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

2

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°

3

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)

4

ZOOM ON THE TRACE (SIMULATIONS)

Linear scaleorders 1 and 2

Log scaleorders 1,2,3 at overlap region (>2.0 microns)

5

• 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

6

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)

7

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

8

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

9

EXPECTED SCIENCE TARGETS

Figure courtesy of George Ricker (TESS PI)

NIRISS Saturation limit

10

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

3

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

11

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

12

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

13

EXPECTED SCIENCE TARGETS

Figure courtesy of George Ricker (TESS PI)

NIRISS Saturation limit

NIR

ISS

Sat

urat

ion

Lim

it

14

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

15

INTEGRATION EFFICIENCY

eff_usingBIAS = (NG-1)/NGeff_traditional = (NG-1)/(NG+1)

16

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

17

CONTAMINATION BY FIELD STARSCase of GJ 1214b – Field Orientation.Contamination can be mitigated

10°5°0° 15° 20° 25° 30° 35°

18

2nd and 1st orders cross contamination

SPECTRAL ORDERS CROSS CONTAMINATION

19

• 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

20

• 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)

21

• 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

22

• Ground-based NL characterization during CV3.

• Will be verified during commissioning on orbit.

NON-LINEARITY CALIBRATION

23

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.

24

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

25

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

28

• 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

29

On the Web• Visit http://jwst.astro.umontreal.ca/?page_id=213• More will be posted soon. Data challenges.

SOSS Simulations

30

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.

31

• NIRISS• 100 transits!• noise floor 15 ppm

SOSS 1-D Simulation of GJ 1132b