Wide-Field Slitless Spectr scopy
with NIRISS
Van Dixon
STScI
TIPS/JIM
2012 September 20
NIRISS at a Glance
Parameter
Value
Array 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
TIPS/JIM
2012 September 20
Dual-Wheel Configuration
GR150R
TIPS/JIM
2012 September 20
GR150C
Dual-Wheel Configuration
GR150R
GR150C
TIPS/JIM
2012 September 20
GR150C
GR150R
Blaze Function for GR150 Grisms
4. Blaze funct ion of t he GR150 grism measured on a flight -like grism. T he flight prisms are expect ed t o ha
performance.
TIPS/JIM
2012 September 20
First-Order Sensitivity: Emission Lines
Minimum line flux to reach S/N = 10 in 10 ks
TIPS/JIM
2012 September 20
First-Order Sensitivity: Emission Lines
z=9
z = 17
Redshifted Lyman α
TIPS/JIM
2012 September 20
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).
TIPS/JIM
2012 September 20
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
TIPS/JIM
2012 September 20
Location of Each Order (F150W)
2040 pixels
Direct Image
-1
-601 -512
0
-227
+1
0 53 148
1.03 1.3 1.7 (μm)
TIPS/JIM
2012 September 20
+2
336
+3
520
619
893
Model Object Table
#
#
#
#
#
#
#
#
#
#
1
2
3
4
5
6
7
1 NUMBER
2 X_IMAGE
3 Y_IMAGE
4 A_IMAGE
5 B_IMAGE
6 THETA_IMAGE
7 MAG_F1537
8 SPECTEMP
9 Z
10 MODIMAGE
900.00 1500.00
900.00 1250.00
900.00 1000.00
900.00 750.00
900.00 700.00
900.00 650.00
900.00 600.00
x
y
TIPS/JIM
2012 September 20
Object number
X coordinate (pixels)
Y coordinate (pixels)
Length of semi-major axis (pixels)
Length of semi-minor axis (pixels)
Angle of major axis
Magnitude at 1537 nm (WFC3 F160W)
Key to spectral template
Redshift
Key to model image
0.6 0.6
0.0 22.5 0 0.0
0
30.0 10.0 45.0 18.0 0 0.0
0
18.0 10.0 -30.0 18.0 1 0.0
1
5.0 2.0 60.0 18.0 2 0.0
0
5.0 2.0 60.0 18.0 2 0.075 0
5.0 2.0 60.0 18.0 2 0.15 0
5.0 2.0 60.0 18.0 2 0.22 0
a
b
θ
mag sp
z
image
Dispersed Image (log stretch)
-1
0
+1
2040 pixels
TIPS/JIM
2012 September 20
+2
+3
Millennium Run Observatory
Semi-analytic galaxy-formation models based on the
Millennium Run dark-matter simulations
H-band image using the WFC3/F160W bandpass
Overzier et al. 2012 (MNRAS, in press)
TIPS/JIM
2012 September 20
Direct Image w/ NIRISS PSF
TIPS/JIM
2012 September 20
Direct Image w/ Background (100 s)
TIPS/JIM
2012 September 20
Source Extractor Apertures
TIPS/JIM
2012 September 20
Dispersed Image: Horizontal
TIPS/JIM
2012 September 20
Dispersed Image: Vertical
TIPS/JIM
2012 September 20
aXe2web
TIPS/JIM
2012 September 20