Klaus Pontoppidan, on behalf of the STScI ETC teamJWST @ STScI

Proposal Planning Workshop, May 15, 2017

Pandeia: The JWST Exposure Time Calculator

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Current version: 1.0 (January 2017)

jwst.etc.stsci.edu

for DD-ERS/GTO/GO Cycle 1

Fun fact: When an update occurs, your old workbooks will be saved as read-only

Planned update releases (version 1.1 ~June 1, including target acq + numerous updates, bug fixes and enhancements)

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SOME HISTORY

• Originally, it was envisioned that the JWST ETC would be similar to the HST ETC.

• It became clear that using the HST ETC as a basic design was not viable.

• Works ok for imaging, but not for most of the advanced JWST modes, JWST detector noise and to support modern user interaction and collaborations

• Something more modern/complex was needed. This was ~2012.

• Imaging • NIRCam SW+LW • NIRISS • MIRI • NIRSpec TA (through MSA mesh)

• Slit spectroscopy • NIRSpec FS • MIRI LRS

• IFU spectroscopy • NIRSpec IFU • MIRI MRS

• Multi-object spectroscopy • NIRSpec MSA

• Slitless spectroscopy • NIRISS WFSS • NIRISS SOSS • MIRI LRS

• Coronagraphy • NIRCam spots + wedges • MIRI Lyot • MIRI FQPMs

• Sparse Aperture Masking Interferometry • NIRISS AMI

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SOME HISTORY, CONT’D2012 Pandeia prototype2013 Recommendations to satisfy the ETC requirements

within a feasible framework.

1. The ETC should use a three-dimensional (two spatial and one spectral) framework.

2. The ETC should use a PSF library, for instance as generated by the WebbPSF tool.

3. The noise propagation should include correlated noise.

4. Post-observation additions of multiple exposures, subtraction of backgrounds, extraction of photometry and spectroscopy […] should be explicitly modeled under the unifying concept of “observing strategy”.

5. The ETC engine should have a stable API supporting different clients, including APT, a user web form and a standalone application.

6. ETC calculations should be sufficiently fast to support efficient comparative parameter studies, as well as inverse problems.

7. The ETC should be template-based for all three main inputs (instrument configuration, target design, observing strategy).

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ETC enginePython library

JWST reference database

Separate from other JWST reference data

Throughputs noise properties

PSFs

Web application @

jwst.etc.stsci.edu

User interface relevant for most users of the ETC

Collaborative functionality

JWST Background Model

Currently only available through the web

application

THE PANDEIA PROJECT

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Workbooks: Organize and save your ETC calculations

Build your own sources and scenes library

Analyze and compare different instruments and modes

Share your work with your team

WEB APPLICATION: A COLLABORATIVE WORK ENVIRONMENT

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• JWST code project called “Pandeia”

• Under the hood: 3D sensitivity engine

• Includes PSFs, correlated noise other detector effects, effects of background subtraction and extraction.

• Pointing+time dependent JWST background model.

• 100% data driven

ETC ENGINE

Reference: Pontoppidan, Pickering, et al. 2016, SPIE

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Python module

More functionality than available in the web application

Advanced scripting

pip install pandeia.engine

http://ssb.stsci.edu/pandeia/engine/1.0/

examples at https://github.com/spacetelescope/JWSTUserTraining2016

ETC ENGINE CODE AVAILABLE

Fun fact: The engine is a compact, general code, currently only ~12,500 lines, excluding standard libraries.

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ENGINE ALGORITHM: SCENE PROJECTIONSImage projection

Slit spectroscopy projection

Slitless spectroscopy projection

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Flux rate in pixel iTotal flux rate in measurement

scalar weight of pixel iVariance of measurement

covariance matrix of pixels (1,2,...,N)

ETC Strategies: Extracting signal and propagating noise

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Strategy example: Aperture photometry

Aperture correctionPixel flux rate in aperture

A = extraction region/aperture

B = sky region

Pixel flux rate in background

Pixel weights

Negative because the background is subtracted

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PSF LIBRARY•Uses WebbPSF to calculate theoretical point spread functions, including realistic wavefront errors. (M. Perrin, https://pythonhosted.org/webbpsf/).

•Almost 3000 individual monochromatic PSFs.

•Subsampled by integer factor of pixel size.

•Fun fact: The observed PSF depends on the color of the astronomical source.

NIRCam SW imaging PSFs

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PANDEIA REFERENCE DATAPandeia_data Observatory (JWST)

Extinction

SED (template spectra)

Normalization (external bandpasses)

Various defaults

MIRI

NIRCam

NIRISS

NIRSpec

Telescope

Detectors(IPC+correlation)

config.json

Spectral efficiency

Dispersion

Filters

Optical efficiency

Detector QE+QY

PSFs

Fun fact: The ETC reference data is a ~1.5 GB tarball. Almost all of that are PSF libraries.

