- JWST Cycle 1 Proposal Opportunities
- JWST Cycle 1 Guaranteed Time Observations Call for Proposals
- • JWST Director's Discretionary Early Release Science Call for Proposals
- • JWST Call for Proposals for Cycle 1
- James Webb Space Telescope Call for Proposals for Cycle 1
- •JWST Cycle 1 Proposal Checklist and Resources
- •JWST Cycle 1 Proposal Policies and Funding Support
- JWST Cycle 1 Proposal Categories
- •JWST Cycle 1 Observation Types and Restrictions
- •JWST Cycle 1 Proposal Preparation
- •JWST Cycle 1 Single-Stream Proposal Process
- •JWST Cycle 1 Special Submission Requirements
- •JWST Cycle 1 Observation Mode Restrictions
- •JWST Cycle 1 Proposal Selection Process
- •JWST Cycle 1 Awarded Program Implementation
- •JWST Cycle 1 Proposal Science Categories and Keywords
- JWST General Science Policies
- • JWST Observing Overheads and Time Accounting Policy
- • JWST Duplicate Observations Policy
- • JWST Science Parallel Observation Policies and Guidelines
- • JWST Observing Program Modification Policy
- • Policies for the Telescope Time Review Board
- • JWST Target of Opportunity Program Activation
- NASA-SMD Policies and Guidelines for the Operations of JWST at STScI
- •Policy 1 - Limitations on the Use of Funds for the Research of General Observers and Archival Research
- •Policy 2 - Data Rights and Data Dissemination
- •Policy 3 - Data Requests and Facilities
- •Policy 4 - Post-Launch Commissioning of JWST
- •Policy 5 - Clarification of Extensions of Exclusive Access Data to Public Affairs Activities
- •Policy 6 - Distribution of JWST Science Data Obtained from Investigations Other Than Those Selected Through the Peer-review Process
- •Policy 7 - NASA Needs for Support for Other Missions
- •Policy 8 - Definition of Observing Time
- •Policy 9 - Allocation of Guaranteed Observing Time to Scientists Selected Under AO 01-OSS-05 and Through NASA-ESA-CSA Agreements
- •Policy 10 - Redistribution of Guaranteed Observing Time Among Observers
- •Policy 11 - Protection of Science Programs Associated With Guaranteed Time
- •Policy 12 - Education and Public Outreach
- Methods and Roadmaps
- JWST Imaging
- • JWST Slit Spectroscopy
- • JWST Slitless Spectroscopy
- JWST High-Contrast Imaging
- •Contrast Considerations for JWST High-Contrast Imaging
- •JWST Coronagraphic Observation Planning
- •JWST Coronagraphic Sequences
- •JWST Coronagraphy in ETC
- •JWST High-Contrast Imaging in APT
- •JWST High-Contrast Imaging Inner Working Angle
- •JWST High-Contrast Imaging Optics
- •JWST Small Grid Dither Technique
- •MIRI-Specific Treatment of Limiting Contrast
- •NIRCam-Specific Treatment of Limiting Contrast
- •NIRISS AMI-Specific Treatment of Limiting Contrast
- •Selecting Suitable PSF Reference Stars for JWST High-Contrast Imaging
- JWST Integral Field Spectroscopy
- JWST MOS Spectroscopy
- JWST Time-Series Observations
- •Overview of Time-Series Observation (TSO) Modes
- •Noise Sources for Time-Series Observations
- •Sensitivity of Time-Series Observation Modes
- •Bright limits of Time-Series Observation Modes
- •Preparing Time-Series Observations with JWST
- •Target Acquisition for Time-Series Observations
- •NIRCam-Specific Time-Series Observations
- •NIRISS-Specific Time-Series Observations
- •MIRI-Specific Time-Series Observations
- JWST Moving Target Observations
- •Moving Target Roadmap
- •Field of Regard Considerations for Moving Targets
- •Instrument-Specific Considerations for Moving Targets
