The next generation exposure time
calculator for JWST
Recommendations and science concept
The MESA ETC WG:
Elizabeth Barker (coordinator), Klaus Pontoppidan, Mike Regan, Diane
Karakla, Christine Chen, Jen Lotz, Massimo Robberto, Andre Martel, Vicki
Laidler, Chris Hanley
We are required to support all JWST modes.
(S&OC level 3)
NIRCAM
•
•
•
Imaging
Coronagraphy
Target Acq
NIRSPEC
•
•
•
•
•
•
•
•
MIRI
•
•
MOS PRISM
MOS R=1000
MOS R=2700
IFU PRISM
IFU R=1000
IFU R=2700
Fixed slit
Target Acq
•
•
•
•
Imaging
Lyot
coronagraphy
4QPM
coronagraphy
Medium
resolution IFU
Lowresolution
single object
Target Acq
NIRISS
•
•
•
•
•
Imaging
Wide-field
slitless
spectroscopy
Single-object
slitless
spectroscopy
Aperture
masking
Target Acq
FGS
•
•
Guiding
Calibration
TBD
ETC development is a close
collaboration between the
ETCWG and the SSB.
Detailed test
implementation
Productio
n ETC
“Scamp”
WIT
ETCWG
SSB
“blue sky”
concept
ETCWG
Recommendation
Level 4
Requirements
(what)
Level 5
Requirements
(how)
Recommendation
The ETC should use a three-dimensional (two
spatial and one spectral) framework.
Recommendation
The ETC should use a grid of reference PSFs, for
instance as generated by the WebbPSF tool.
A 3D ETC enables support for all
JWST observing modes.
A few arcsec
Y
X
Recommendation
The noise propagation should include
correlated noise.
The ETC models correlated detector noise.
Total flux rate in measurement
Flux rate in pixel i
scalar weight of pixel i
Variance of measurement
covariance matrix of pixels (1,2,...,N)
Example: Aperture photometry
A = extraction region/aperture
B = sky region
Aperture correction
Pixel flux rate in aperture
Pixel flux rate in background
Pixel weights
Negative because the background is subtracted
The ETC requires 3 classes of input.
Target
- Spectrum
- Shape
- Observing epoch
- RA/DEC
Instrument
transmission
Background
Source 1
Source 2
Source 3
...
PSF calculation
- WebbPSF
- Grid of monochromatic
PSF reference files
- 3D cube
Instrument
config
APT parameters
Detector
projection
- Imager
- Slit spectrum
- IFU
- ...
Scene
- 3D cube
- (x,y,wavelength)
- (x,y) sampled as
- integer fraction of
pixel scale
Strategy
- Background
subtraction
- Extraction algorithm
Linear
combination
weights
2D Signal
e-/s/pixel
Extraction
2D Noise
- Full covariance
matrix
- Up-the-ramp noise
- Total signal
- Error propagation
Scalar/1D
Signal-to-noise
The ETC engine may have to
support several clients.
APT
Web
applet/form
ETC engine
A python library
Stand-alone
app
Recommendation
The ETC engine should have a stable API,
capable of supporting different clients,
including APT, a user web form and a
standalone graphical application.
https://confluence.stsci.edu/display/JWST/MESA+ETC