Post on 08-Jan-2018
description
NIRISS and Transit Spectroscopy(What you should know before observing with
NIRISS)
Loïc Albert for the NIRISS TeamEnabling Transiting Exoplanet Science with JWST
November 16-18 2015
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λ
SINGLE OBJECT SLITLESS SPECTROSCOPY WITH NIRISS"The SOSS has a slitless cross-dispersed grism used simultaneously in orders 1 and 2 with a weak cylindrical lens producing traces defocussed along the spatial direction."
22 pixels
Simulated MonochromaticPSF (1300 nm)
tilt = 3.5°
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Superimposed, position of the source without grism (direct image)
Order 0
Order 1
Order 2
Actual NIRISS Field of View
Mosaic of CV1RR images exploring the focal plane with and without the GR700XD grism.
ACTUAL SOSS TRACES AND ACQUISITION SPOT (rotated 90° CCW)
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ZOOM ON THE TRACE (SIMULATIONS)
Linear scaleorders 1 and 2
Log scaleorders 1,2,3 at overlap region (>2.0 microns)
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• 4 hr clock time, incl. 30 min setup• J=8 mag target (Teff = 3200 K)• ~80-150 ppm per 2-pixel resol.
element• Assuming Poisson noise limit• R ~1000 resolution (see below)• Standard mode, 0.6 to 2.8 um in
one shot
Sensitivity at Native Resolution
Resolving Power (2 pixels)
SENSITIVITY AND RESOLVING POWER
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SOSS SPECIFICATIONSSpectral Range 0.6-2.8 micron (1st order) + 0.6-1.4 micron (2nd order)
Resolving Power R=700-1000 (1 nm/pixel in order 1, 0.5 nm/pixel in order 2)
Trace Width 22 pixels
Pixel Scale 0.065 arcsec/pixel
Full Well Depth 75 000 e-
Blaze Wavelength 1.25 microns (m=1) and 0.68 microns (m=2)
Throughput (BOI) ~20% (OTE+NIRISS+Detector)
Trace Rotation Repeatability Between Sequences
~0.15 degrees
Second Order Contamination On First Order Red End
<1% (Teff dependent)
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① Acquisition through the NRM (T=15%) and F480M filter using a 64x64 sub-array (saturating for J<~5).
② Single pass acquisition with precision of ~1/10 pixel.
③ Grism in. Repeatability of wheel is 0.15 degrees. Baseline is no fine tuning of the trace position.
④ Observing Sequence: A single exposure (FITS file) containing large nbr of integrations to maximize efficiency. No loss between integrations beside array reset.
OPERATIONS CONCEPT – TARGET ACQUISITION
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OPERATIONS CONCEPT – DETECTOR READOUT R
ES
ET
(BIA
S L
EV
EL)
RE
AD
1
RE
AD
2
RE
AD
N
...
NGROUP=2
NGROUP=1
NGROUP=N
MINIMUM INTEGRATION
Correlated Double Sampling (CDS):Flux = READ2 – READ1efficiency = (⅓)*tframe = 33%
OR
Ngroup=1:
Flux = READ1 – BIASefficiency = (½)*tframe = 50%
(where BIAS is obtained from the first READ in a DARK sequence)
Pro: Brighter saturation limit.Con: BIAS level constant?
TIME
0 tframe 2tframe 3tframe (N+1)tframe
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EXPECTED SCIENCE TARGETS
Figure courtesy of George Ricker (TESS PI)
NIRISS Saturation limit
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7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0
Ons
et o
f Sat
urat
ion
in O
rder
1
Ons
et o
f Sat
urat
ion
in O
rder
2
Evo
lvin
g Fr
om P
eaks
to V
alle
y
1.64
μm 1.
88 μ
m
1.46
μm 1.
76 μ
m 1.98
μm
No Saturation in first order tra
ce
Some saturation in the peaks of order 1
No saturation in order 2
Ons
et o
f Sat
urat
ion
in O
rder
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SATURATION MAGNITUDE (J Band - NGROUP=1 - NOMINAL Sub-Array)
0.6 μm
1.4 μm
0.85 μm
2.8 μm
• J-Band Vega• Teff = 5800 K• NGROUP = 1• 75000 e- Saturation• 256x2048 Sub-Array
Assumptions
6.75 6.507.0 5.50
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SATURATION MAGNITUDE (J Band - NGROUP=1 - BRIGHT Sub-Array)7.0 6.0 5.0 4.0 3.0 2.0
0.85 μm
Saturation OnsetJ=5.75
2.85 μm
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SATURATION MAGNITUDE SUMMARY
J NG Sub Coverage Warnings
>8.0 2+ 256 full 0.6-2.8 None
7.25-8.0 1 256 full 0.6-2.8 Bias drift uncertainty
6.75-7.25 1 256 full 0.6-2.8 Sat. pix. in "horns" for λ=0.96-1.46 μm
6.25-6.75 1 256 full 0.6-2.8 Can recover λ<1.40 μm from order 2
5.75-6.25 1 256 0.6-1.4+2.0-2.8 Order 1 saturated for λ<2.0 μm
>6.0 1 80 1.0-2.8 (order 1) Bias drift uncertainty
5.5-6.0 1 80 1.0-2.8 (order 1) Sat. pix. in "horns" for λ=0.96-1.46 μm
<5.5 1 80 Less of blue end λ=1.2μm@5.25, 1.8@4.5, etc
Coverage and Risks at Various Magnitudes• For other NGROUP values: Δmag = 2.5*log10(NGROUP)NG=2 Δmag = 0.75 fainterNG=3 Δmag = 1.20 fainter• Switch between NOMINAL and
BRIGHT modes: Δmag = 1.26 to FULLARRAY: Δmag = 0.75
Scaling Laws
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EXPECTED SCIENCE TARGETS
Figure courtesy of George Ricker (TESS PI)
NIRISS Saturation limit
NIR
ISS
Sat
urat
ion
Lim
it
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FAINT LIMITING MAGNITUDE
• 4 hr clock time• 15 ppm noise floor• Teff = 3200• At full spectral resolution
J=14
J=7
J=10
J=12
Order 2
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INTEGRATION EFFICIENCY
eff_usingBIAS = (NG-1)/NGeff_traditional = (NG-1)/(NG+1)
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GJ 1214 simulation
LHS 6343 simulation
This roll angle makes order 0 of star A contaminate our science sub-array for GJ 1214.
