Wide-Field Slitless Spectr scopy with NIRISS

Wide-Field Slitless Spectr scopy with NIRISS

Van DixonSTScI

TIPS/JIM2012 September 20


NIRISS at a Glance

Parameter ValueArray Size 2048 x 2048 (2040 x 2040 active)

Pixel size 18 μm x 18 μm

Dark rate < 0.02 e-/s

Noise 23 e- (correlated double sample)

Gain 1.5 e-/ADU

Field of View 2.2’ x 2.2’

Plate scale in x 0.0654 arcsec/pixel

Plate scale in y 0.0658 arcsec/pixel


Dual-Wheel Configuration

GR150R GR150C


Dual-Wheel Configuration

GR150R GR150C

GR150C GR150R


Blaze Function for GR150 Grisms


First-Order Sensitivity: Emission LinesMinimum line flux to reach S/N = 10 in 10 ks


First-Order Sensitivity: Emission Lines

z = 9 z = 17

Redshifted Lyman α


The Software

The aXe software package was developed to support slitless spectroscopy with ACS and WFC3 (Kümmel et al. 2009).

aXeSIM generates synthetic direct and dispersed images of the sky.aXe extracts and calibrates spectra.

aXedrizzle allows for the combination of dithered images.aXe2web displays the extracted spectra in a convenient HTML format.

aXe was originally developed by the Space Telescope - European Coordinating Facility (ST-ECF). The code is now supported by STScI.

Source Extractor builds a catalog of objects from an astronomical image (Bertin & Arnouts 1996).


Input for aXeSIM Configuration file

• Location, trace, and dispersion of each spectral order Sensitivity (including grating)

• e- per erg/cm2 Throughput (w/out grating; used to make direct image) Flat Field (may be wavelength-dependent) Master Sky Background Image Input Catalog (output from Source Extractor)

• Specify image size and shape or point to image file Postage-stamp images Spectra


Location of Each Order (F150W)

-1 0 +1 +2 +3

-601 -512 -227 0 53 148 336 520 619 893

Direct Image

2040 pixels

1.03 1.3 1.7 (μm)


Model Object Table

# 1 NUMBER Object number# 2 X_IMAGE X coordinate (pixels)# 3 Y_IMAGE Y coordinate (pixels)# 4 A_IMAGE Length of semi-major axis (pixels)# 5 B_IMAGE Length of semi-minor axis (pixels)# 6 THETA_IMAGE Angle of major axis# 7 MAG_F1537 Magnitude at 1537 nm (WFC3 F160W) # 8 SPECTEMP Key to spectral template# 9 Z Redshift# 10 MODIMAGE Key to model image 1 900.00 1500.00 0.6 0.6 0.0 22.5 0 0.0 0 2 900.00 1250.00 30.0 10.0 45.0 18.0 0 0.0 0 3 900.00 1000.00 18.0 10.0 -30.0 18.0 1 0.0 1 4 900.00 750.00 5.0 2.0 60.0 18.0 2 0.0 0 5 900.00 700.00 5.0 2.0 60.0 18.0 2 0.075 0 6 900.00 650.00 5.0 2.0 60.0 18.0 2 0.15 0 7 900.00 600.00 5.0 2.0 60.0 18.0 2 0.22 0

x y a b θ mag sp z image


Dispersed Image (log stretch)

-1 0 +1 +2 +3

2040 pixels


Millennium Run Observatory

H-band image using the WFC3/F160W bandpassOverzier et al. 2012 (MNRAS, in press)

Semi-analytic galaxy-formation models based on the Millennium Run dark-matter simulations


Direct Image w/ NIRISS PSF


Direct Image w/ Background (100 s)


Source Extractor Apertures


Dispersed Image: Horizontal


Dispersed Image: Vertical


aXe2web

Wide-Field Slitless Spectr scopy with NIRISS

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