- •Moving Target Recommended Strategies
- •JWST Moving Target Observing Procedures
- •JWST Moving Target Calibration and Processing
- •JWST Moving Target Ephemerides
- JWST Moving Targets in APT
- •JWST Moving Targets in ETC
- •JWST Moving Target Useful References and Links
- •Overheads for Moving Targets
- •JWST Moving Target Policies
- NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids
- JWST Parallel Observations
- JWST Target of Opportunity Observations
- Observatory Functionality
- • JWST Position Angles, Ranges, and Offsets
- • JWST Instrument Ideal Coordinate Systems
- JWST Background Model
- • JWST Guide Stars
- • JWST Mosaic Overview
- • JWST Dithering Overview
- JWST Duplication Checking
- JWST Observing Overheads and Time Accounting Overview
- •JWST Observing Overheads Summary
- •JWST Slew Times and Overheads
- JWST Instrument Overheads
- Observing Overheads for NIRCam Imaging
- • JWST Data Rate and Data Volume Limits
- Observatory Hardware
- • JWST Observatory Overview
- • JWST Observatory Coordinate System and Field of Regard
- • JWST Field of View
- • JWST Orbit
- JWST Spacecraft Bus
- • JWST Pointing Performance
- • JWST Telescope
- • JWST Wavefront Sensing and Control
- • JWST Momentum Management
- • JWST Integrated Science Instrument Module
- • JWST Solid State Recorder
- • JWST Target Viewing Constraints
- • Fine Guidance Sensor, FGS
- JWST Exposure Time Calculator Overview
- • JWST ETC New User Guide
- JWST ETC Calculations Page Overview
- •JWST ETC Creating a New Calculation
- •JWST ETC Backgrounds
- •JWST ETC Wavelength of Interest/Slice
- •JWST ETC Batch Expansions
- JWST ETC Strategies
- JWST ETC Target Acquisition
- JWST ETC Outputs Overview
- JWST ETC Workbooks Overview
- JWST ETC Pandeia Engine Tutorial
- • JWST ETC Point Spread Functions
- • JWST ETC Instrument Throughputs
- • JWST ETC Residual Flat Field Errors
- • JWST ETC NIRCam Imaging
- Astronomers Proposal Tool
- • JWST Astronomers Proposal Tool Overview
- APT Workflow
- Additional APT Functionality
- Getting Help with APT
- Other Tools
- Mid Infrared Instrument
- • MIRI Overview
- MIRI Observing Modes
- MIRI Instrumentation
- MIRI Operations
- MIRI Target Acquisitions
- MIRI Dithering
- MIRI Mosaics
- •MIRI MRS Simultaneous Imaging
- MIRI Time Series Observations
- MIRI Predicted Performance
- MIRI APT Templates
- MIRI Observing Strategies
- MIRI Example Programs
- •MIRI Coronagraphy of GJ 758 b
- MIRI Imaging, MIRI MRS, and NIRSpec IFU Observations of SN1987A
- •MIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk
- •MIRI IFU and NIRSpec Observations of Cas A
- MIRI MRS Spectroscopy of a Late M Star
- MIRI MRS and NIRSpec IFU Observations of Cassiopeia A
- Near Infrared Camera
- • NIRCam Overview
- NIRCam Observing Modes
- NIRCam Instrumentation
- •NIRCam Field of View
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- •NIRCam Filters for Coronagraphy
- •NIRCam Grisms
- •NIRCam Weak Lenses
- NIRCam Detectors
- NIRCam Operations
- NIRCam Dithers and Mosaics
- •NIRCam Coronagraphic PSF Estimation
- •NIRCam Coronagraph Astrometric Confirmation Images
- •NIRCam Apertures
- NIRCam Target Acquisition Overview
- NIRCam Predicted Performance
- NIRCam APT Templates
- NIRCam Observing Strategies
- NIRCam Example Programs
- NIRCam Deep Field Imaging with MIRI Imaging Parallels
- NIRCam Imaging and NIRISS WFSS of Galaxies Within Lensing Clusters
- •NIRCam WFSS Deep Galaxy Observations
- •NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris Disk