star A
GJ 1214
LHS 6343 is a binary star with separation of 0.6". Creates a double trace.
Slitless spectroscopy=
field star contamination
CONTAMINATION BY FIELD STARS
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CONTAMINATION BY FIELD STARSCase of GJ 1214b – Field Orientation.Contamination can be mitigated
10°5°0° 15° 20° 25° 30° 35°
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2nd and 1st orders cross contamination
SPECTRAL ORDERS CROSS CONTAMINATION
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• The SOSS mode requires one Pupil Wheel (PW) movement (F480M to GR700XD) between Target Acquisition and Science Observation.
• Repeatability of PW is 1 resolver step ~0.15° which introduces same rotation on spectral traces ~5 pixels between blue and red ends of order 1 trace. Applies only for multi-visits targets.
• Fine Guidance Sensor (FGS) guides on a single star with rms = 6 mas (~1/10th NIRISS pixel).
• Star Tracker responsible of the spacecraft field angle stability (accuracy TBD) within a visit. Would introduce an x-y offset to our traces.
MULTI-VISIT REPEATABILITY
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• NIRISS has no internal lamp for flat field calibration.
• Ground-based pixel flats through imaging filters (F090, F140, F150, F158, F200, F277 and redder). Can interpolate if λ dependency is small.
• On orbit, use A0 calibrators. Dither the calibrator by a few pixels to cover a wider pixel area.
FLUX CALIBRATION (PIXEL TO PIXEL FLAT)
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• Calibration Targets– M stars and compact SMC Planetary Nebula
• Self Calibration– Extract the spectrum based first on rough λ-
solution then bootstrap using an atmosphere model spectrum.
WAVELENGTH CALIBRATION
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• Ground-based NL characterization during CV3.
• Will be verified during commissioning on orbit.
NON-LINEARITY CALIBRATION
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Detector-related noise– Intra-pixel sensitivity– Non-linearity coefficients
uncertainty– Gain varying in detector
epoxy voids– kTC noise for NGROUP=1– Temperature-induced
instability– 1/f noise– Cross-talk and PSF
smearing
WHERE THE DEVIL IS...
What noise floor will the SOSS achieve? WFC3 is ~20-30 ppm.
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WHERE THE DEVIL IS... IPS (INTRA-PIXEL SENSITIVITY)
IPS (Intra-pixel sensitivity)
Tim Hardy, HIA, Engineering Detector
Sub pixel sensitivity variations at 940 nm
10 actual pixels
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WHERE THE DEVIL IS... GAIN CHANGE IN VOIDS
Gain different in detector epoxy voids(~1% effect)
SOSS subarray
Flux dependent on FPA temperature:ΔF ≈ -10-3 ΔT
Ramping occurs at the start of every exposure (after idling) and lasts for <5 minutes
Flux not dependent on ASIC temperature
Flux stability at a premium. To achieve 1 ppm flux stability requires FPA control to 1 mK.
WHERE THE DEVIL IS... DETECTOR TEMPERATURE
Modelled zodiacal light background Modelled 1/f noise
Background components that need to be subtracted
• Scattering on optics (10-3 and uniform, ref. Rohrbach simulations)
• OTE thermal emission?
WHERE THE DEVIL IS... 1/F NOISE
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• Develop optimal trace extraction algorithms for SOSS (deal with trace contamination, trace rotation, etc).
• Inject correlated noise or systematics to test analysis algorithms.
• Be prepared for First Light.• Help the community be prepared to use
NIRISS.
SOSS Simulations
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On the Web• Visit http://jwst.astro.umontreal.ca/?page_id=213• More will be posted soon. Data challenges.
SOSS Simulations
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Summary & Perspective• The SOSS mode on NIRISS was designed
specifically for transit spectroscopy.• It covers 0.6-2.8 microns @ R~500-1000 in a single
snapshot.• It can target J>=6.5-7.0 stars before saturating.• Data simulations are on our web site – try your favorite
extraction method on it – feedbacks welcomed.• Most difficult hurdle is 2nd to 1st order trace
contamination as well as potential detector-related correlated noise sources.
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• NIRISS• 100 transits!• noise floor 15 ppm
SOSS 1-D Simulation of GJ 1132b