of 100x100. The array can also be masked to reduce the field-of-view while enabling a mix of higher spectral resolutions and bandpasses. The coarser sampling scales (25 & 50 mas) are provided by the Slicer channel. The slicer channel is built around an image slicer that chops the field into 88 separate samples. Both channels share a common set of gratings, camera and detector.

Shown in its test cryostat Caltech has designed the mechanism for the IFS slicer periscope mirror. The mechanism was moved 45,000 times to demonstrate it meets its lifetime requirements while preserving its repeatability and accuracy. The periscope mirror assembly is the largest of four analogous mechanisms found in the IFS. Successful testing completed November 2019.

The IRIS final design approach involves a standard mechanical design iteration where the preliminary designs are expanded based upon evolving definition of the performance budgets. The designs are then analyzed for thermal and structural considerations via extensive finite element analysis. Mechanisms, bonding and alignment of optical components were then prototyped to confirm real performance and verify the analysis. If required minor design changes are then incorporated and the final detailing of the assemblies is completed. (top) Detailed CAD models for the IRIS Slicer channel TMA collimator highlighting the optics, mounting features, bench plate and thermal strapping. (bottom left) Detailed CAD model of the Slicer channel deployable periscope mirror. (Bottom right) CAD model zoom on the adjustment features for one of the Slicer channel optical mounts.

The Lenslet mask slide serves to mask the IFS Lenslet array and features settings enabling exposures of the full array, a field reduced array and calibration settings. UCLA has designed and is prototyping the Lenslet mask to complete ensure actual performance matches analysis results. The mask derives some heritage from a similar Lenslet mask used with Keck’s OSIRIS instrument.

UCLA is providing the grating turret for the IFS. The turret houses up to 14 gratings and will enable spectral resolving powers in the 4,000 – 10,000 range for both channels of the IFS. To ensure the demanding stability and repeatability requirements are met UCLA has undertaken extensive prototyping of the assembly. (right image) The turret structure will house the 14 gratings and provides 360° rotation to grating selection. (left image) Each grating is mounted on its own housing. Upon selection the individual gratings will be pushed forward into an observing configuration.

Imager

The near-infared (0.84 - 2.4 μm) imager will have a spatial sampling of 0.004" per pixel with a total field of view of 34" x 34". The optical design has been optimized to achieve the lowest wavefront error (~40 nm) in order to sample the high spatial resolutions achieved from the adaptive optics system, NFIRAOS. The leading science cases for the imager require a high level of astrometric precision (e.g., Galactic Center, star forming regions, and the Solar Systems). The imager is expected to achieve an astrometric accuracy of 2 mas for absolute astrometry and 50 μas for differential astrometry in a 100 s exposure, with a systematic floor of 10 µas.

Assembly and alignment of IRIS’s science channel opto-mechanical assemblies occurs at room temperature; however, the operating temperature is several hundred Kelvin lower. This will cause a variable contraction for the different materials and could introduce stress deformation into the optical surfaces. This deformation would manifest as an additional error term and degrade IRIS’s imaging quality. NAOJ has completed testing to measure the thermal deformation effects using sample coated substrates. The effects were measured by cooling a multilayered, coated substrate and measuring the surface figure errors. Shown in the left images the mirror sizes are 160x160mm. The prototyping efforts enable NAOJ to compare their previous analysis against the actual observed results and adjust their final design accordingly.

The first step in retiring risk was for NAOJ to verify their analysis and confirm the extent of deformation imparted to the optic from the bond pad and adhesive. The effects of optic deformation were first gauged by cooling a smaller sample optic and bond pad and measuring the surface figure. Shown below the optic sizes were 25mm in diameter. During this exploration a number of bond pad materials were explored. As shown in the right image the testing also included extensive destructive testing to also validate the strength of our bonded optics to ensure the IRIS will survive both its shipping and seismic requirements.

The imager cold-stop final mechanical design and detailing is complete (upper image). The cold-stop needs to track the formed pupil with a high degree of accuracy to ensure suppression of the background. Similar to all of IRIS’s cryo-mechanism to minimize instrument downtime the cold-stop also has demanding lifetime requirements. The mechanism was driven full rotation 12,000 times and then assessed for wear. The results of the prototyping were then incorporated into the final detailing of the design. The lower two images illustrate some of the components manufactured for the prototyping effort including the flywheel and spring supported worm gear. The lower right image shows the isolated worm gear coated in dicronite.

One of the challenges of generating an instrument with high angular resolutions (0.004") is compensating for the dispersion that occurs within Earth's atmosphere. In order to counter this effect IRIS will house an atmospheric dispersion corrector (ADC) in front of IRIS science's dewar. It will use real-time knowledge of atmospheric conditions (temperature, pressure, humidity) and optical elements to correct for dispersion over varying observing elevations.

Science Cryostat

The IRIS science channel instruments, the imager and IFS, are housed in a cryogenic vacuum container – the IRIS Science Cryostat (Caltech). After the air is evacuated the science cryostat is cooled to the boiling point of liquid nitrogen (~ -200°C at the altitude of the summit). The science cryostat is reconfigurable so that its upper and lower halves can be separated such that both can be evacuated and cooled separately. This functionality enables separate integration and testing of both the imager and IFS.

Caltech is currently testing a half-scale prototype of the science cryostat cold plate. The cold plate provides the reservoir for the liquid supplied by the Observatory and provides the primary thermal interface to ensure heat is removed from the system. The prototyping activities have been undertaken to validate earlier thermal models showing the heat flow and stability of the instrument.