CHARIS
The Coronagraphic High Angular Resolution Imaging Spectrograph (CHARIS) is an IFS for the Subaru telescope. It sits behind the SCExAO coronagraphic and extreme AO system and AO188 adaptive optics systems at the Subaru telescope.
HD1160 data cube. The central star is blocked by the coronagraph. The four bright speckles are the astrogrid used for photometric and astrometric calibration. A faint companion is visible on the left part of the image (8-oclock)
Capabilities - Overview
Targets with an H-band magnitude brighter than 1 can only be observed with the internal CHARIS ND3 filter in-beam, which restricts the user to broadband mode.
The current instrument capabilties are below:
| Basic Parameters | ||
|---|---|---|
| Field of view | 2.07''x2.07'' | |
| 2.07''x1.0'' | Spectro-polarimetric mode (Wollaston prism in) | |
| Plate Scale | 16.2 mas/lenslet | |
| Recommended Satellite Spot Contrast | 1x10-1 - 1x10-3 | Tunable |
| Satellite Spot Location | 15.9 lambda/D (~0.63'' at 1550 nm) | Scales linearly with lambda |
| Detector | Hawaii 2RG | See CHARIS instrument description |
| Wavelength coverage | 1154 nm to 2387 nmm | SCExAO feeds CHARIS with wavelengths longer than 950nm |
| CHARIS throughput | Low resolution mode: 65-70% | ~15% from atmosphere to detector |
| High resolution mode: 55-60% | ||
| Spectral resolution | Low resolution mode: R~19 | J, H and K bands combined |
| High resolution mode: R~70-90 | J, H or K band | |
| Coronagraph modes | Lyot, vAPP, PIAACMC, Vortex | See SCExAO coronagraph section |
| Angular Differential Imaging | YES | SCExAO operates in fixed pupil mode |
| Speckle control | YES (static WF map) | Speckle control performed using SCExAO internal science camera |
| Tip-Tilt control | YES | Tip-Tilt control performed using SCExAO Low Order Wavefront Sensor |
| Spectro-polarimetric imaging | YES | A Wollaston prism, combined with a field stop. See Spectro-Polarimetric Imaging section |
| Dithering | ±2'' | It is now possible to offset the field-of-view with respect to the guide star |
Spectro-Polarimetric Imaging
Spectro-Polarimetric Imaging is enabled by a Wollaston prism, field stop, and half-waveplate (HWP) rotation for slow modulation. This mode is refered to as CHARIS-PDI (Polarization Differential Imaging).See the CHARIS-PDI webpage for details and data calibration.
Observers using PDI should account for the following factors when planning observations:- Field of View is reduced to 1"x2" by the field stop.
- Image splitting by the Wollaston prism produces two images in orthogonal linear polarizations. This usually calls for exposure times to be doubled to avoid incurring additional readout noise.
- Throughput is reduced by the HWP and Wollaston prism. See table below.
| Spectro-Polarimetric Imaging Throughput (relative to non-PDI observations) | |||
|---|---|---|---|
| Band | HWP | Wollaston | total |
| J (1.24um) | 96.1% | 93.2% | 89.6% |
| H (1.63um) | 94.3% | 95.7% | 90.2% |
| K (2.14um) | 92.2% | 92.2% | 85.0% |
Astrometric and photometric calibration
Calibration speckles ("astrogrid") can be added in the image by way of fast DM modulation. The spot brightess and position are accurately referenced to the central PSF core, providing astrometric and photometric calibration in coronagraphic images where the central star is blocked.Observers can choose the astrogrid contrast, angular separation and number of spots. Within a CHARIS exposure, the astrogrid phase is rapidly modulated between 0 and PI to ensure the reference speckles are incoherent with the existing PSF halo. Additionally, the spot pattern can be switched between CHARIS exposured for subtraction of the incoherent speckle halo. See table below for parameters.
| Astrogrid settings | |
|---|---|
| Parameter | Value(s) |
| Angular separation from PSF core | 5.3 λ/D, 7.5 λ/D, 11.2 λ/D, 15.9 λ/D or 22.5 λ/D |
| Brightness relative to central PSF | 1e-4 to 1e-2 |
| Number of spots | 2 or 4 |
| Phase modulation | 2 kHz |
| Spatial modulation (optional) | Synchronized with CHARIS exposures |
| Coronagraph | Lyot, Vortex |
On-sky calibration photometric and astrometric precision has been estimated by measuring the flux ratio and relative position between calibration spots. Numbers are shown in the table below, and indicate the astrometric and photometric calibration precision for bright targets. Photon noise and readout noise may dominate on faint targets. Precision scales as the inverse square as exposure time.
| Astrogrid calibration precision (measured on star Beta Leo with 8.8nm DM modulation, 30mn observation) | ||
|---|---|---|
| Wavelength band | Photometry | Astrometry |
| J | 0.26 % | 1.7 mas |
| H | 0.33 % | 1.67 mas |
Wavelength modes
| Filters | Dispersion Modes | |||||
|---|---|---|---|---|---|---|
| Band | 50% Transmission Range (nm) | In-Band Filter Transmission | Band | Avg. Spectral Resolution (2x2 pixels) | # Output Channels | CHARIS Internal Throughput |
| J | 1176-1328 | >80% | J | R=75.2 | 15 | >40% |
| H | 1490-1783 | >90% | H | R=65.2 | 20 | >40% |
| K | 2019-2375 | >90% | K | R=77.1 | 17 | >40% |
| Broadband | 1154-2387 | >93% | Broadband | R=18.4 | 22 | >53% |
| Cold ND3 | 1165-2365 | >OD 3.258 | Broadband | R=18.4 | 22 | - |
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