NIRISS and Transit Spectroscopy (What you should know before observing with NIRISS) Loïc Albert for the NIRISS Team Enabling Transiting Exoplanet Science with JWST November

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 Monochromatic PSF (1300 nm) tilt = 3.5° 2

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) 3

4 ZOOM ON THE TRACE (SIMULATIONS) Linear scale orders 1 and 2 Log scale orders 1,2,3 at overlap region (>2.0 microns)

4 hr clock time, incl. 30 min setup J=8 mag target (T eff = 3200 K) ~ 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 5

SOSS SPECIFICATIONS Spectral Range micron (1st order) micron (2nd order) Resolving PowerR= (1 nm/pixel in order 1, 0.5 nm/pixel in order 2) Trace Width22 pixels Pixel Scale0.065 arcsec/pixel Full Well Depth e- Blaze Wavelength1.25 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% (T eff dependent) 6

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 7

OPERATIONS CONCEPT – DETECTOR READOUT RESET (BIAS LEVEL) READ 1READ 2 READ N... N GROUP =2 N GROUP =1 N GROUP =N MINIMUM INTEGRATION Correlated Double Sampling (CDS): Flux = READ2 – READ1 efficiency = (⅓)*tframe = 33% OR Ngroup=1: Flux = READ1 – BIAS efficiency = (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 t frame 2t frame 3t frame (N+1)t frame 8

9 EXPECTED SCIENCE TARGETS Figure courtesy of George Ricker (TESS PI) NIRISS Saturation limit

Onset of Saturation in Order 1 Onset of Saturation in Order 2 Evolving From Peaks to Valley 1.64 μm 1.88 μm 1.46 μm 1.76 μm 1.98 μm No Saturation in first order trace Some saturation in the peaks of order 1 No saturation in order 2 Onset of Saturation in Order 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 T eff = 5800 K NGROUP = e- Saturation 256x2048 Sub-Array Assumptions

SATURATION MAGNITUDE (J Band - NGROUP=1 - BRIGHT Sub-Array) μm Saturation Onset J= μm 11

SATURATION MAGNITUDE SUMMARY JNGSubCoverageWarnings > full None full Bias drift uncertainty full Sat. pix. in "horns" for λ= μm full Can recover λ<1.40 μm from order Order 1 saturated for λ<2.0 μm > (order 1)Bias drift uncertainty (order 1)Sat. pix. in "horns" for λ= μm <5.5180Less of blue etc Coverage and Risks at Various Magnitudes For other NGROUP values: Δmag = 2.5*log 10 (NGROUP) NG=2  Δmag = 0.75 fainter NG=3  Δmag = 1.20 fainter Switch between NOMINAL and BRIGHT modes: Δmag = 1.26 to FULLARRAY: Δmag = 0.75 Scaling Laws 12

13 EXPECTED SCIENCE TARGETS Figure courtesy of George Ricker (TESS PI) NIRISS Saturation limit NIRISS Saturation Limit

FAINT LIMITING MAGNITUDE 14 4 hr clock time 15 ppm noise floor Teff = 3200 At full spectral resolution J=14 J=7 J=10 J=12 Order 2

INTEGRATION EFFICIENCY 15 eff_usingBIAS = (NG-1)/NG eff_traditional = (NG-1)/(NG+1)

GJ 1214 simulation LHS 6343 simulation This roll angle makes order 0 of star A contaminate our science sub-array for GJ 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 16

CONTAMINATION BY FIELD STARS Case of GJ 1214b – Field Orientation. Contamination can be mitigated 10° 5° 0° 15° 20° 25° 30°35° 17

2 nd and 1 st orders cross contamination SPECTRAL ORDERS CROSS CONTAMINATION 18

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 19

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) 20

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 21

Ground-based NL characterization during CV3. Will be verified during commissioning on orbit. NON-LINEARITY CALIBRATION 22

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.

WHERE THE DEVIL IS... IPS (INTRA-PIXEL SENSITIVITY) 24 IPS (Intra-pixel sensitivity) Tim Hardy, HIA, Engineering Detector Sub pixel sensitivity variations at 940 nm 10 actual pixels

WHERE THE DEVIL IS... GAIN CHANGE IN VOIDS 25 Gain different in detector epoxy voids (~1% effect) SOSS subarray

Flux dependent on FPA temperature: ΔF ≈ 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 backgroundModelled 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

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 28

On the Web Visit More will be posted soon. Data challenges. SOSS Simulations 29

Summary & Perspective The SOSS mode on NIRISS was designed specifically for transit spectroscopy. It covers R~ in a single snapshot. It can target J>= stars before saturating. Data simulations are on our web site – try your favorite extraction method on it – feedbacks welcomed. Most difficult hurdle is 2 nd to 1st order trace contamination as well as potential detector-related correlated noise sources. 30

31 NIRISS 100 transits! noise floor 15 ppm SOSS 1-D Simulation of GJ 1132b