1. Imaging with the James Webb Space Telescope
Christine Chen (STScI)
JWST Deputy Project Scientist
Based on work by many people across the JWST project
Time change for next semester
First in a series of practical talks about various instrument modes
Introduce self – former miri instrument scientist

2. Contributions from ESAC Workshop Presenters…
Conference presentations posted on the web at the following url:

3. Wavelength Coverage From G. Kriss
The long and short of it---long wavelengths, use MIRI; short wavelengths, use NIRCam.
From G. Kriss

4. Imaging Instruments in the JWST Focal Plane
Approximately 9’x18’
133”
NIRISS
From G. Kriss

5. Imaging Fields of View Comparison
65 mas pixels
110 mas pixels
Short Wavelength Camera
32 mas pixels
Long Wavelength Camera
65 mas pixels
From G. Kriss

6. Outline Instrument Overviews Dither Patterns Readout Patterns
Sensitivities and Saturation Limits
Subarrays
Parallel Observations

7. The Near-Infrared Camera (NIRCam)
Pocket Guide
Short Wavelength Channel:
Field of View: 2x2.2’x2.2’
Plate Scale: 32 mas pix-1
Nyquist Sampled at 2 μm
Long Wavelength Channel:
Plate Scale: 65 mas pix-1
Nyquist Sampled at 4 μm

8. 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
Provides a powerful “parallel” capability
Matched NIRCam filters; comparable sensitivity
Increases areal coverage of the sky when paired with NIRCam
Provides near-IR imaging at lower data rate than NIRCam
Pocket Guide

9. The Mid-Infrared Instrument (MIRI)
4QPM Coronagraphs
10.65µm
11.4µm
15.5µm
24 x 24 arcsec.
Low Resolution Spectrometer Slit
5 x 0.6 arcsec
Imager
75 x 113 arcsec
Medium Resolution Spectrometer
> 3.5 x 3.5 arcsec
Lyot Coronagraph Mask 23mm
30” x 30”
(arcminutes on sky)
Field of View: 74”x113”
Plate Scale: 110 mas pix-1
Nyquist Sampled at 7 μm
Pocket Guide
Encyclopedia

10. 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%

11. NIRCam Primary Dithers
28:1%
29: 6%
30: 23%
31: 38%
32: 25%
33: 7%
34: 1%

12. NIRCam Primary Dithers
(2) INTRAMODULE – Objects smaller than the individual modules (<110”),

13. 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”
Confusing “Small” dither for large targets
The primary use for this is improving the flat field

14. 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

15. 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

16. MIRI: Reuleaux Triangle
Based on IRAC Dither pattern Designed for unresolved or barely resolved sources
Dither Positions
Recommended Pattern Sizes

17. 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

18. 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

19. Maximum Data Rate Instrument ndetectors nx ny tframe (sec)
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 but for good scientific cause a locally high data rate is possible
Instrument
ndetectors nx ny
tframe
(sec)
Data in 12 hrs (Gbits)
NIRCam SW RAPID 8
2048
10.74
2160.
NIRCam LW RAPID 2
540.
NIRISS
NISRAPID 1
270.
MIRI FASTMODE
( reference outputs)
1024
2.775
330.
Hope to report in APT how data rate in the program compares with nominal downlink rate (really on a visit basis)

20. 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

21. 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
Deep8 may have an advantage in fewer cosmic rays compared with Medium8

22. NIRCam Readout Patterns
Highlight that the number of groups for DEEP2 AND DEEP8 actually goes to n=20 even though it’s not shown on the table