- •NIRCam Coronagraphy of HR8799 b
- NIRCam Grism Time-Series Observations of GJ 436b
- NIRCam Time-Series Imaging of HAT-P-18 b
- Near Infrared Imager and Slitless Spectrograph
- • NIRISS Overview
- NIRISS Observing Modes
- NIRISS Instrumentation
- NIRISS Operations
- NIRISS Predicted Performance
- NIRISS APT Templates
- NIRISS Observing Strategies
- NIRISS Example Programs
- NIRISS AMI Observations of Extrasolar Planets Around a Host Star
- NIRISS SOSS Time-Series Observations of HAT-P-1
- NIRISS WFSS with NIRCam Parallel Imaging of Galaxies in Lensing Clusters
- Near Infrared Spectrograph
- NIRSpec Overview
- NIRSpec Observing Modes
- NIRSpec Instrumentation
- •NIRSpec Optics
- •NIRSpec Dispersers and Filters
- NIRSpec Detectors
- •NIRSpec Micro-Shutter Assembly
- •NIRSpec Integral Field Unit
- •NIRSpec Fixed Slits
- NIRSpec Operations
- NIRSpec Dithers and Nods
- NIRSpec MOS Operations
- NIRSpec IFU Operations
- •NIRSpec FS Operations
- •NIRSpec BOTS Operations
- NIRSpec Target Acquisition
- NIRSpec Predicted Performance
- NIRSpec APT Templates
- NIRSpec Multi-Object Spectroscopy APT Template
- •NIRSpec MOS Proposal Checklist
- •NIRSpec MSA Planning Tool, MPT
- NIRSpec MPT - Catalogs
- •NIRSpec MPT - Planner
- NIRSpec MPT - Manual Planner
- •NIRSpec MPT - Plans
- •NIRSpec MPT - Parameter Space
- •NIRSpec MSA Spectral Visualization Tool Help
- •NIRSpec Observation Visualization Tool Help
- •NIRSpec IFU Spectroscopy APT Template
- •NIRSpec Fixed Slit Spectroscopy APT Template
- •NIRSpec Bright Object Time-Series APT Template
- •NIRSpec FS and IFU Mosaic APT Guide
- NIRSpec Multi-Object Spectroscopy APT Template
- NIRSpec Observing Strategies
- •NIRSpec Background Recommended Strategies
- •NIRSpec Bright Spoilers and the IFU Recommended Strategies
- •NIRSpec Detector Recommended Strategies
- •NIRSpec Dithering Recommended Strategies
- •NIRSpec MOS Recommended Strategies
- •NIRSpec MSA Leakage Subtraction Recommended Strategies
- •NIRSpec Target Acquisition Recommended Strategies
- NIRSpec Example Programs
- NIRSpec IFU and MIRI MRS Observations of Cassiopeia A
- NIRSpec BOTS Observations of GJ 1214b
- NIRSpec IFU, MIRI Imaging, and MIRI MRS Observations of SN1987A
- NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids
- NIRSpec MOS Deep Extragalactic Survey
- •NIRSpec MOS Observations of NGC 346
- •NIRSpec and MIRI IFU Observations of Cas A
- Understanding Data Files
- Obtaining Data
- Data Processing and Calibration Files
- JWST Data Reduction Pipeline
- • Primer and Tutorials
- • Pipeline User's Guide
- • Software Reference Documentation
- Algorithm Documentation
- • Obtaining and Installing Software
The JWST Exposure Time Calculator (ETC) allows observers to simulate sky scenes and perform signal to noise calculation for a given telescope/instrument configuration. We show the step-by-step instructions to perform ETC calculations for observations following a real science case, namely IR imaging and spectroscopy IFU observations of Supernova 1987A.
Exposure Time Calculator (ETC) instructions
These instructions are based on the ETC workbook demonstration presented at the 2017 Proposal Planning Workshop. The instructions show in detail the steps needed to create the sources, setup the instrument and run a simulation
The main goals of the ETC calculations are :
- To create an extended source using composite extended shapes and simple spectra and specify the flux and normalization parameters.
