Step-by-Step ETC Guide for NIRSpec IFU and MIRI MRS Observations of SN1987A
A walk-through of the JWST ETC for the Example Science program MIRI MRS and NIRSpec IFU Observations of SN1987A is provided, demonstrating how to select exposure parameters for this observing program.
The JWST Exposure Time Calculator performs signal-to-noise (SNR) calculations for the JWST observing modes. Sources of interest are defined by the user and assigned to scenes that are used by the ETC to run calculations for the requested observing mode.
For the MIRI and NIRSpec Observations of SN 1987A example science program, we focus on selecting exposure parameters for both MIRI MRS and the NIRSpec IFU. We start by defining a scene with an extended source that represents part of the 2" diameter equatorial ring (ER) of SN 1987A and another source representing the ejecta in the center of the ring. We show how to run ETC calculations to achieve the desired SNR for MIRI MRS and the NIRSpec IFU. We would like to acknowledge that a number of the ETC calculations we describe here were performed by Ryan Lau (JAXA/ISAS).
The optimal exposure parameters (e.g., the number of groups and integrations) are the input needed for the Astronomer's Proposal Tool APT observation template, which is used to specify an observing program and submit proposals.
The ETC workbook associated with this example science program is called #27: MIRI MRS and NIRSpec IFU Observations of SN 1987A and can be selected from the Example Science Program Workbooks dropdown parameters in the ETC Workbooks page. The nomenclature and reported SNR values are based on ETC 1.5. There may be subtle differences if using a different version of ETC.
This ETC workbook provides an example of building a complex scene in the ETC with multiple emission sources. For a simpler calculation, a single extended ellipse-shaped emission structure with an appropriately scaled surface brightness could have been used for a rough estimate of exposure sensitivity. In general, it is a good idea to keep an ETC scene as simple as possible. We chose to demonstrate a multi-component scene that showcases the ability of the JWST ETC to build realistic, complex calculations. For an example of a simplified ETC scene construction on an extended source, please see Step-by-Step ETC Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A.
Define Sources and Scenes in the ETC
Define Sources for MIRI MRS
We set up a scene called SN 1987A (MIR) (Scene ID #1) with multiple sources to represent the equatorial ring (ER) and central ejecta of SN 1987A, as defined below. Clicking on the SN 1987A (MIR) scene in the Select a Scene pane will display the Scene Sketch in the lower left corner of the ETC.
Equatorial ring (ER)
For the sources representing the ER (each called "ring seg. #N (MIR)" in the ETC workbook, with N between 1 and 8 inclusive), we used 8 extended sources; for each, under the Shape tab in the Source Editor, the Flux distribution was set to 2D Gaussian, the Normalization choices was set to Integrated Flux, σx is set to 0.1 (arcsec), and σy is set to 0.3 (arcsec). For each source, we defined the continuum as a Blackbody spectrum in the Continuum tab, assigning a blackbody temperature (Tb) of 370 K. This temperature represents an average temperature between the 525 K and ~190 K components of the dust models of SN 1987A reported by Matsuura et al. (2019).
We estimate the flux density of each ring component by equally dividing the SN 1987A flux density of 88 mJy from the SOFIA FORCAST 19.7 μm measurement (Matsuura et al. 2019) among the 8 sources composing the ring. We thus normalize the spectra in the Renorm tab to 10 mJy at 18 μm (choosing the option to Normalize at wavelength).
To emulate a ring geometry, we apply the following offsets (in arcseconds) and orientations (in degrees) within the scene:
|Source ID #||Source name||X offset||Y offset||Orientation|
|1||ring seg. #1 (MIR)||0.8||0||0|
|5||ring seg. #2 (MIR)||-0.8||0||0|
|6||ring seg. #3 (MIR)||0||0.5||90|
|7||ring seg. #4 (MIR)||0||-0.5||90|
|8||ring seg. #5 (MIR)||-0.5||-0.4||70|
|9||ring seg. #6 (MIR)||-0.5||0.4||-70|
|10||ring seg. #7 (MIR)||0.5||0.4||70|
|11||ring seg. #8 (MIR)||0.5||-0.4||120|
Bouchet et al. (2004) observed the ejecta in the center of the right to be point-like in their N-band image. We thus define the central ejecta (named Core (MIR), ETC source ID #12) as a Point source under the Shape tab. Similar to the equatorial ring, the spectrum of the ejecta is set to a Blackbody Spectra in the Continuum tab, with a temperature of Tb = 100 K. The ejecta temperature of 100 K is consistent with the temperature range of 90–100K reported by Bouchet et al. (2004) for SN 1987A ejecta, and it is also roughly consistent with the 85 K component in the Matsuura et al. (2019) dust models.
We normalized the spectrum to 0.1 mJy at 10 μm in the Renorm tab. This is consistent with assuming that the 0.32 mJy flux density enhancement reported by Bouchet et al. (2004) in the center of the ER in their N-band image has decreased in flux over time (Bouchet et al. 2004 noted the observed decrease over time of the 10 μm flux density).
