Imaging with the James Webb
Space Telescope
Christine Chen (STScI)
JWST Deputy Project Scientist
Based on work by many people across
the JWST project
Contributions from ESAC Workshop
Presenters…
Conference
presentations
posted on the
web at the
following url:
http://www.cos
mos.esa.int/we
b/jwst-2016esac
Wavelength Coverage
From G. Kriss
Imaging Instruments in the
JWST Focal Plane
133”
NIRISS
From G. Kriss
Imaging Fields of View Comparison
Short Wavelength Camera
32 mas pixels
65 mas pixels
110 mas pixels
Long Wavelength Camera
65 mas pixels
From G. Kriss
Outline
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Instrument Overviews
Dither Patterns
Readout Patterns
Sensitivities and Saturation Limits
Subarrays
Parallel Observations
The Near-Infrared Camera (NIRCam)
Pocket Guide
https://jwst.stsci.edu/files/live/sites/jwst/files/home/
instrumentation/nircam/technical%20documents/NIR
Cam-pocket-guide.pdf
Short Wavelength Channel:
Field of View: 2x2.2’x2.2’
Plate Scale: 32 mas pix-1
Nyquist Sampled at 2 μm
Long Wavelength Channel:
Field of View: 2x2.2’x2.2’
Plate Scale: 65 mas pix-1
Nyquist Sampled at 4 μm
The Near-Infrared Imager and Slitless
Spectrograph (NIRISS)
Field of View: 2.2’x2.2’
Plate Scale: 65 mas pix-1
Nyquist Sampled at 4 μm
Pocket Guide
https://jwst.stsci.edu/files/live/si
tes/jwst/files/home/instrumenta
tion/niriss/technical%20docume
nts/NIRISS-pocket-guide.pdf
The Mid-Infrared Instrument (MIRI)
4QPM
Coronagraphs
10.65µm
11.4µm
15.5µm
24 x 24 arcsec.
Imager
75 x 113 arcsec
Low Resolution
Spectrometer Slit
5 x 0.6 arcsec
Medium Resolution
Spectrometer
> 3.5 x 3.5 arcsec
(arcminutes on sky)
Pocket Guide
https://jwst.stsci.edu/files/live/sites/jws
t/files/home/instrumentation/miri/tech
nical%20documents/miri-pocketguide.pdf
Encyclopedia
http://ircamera.as.arizona.edu/MIRI/en
cyclopedia.htm
Lyot Coronagraph
Mask 23mm
30” x 30”
Field of View: 74”x113”
Plate Scale: 110 mas pix-1
Nyquist Sampled at 7 μm
NIRCam Primary Dithers
Purpose: Provide even spatial coverage across module and detector
gaps
Available Patterns: (1) FULL – Large fields without gaps, including
mosaics (tiled pointings of larger areas),
3:18% - 2:71% - 1:11%
NIRCam Primary Dithers
28:1%
29: 6%
30: 23%
31: 38%
32: 25%
33: 7%
34: 1%
NIRCam Primary Dithers
(2) INTRAMODULE – Objects smaller than the individual modules
(<110”),
NIRCam Primary Dithers
(3) INTRASCA – Objects smaller than the individual SCA detectors (<50” or
<100”) when optimizing for short or long wavelength observations,
respectively
Dither Size
Science Target Size
LWC
SWC
Large
<25%
<16”
<32”
Medium
<50%
<33”
<66”
Small
<75%
<50”
<100”
NIRCam: Secondary Dithers
Purpose: (1) Provide sub-pixel
sampling to improve image
reconstruction and (2) mitigate
the effect of bad pixels
Available Patterns: Even
sampling using a specified
number of secondary dithers
(Ns=1-64). If Ns<10, the all of
the offsets will fit within a 10
pixel x10 pixel box.
Instrument Requirement:
Distortion at the edge of the
field <2% that of the center
NIRISS Dithering
The NIRISS Imaging template is intended to support pure or
coordinated parallels only, since NIRCam is the better
choice for all general imaging applications (e.g. bigger field
of view, simultaneous blue/red channels, better PSF
sampling)
• Template does not support the use of subarrays
• NIRISS will necessarily follow dither pattern / mosaic
strategy of “primary” instrument
MIRI: Reuleaux Triangle
Based on IRAC Dither pattern
Designed for unresolved or barely resolved
sources
Dither Positions
Recommended Pattern Sizes
MIRI: Two- and Four-Point Dither Patterns
Optimized for sub-pixel sampling at the shortest wavelengths and dithering along the
long direction of the imager FOV
1) Small point
source pattern
optimized for
F560W and
F770W
2) Small
extended
source pattern
optimized for
F560W and
F770W
3) Subarray
(SUB64)
pattern
MIRI: Cycling Pattern
Designed to be flexible
Based on IRAC Cycling
Pattern (positions drawn
randomly from Gaussian
distribution)
Observers may choose
starting position in table (of
311 position) and the
number of positions desired.
The pattern wraps for
numbers of positions >311.
Includes 0.5 pixel
subsampling
Will provide a limited-access
“sparse cycling” option to
select table positions
Maximum Data Rate
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JWST can downlink 229 Gbits of data in a nominal 4 hour contact with the Deep Space
Network.
Current plans allow for two 4-hour contacts per day.
If the requested data rate exceeds the downlink capability then the program will be more
difficult to schedule and runs the risk of being delayed æbut for good sscientific
ö cause a locally
high data rate is possible
ç 43200
÷
æ 16 bits ö
Data Rate = ndet ectors (nxny pix) ç
֍
è pix øç
ç
è
half day ÷
t frame
÷
÷
ø
Instrument
ndetectors
nx
ny
tframe
(sec)
Data in 12
hrs (Gbits)
NIRCam
SW RAPID
8
2048
2048
10.74
2160.
NIRCam
LW RAPID
2
2048
2048
10.74
540.
NIRISS
NISRAPID
1
2048
2048
10.74
270.
MIRI
FASTMODE
1
(1032+258
reference
outputs)
1024
2.775
330.
NIRCam Exposure Specification
Nframe is the number of frames averaged in a group
Nskip is the number of frames skipped in a group
Ngroup is the number of groups in an integration
Nint is the number of integrations in an exposure
NIRCam Readout Patterns
Readout patterns consist of groups with 1, 2, 5, 10 or 20
frames
On-board electronics can average 2, 4, or 8 frames within a
group
NIRCam Readout Patterns