- To present example signal to noise ratio (SNR) calculations for:
Defining the sources
SN1987A consists of a bright ring, enclosing the dust ejecta material. The outer region of the dust ejecta includes several knots of shocked emission. The geometry of the dust source could be defined as:
- A dust ring with a diameter of ~1.1" and a thickness of ~0.2" for MIRI calculations.
- The central dust continuum ejecta that occupy ~4 MIRI MRS pixels.
- Discrete knots of shocked emission, for NIRSpec calculations to investigate molecular hydrogen emission at 2.12 μm.
Using the ETC web interface, it is possible to define sources within a scene. The ring part of the supernova is defined as an extended source with a flat-flux distribution and using a circular shape with a radius of 0.63".
The Source Editor panel (right side of Figure 1) includes a set of tabs that allow the user to define the characteristics of the sources in the scene. Figure 1 shows the first step in defining a new source in the scene. We start by assigning it a source identification ID, in this case the ID "ring".
The Continuum tab allows us to select an SED for the source. In this case we model the source with a Blackbody Spectrum of 400 K as shown in Figure 2.
In the normalization (Reform) tab, we can normalize the spectrum to have a flux density of 80 mJy at 10 μm as shown in Figure 3.
The shape of the source is defined in the Shape tab. In this case we select an extended source with a flat flux distribution and we give it the shape of a disk with a 0.63" radius. See Figure 4.
The ETC web interface creates a simple sketch of the scene and displays it at the bottom of the GUI. The spectrum of the source is also plotted when selected as shown in Figure 5.
Following the same procedure, it is possible to define another source in the scene, in this case the "ejecta", which we can assume is a point source with a Blackbody SED of 100 K, and with its flux density normalized to 0.1 mJy at 10 μm. Figure 6 shows the ETC workbook with both sources selected.
A third source in the scene is defined with a name "shock knot1. This will be a source of pure line emission at 2.12 μm to simulate shocked molecular hydrogen in an extended knot. This source is added in a similar manner to the instructions described above. The detailed steps for this are:
- Add a "new" source in the 'select a source' region of the ETC.
- In the ID area for the source, name the new source "shock knot1"
- Choose 'no continuum' in the "Continuum" tab for the source definition.
- In the "Renorm" tab, select 'Do not renormalize'.
- In the "Lines" tab, fill out the appropriate parameters for molecular hydrogen H2 emission: 2.12 μm wavelength, line width of 50km/s (a possible value for shocked H2 lines), and a line flux of 1.0e-16 ergs/cm2/s. The intrinsic flux of the expected emission may not be well known. The ETC can be used to estimate S/N on a range of source fluxes by changing this line emission flux, or by adding additional sources of varying flux levels.
- Click "Add" to add the line to the source. Additional lines can be added to build up a more complex emission line spectrum, but here we describe only the 2.12 μm H2 feature.
- In the "Shape" tab, keep the default point source for a compact H2 knot of emission.
- In order to offset this simulated H2 line to the outer region of the dust source, the 'shock knot1' source must be added to the scene. Highlight 'shock knot1' in the "Select a Source" region, and add it to the SN1987A using the "Add a Source" (blue) button in the "Select a Scene" area.
- Now, offset the source in the scene to the outer region of the dust emission. Do this in the "Offset" tab, by adding and saving x, y offset values of 0.4, -0.4.
Figure 7 shows the ETC workbook with all sources selected.
Overview of NIRSpec calculations
Once the sources are defined, it is possible to perform many signal to noise calculations on the scene using the different JWST instruments. This is carried out under the Calculations tab in the ETC view. For each calculation, few additional parameters need to be set under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.
For this example we used the Medium background configuration as shown in Figure 8.
Instrument Setup tab:
Based on the science goals, the selection for Grating/Filter pair should be G235M/F170LP as shown in Figure 9. Note that the ETC automatically shows the Total System Throughout for this configuration in its corresponding wavelength range.
Detector Setup tab:
In order to achieve the desired exposure time, the number of Groups is set to 10, and the number of Integrations is set to 2. Multiple integrations are used in this case to ensure that uncertain source fluxes will not saturate in the allocated exposure time. The Readout pattern used for this example is NRS IRS2, see Figure 10. The exposure time is automatically calculated and displayed. For this example the total exposure time is ~25 minutes.
