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Science ready Capabilities (Can be used from S14B onwards):

The SCExAO instrument is continuously growing and is therefore being commissioned in phases. As of January 2014 only phase I engineering has been completed and therefore only the modes tested within that phase can be used for science. These include the following:

  • Coronagraphs: The SCExAO instrument is equipped with several coronagraphs, which operate across the near-IR (y-K bands). Those that can be used include the Phase induced amplitude apodisation (PIAA), Vector Vortex, Four-Quadrant Phase Mask and Shaped pupil versions.

  • Tip/tilt (low order) control: The low order wavefront sensor (LOWFS) uses the rejected light from the coronagraphs in the near-IR to fine-tune the tip/tilt for optimum starlight rejection.

  • Coherent differential imaging: The speckle nulling routine allows for quasi-static speckles due to aberrations in the optics, or diffraction from the spiders and obstructions to be systematically beaten down. The performance of this mode is limited by the current generation of cameras (See below).

  • High frame rate imaging: The internal near-IR camera in SCExAO can run at up to 346 Hz offering the possibility to collect fast frames for lucky imaging or advanced PSF calibration of long exposures taken with HiCIAO.

  • Deep exposure imaging: HiCIAO is used to take longer exposures of a given target once it has passed through SCExAO. It can be used in conjunction with all the other modes described above or separate to them.

Please note SCExAO is a booster stage for the initial AO correction provided by AO188. In good seeing AO188 offers 20-40% Strehl ratios in the H-band. Please use this value when preparing proposals.

SCExAO Phase I capabilities and performance summary table

Target acquisition and Image quality
Strehl Ratio, FWHM 30-40% in H-band (median seeing), See AO188 performance SCExAO phase I provides no significant
improvement in image quality over AO188
Target Acquisition and
setup overhead
AO188 target acquisition time + 10 mn Does not include fine tuning of speckle control loop
Seeing limit ~1.2" See AO188 performance
Faint limit R=8, H=5 Performance improves steeply with brighter targets
Bright limit R=-1 Neutral Density filters can be placed in front of wavefront sensor and HiCIAO when observing bright targets but not brighter than R=-1
Exclusion angle around the moon 15 degrees
Cameras: HiCIAO
Detector Hawaii 2RG See HiCIAO instrument description
Wavelength coverage See HiCIAO instrument description SCExAO feeds HiCIAO with wavelengths longer than 950nm
Field-of-view, plate scale ~5", 8.3 mas/pix; SCExAO preserves HiCIAO's plate scale and FOV
SCExAO throughput to HiCIAO Up to 60% (y-H band) Assumes 50%/50% beam splitter between HiCIAO and
SCExAO science camera, does not include PIAA lenses
Coronagraph modes PIAA, Vortex, 4QPM,
Shaped Pupil
Angular Differential Imaging YES SCExAO operates in fixed pupil mode
Active Speckle control YES Speckle control performed using SCExAO
internal science camera
Tip-Tilt control YES Tip-Tilt control performed using SCExAO
Low Order Wavefront Sensor
Spectral Differential Imaging NO Will be offered at a later time
Polarimetric imaging NO Will be offered at a later time
Cameras: Internal SCExAO science camera
Detector InGaAs, 320x256 Axiom Optics OWL SW1.7HS
Wavelength coverage J, H band 0.9-1.7 μm
Field-of-View 3.8"x 3.0" 11.94 mas/pixel
Frame rate 176 Hz max adjustable
Readout Noise 115 e-
Phase I High Contrast Imaging Performance
Inner Working Angle 1 to 3 l/D Function of coronagraph configuration
PIAA Coronagraph optics throughput 52%
Detection contrast threshold:
1-4 λ/D (40-160 mas)
1e-4 H band, Approximate value, assumes bright star
and PSF post-calibration
Detection contrast threshold:
4-20 λ/D (160-800 mas)
1e-5 H band, Approximate value, assumes bright star
and PSF post-calibration
Phase II High Order Wavefront Correction
Extreme AO HOWFS development is progressing well and we should be able to offer additional wavefront correction on top of AO188 in S14B. However, at this point we can not commit to a Strehl ratio.


Coronagraphs

Coronagraph type PIAA Vortex 4QPM Shaped pupil
Inner working angle (λ/D) 1.5 1 1 3
Waveband(s) y-K H H y-K

Cameras

Speckle nulling is performed with the camera used for high frame rate imaging. This camera is the same as the one used for the LOWFS. They are both Axiom Optics OWL SW1.7HS units, which have an InGaAs CMOS array with 320x256 pixels that are 30x30 μm in size. These cameras can run full frame at 176 Hz and have a read noise of 115 and 140 e- respectively. The dark current is such that the maximum exposure is ~1 s. Please note that the speckle nulling routine operates on the speckles which are ~1000 x fainter than the central PSF and hence this must be taken into consideration when selecting your target, in light of the noise characteristics. A speckle nulling map can be pre-carved off sky and then applied on-sky in the case targets are too faint to operate speckle nulling on. However, this is not as accurate as performing the nulling process on the star of interest. The high frame rate/speckle nulling camera can be used with a series of bandpass filters which include: y-band, J-band, H-band and 50 nm bandwidth centered at 1650 nm.

Pulpit rock Pulpit rock Pulpit rock

(Left) High frame rate science camera/speckle nulling camera. Mounted on a long translation stage to move between
the focal and pupil planes. (Middle) LOWFS camera. (Right) HiCIAO camera.

Calibration source

The internal calibration source consists of a Fianium super continuum source and 2 laser diodes centered at 650 and 1550 nm. The light from the source is delivered via an endlessely single-mode fiber to the bench ensuring a diffraction-limited calibration PSF from the visible to K-band. The brightness and spectral bands of the injected light can be easily controlled. In addition there is a turbulence simulator that can be used for realistic simulations of on-sky performance in the laboratory.

Pulpit rock Pulpit rock Pulpit rock

(Left) Internal calibration source unit (a.k.a rainbow maker). (Middle) Super continuum laser source attached to source box. (Right) Internals of
calibration source box. There is a shutter to turn the light on/off, and three wheels to control the ND in the vis/IR and spectral content.

For more technical details please refer to the publications page.

2014