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SATURATION

Two types of saturation:1. Partial saturation — the ramp saturates before it finishes, but more than the minimum reads are unsaturated

2. Full saturation — the ramp saturates before the minimum number of reads is reached

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Zodiacal and Galactic backgrounds

• JWST uses a dynamic “background model generator” (BMG).

• Zodi+ISM heritage from Spitzer’s operational system (IPAC).

• Data are from COBE/DIRBE.

• ISM “cirrus” uses fixed spectrum, including PAHs. Intensity set by Schlegel et al. extinction maps, which were fits to IRAS + DIRBE/COBE data.

• Wainscoat et al.1998 star counts model.

• The Zodi and Galactic emission is calculated for a given RA & DEC, in one of 2 ways:

1. for a given RA, DEC, date: “Dated background”

2. as a percentile of the backgrounds for those coordinates, over the visibility. “Dateless background”

• Fun fact: Benchmark sensitivities computed for (J2000, 17:26:44 -73:19:56) on June 19, 2019. — “minzodi*1.2”

• Fun fact: If using the ETC engine “stand-alone”, you should still use the web app to create a background.

Zodiacal

ISM

Stars

A. Noriega-Crespo

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Stray light• The background model calculates the

stray light for a given position and date, following Lightsey 2016 SPIE.

• Radiance Transfer Functions (RTFs) from Lightsey, at 0.7, 1, 2, 3, 5, 10, 15 micron.

• Sky map from Galactic + Zody engine (for that position, for that date)

• Interpolate RTFs at other wavelengths.

(Lightsey 2016 SPIE)

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Thermal self-emission

• Thermal emission from JWST itself is the dominant background at ~>15um.

• Sum of 20 blackbodies (P. Lightsey, J. Rigby).

• Produces backgrounds of 0.74 and 174 MJy/SR at 10 and 20 micron. (Reqs. are 3.9 and 200 MJy/SR). Temperatures are as expected: T(SS)=89K, T(PM)=54K (hottest segments).

• Blackbody model agrees with Lightsey calculations at 15, 25 micron to within 0.5%. Thermal-only curve (black line) + Glasse et al. 2015

SPIE (red and blue solid lines.) Zodiacal model of Glasse (dashed lines.) Figure from J. Rigby

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Fun fact: You can download all the ETC inputs/outputs in a convenient format from the downloads tab

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NIRCam F250M/F335M/F460M

MIRI F560W/F770W/F1000WFun fact: You can also use the ETC python module to script

complex, large scenes

USING THE ETC: IMAGING

Red bulge with star-forming knots at z=0.5

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MIRI F560W/F770W/F1000W

Fun fact 1: In spectroscopic modes extraction apertures are one wavelength pixel wide.

USING THE ETC: SPECTROSCOPIC MODESMIRI LRS

NIRSpec MSA

Fun fact 2: You can upload your own spectra, and add lines.

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ETC calculation of a MIRI MRS observation of a protoplanetary disk model.

12 individual calculations (one for each MRS channel+sub-band)

Fun fact 1: The ETC will slow down for more finely sampled input spectra (improved in v1.1).

Fun fact 2: All IFU calculations (NIRSpec+MIRI) will perform a dither to subtract background (pick in-field or off). (simplified in v1.1)

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BENCHMARKING THE ETCETC requirement: 10% in signal-to-noise ratio for 90% of calculations

Compared limiting sensitivities (10 sigma in 10,000 s) for all major instrument modes to independent calculations provided by the JWST instrument teams.

In comparing a 3D calculation with 1D calculations 10% is a reasonable threshold.

Using the same data and same assumptions, get within 10%.

Algorithms are appropriate and accurate Dashed curves are the benchmarks

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MIRI

NIRISS

NIRSpec

NIRCam

VALIDATION COMPARISON

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REAL LIFE ETC RESULTS VERSUS STANDARD SENSITIVITIES

There are many parameters affecting ETC sensitivities

That’s the point of having a versatile tool! Comes with some responsibility.

• Think about your background subtraction • Think about what extraction aperture is optimal (point or extended sources) • Is your background correct for your target? • Different read-out patterns change read noise • The ETC does cut some corners: No distortion

for instance (except for NIRISS SOSS).

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HOW TO GET HELPThe ETC is powerful = a lot of parameters

Documentation: jwst-docs.stsci.edu Known issues: Check the ETC help menu JWST help desk: jwsthelp.stsci.edu Have patience with us - the team is working hard (400+ open tasks)

…more to come