In the Strategy tab it is possible to select the type of nodding with the options "In Scene" and "Off scene". We select "Off scene" so that this extended source has no residual subtraction effects. We select the wavelength 2.12 μm to correspond to run the calculation on the molecular hydrogen emission line. Note that this value has to be in the range determined by the grating/filter combination, in this case 1.66–3.17 in units of μm. We define the calculation to have offsets in x and y that correspond to the location of the extended knot of H2 emission modeled in the 'shock knot1' source. The top section of Figure 11 shows the Strategy tab and the IFU Nod selection.
Once all parameters are in place, simply click on the Calculate button to perform the calculation. This might take some time because the NIRSpec IFU ETC calculates information on the full IFU data cube (which can be saved to disk in the 'downloads' section of the ETC). Figure 11 shows the results of the calculation, a signal-to-noise of 21 is achieved on the H2 emission line. A detailed description of the ETC outputs are described in the article JWST ETC Outputs Overview.
Overview of MIRI calculations
Following a similar approach, we can perform a couple of calculations for the JWST instrument MIRI. We begin with a signal to noise calculation on the mid-infrared dust morphology using the MIRI Imaging mode.
We create a new calculation by selecting MIRI→ Imaging. This sets default values in the calculation space that need to be customized to our specific needs for this project. The few additional parameters that need to be set are under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.
In the Backgrounds tab we set the value to 'Low'. In the Instrument Setup tab we select the MIRI filter F560W. Under the Detector Setup, there are a few options for subarray. We select BRIGHTSKY for this project. From the Readout pattern we select FAST. The number of groups is set to 16 to avoid saturation and eight is the number of integrations. Leave number of exposures equal to one.
Under the Strategy tab, there is only one option which is Imaging Aperture Photometry. We center on the "ring" and modify the radii for the aperture and sky annulus as follows: aperture radius 0.6", inner sky radius 0.7", and outer sky radius 0.9". Once these parameters are set, we can perform the calculation. Figure 12 shows a screen grab of the ETC GUI with the MIRI imaging mode highlighted. The plot in the bottom left shows a recreation of what the scene looks like at the detector level. the signal to noise for this calculation yields a value of 4936.5 in a total exposure time of 355.20 s.
The warning on this calculation can be viewed in the 'Warning' tab in the Reports area. In this case, the warning simply states that the background region used for the subtraction is smaller than the science aperture area. This can lower the signal to noise in this calculation on an extended source area. In actual data processing, a larger spatial region can be defined that will improve noise results in background subtraction.
Next we show an example of MIRI spectroscopy to investigate dust composition in SN 1987A using the Medium Resolution Spectroscopy mode (MRS).
We create a new calculation by selecting MIRI→ MRS. This sets default values in the calculation space that need to be customized to our specific needs for this project. The few additional parameters that need to be set are under the following tabs: Backgrounds, Instrument Setup, Detector Setup, and Strategy.
In the Backgrounds tab we set the value to 'Low'. In the Instrument Setup tab we are presented with four channels. The science line of interest is the 12.8 μm neon line, so we select Channel 3 that spans the wavelength range 11.53-13.48 in units of μm. We leave the Disperser as the default value of 'Short'. Under the Detector Setup, there is only the FULL subarray option. From the Readout pattern we select SLOW. The number of groups is set to 7 to avoid saturation and three is the number of integrations. Leave number of exposures equal to one.
Under the Strategy tab, there are two methods: 'Nod In Scene', or 'Nod Off Scene'. We choose to 'Nod Off Scene' and use and aperture radius of 0.1". We center the calculation on the "ring" source. Once these parameters are set, we can perform the calculation. Figure 13 shows a screen grab of the ETC GUI with the MIRI MRS mode highlighted in yellow. The plot in the bottom left shows a recreation of what the scene looks like at the detector level. The signal to noise for this calculation yields a value of 110.88 in a total exposure time of 501.69 s.
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