Define Source for MIRI simultaneous imaging
We set up a scene called MIRI Simul. Imaging (Scene ID #2) with a single source to represent a star to be observed as part of MIRI simultaneous imaging. Under the Shape tab, we set it to Point. We left entries under the Offset tab equal to 0. Under the Continuum tab, under Spectral Energy Distribution, under Select, we set it to Phoenix Stellar Models, and we chose the M5V 3500 5.0 model, as late M stars are common stars. Under the Renorm tab, we select Normalize in bandpass, and normalize to Johnson K-band magnitude of 14.
Define Sources for the NIRSpec IFU
We set up a scene called SN 1987A (NIR) (Scene ID #4) with multiple sources to represent the 2" diameter ER of SN 1987A and the ejecta in the center of the ring, as defined below.
In order to provide the spectral energy distribution to be used in ETC for the NIRSpec IFU calculations of SN 1987A, we made use of a K-band cube (graciously shared by J. Larsson and C. Fransson) built from IFU observations of SN 1987A using the SINFONI instrument (Larsson et al. 2016). We extracted flux-calibrated spectra from 20 positions along the ER and 1 position at the ejecta in the center of the ER from the SINFONI cube and ingested these into the ETC workbook under the Upload Spectra tab. In the Continuum tab of the Source Editor window, we then can choose which spectral file to use for which source by selecting the Uploaded File option. For the sources representing the equatorial ring, the name of the spectrum chosen began with "knot__", while for the ejecta position, the name of the spectrum began with "ejecta__".
For the sources representing the ER, we defined 20 extended sources, each with a name of the form "ring seg. #N (NIR)" (with N between 1 to 20, inclusive); for each, under the Shape tab, the flux distribution was set to Flat, the normalization was set to Integrated Flux, σx and σy are set to 0.1 (arcsec).
For the 20 extended sources in the ER, under the Offset tab, we set (X offset, Y offset, and Orientation) to: (-0.145, -0.674, 0), (-0.173, 0.532, 0), (-0.364, -0.595, 0), (-0.369, 0.481, 0), (-0.554, 0.375, 0), (-0.571, -0.489, 0), (-0.739, 0.251, 0), (-0.762, -0.332, 0), (-0.829, 0.066, 0), (-0.84, -0.136, 0), (0.023, 0.504, 0), (0.085, -0.691, 0), (0.242, 0.493, 0), (0.326, -0.64, 0), (0.405, 0.38, 0), (0.522, -0.5, 0), (0.59, 0.291, 0), (0.696, -0.332, 0), (0.707, 0.122, 0), and (0.752, -0.107, 0).
For the ejecta in the center of the ER, named ejecta (NIR) (Source ID #32), we defined an extended source since the central ejecta are seen to be extended in near-infrared observations (e.g., see the HST data shown by Bouchet et al. 2006; and see the SINFONI data shown by Larsson et al. 2016). We set the flux distribution to Flat, the normalization to Integrated Flux, and the Semi-Major Axis and Semi-Minor Axis to 0.2 (arcsec).
The x and y offset are set to -0.038 and -0.148 for the central ejecta, as this is its position with respect to the center of the 20 sources defining the ER.
Define Sources for NIRSpec WATA
We set up a scene called NIRSpec WATA (Scene ID #3) with a single source to represent the offset star to be used for the NIRSpec WATA. For this source, under the Shape tab, we set it to Point. We left entries under the Offset tab equal to 0. Under the Continuum tab, under Spectral Energy Distribution, under Select, we set it to Phoenix Stellar Models, and we chose the B3V 19000 4.0 model because this was the closest spectral type to the spectral type of B2 given for this star by Walborn et al. (1993). In order to normalize this stellar photosphere spectrum, we needed a flux density at a given wavelength. By querying the SAGE (Meixner et al. 2006) catalogs of Spitzer Space Telescope IRAC photometry, one finds that the 3.6 μm IRAC flux densities for this source are 0.321 mJy for epoch 1 and 0.247 mJy for epoch 2. We use the average of these, 0.284 mJy, to perform our WATA calculations in ETC. Under the Renorm tab, we choose the Normalize in bandpass option, then chose the Other option under this, then Spitzer and IRAC 3.6 microns, then entered 0.284 in the blank under Normalize in bandpass and selected mJy as the units from the drop-down options.
Run ETC Calculation for IFU
We wish to obtain IFU observations with both the NIRSpec IFU and MIRI-MRS of the 2" arcsecond diameter equatorial ring (ER) and central ejecta of SN 1987A.
Select MIRI Medium Resolution Spectroscopy (MRS) Calculation
MIRI MRS observations are taken in each of 3 grating settings: SHORT, MEDIUM, and LONG. For the observation at each MRS grating setting, all 4 channels are observed simultaneously. By obtaining the observations at each of the 3 grating settings, one obtains the complete 4.9–28.8 μm spectrum.
Since the JWST background is position-dependent, fully specifying background parameters is important for the most accurate SNR calculation. In the Backgrounds calculation panel, we selected Medium for Background configuration, and we entered the coordinates 05:35:27.968 -69:16:11.09 to compute the background for these coordinates.
Select instrument parameters
We seek to detect the ER with SNR > 20 at all MRS wavelengths short-ward of 26.0 μm using a 0.1 arcsecond radius aperture. To achieve this SNR, we run ETC calculations for MIRI MRS spectroscopy for all 12 bands, in order to determine the MRS exposure parameters we need to achieve this SNR threshold. All 4 channels for a particular grating setting have the same exposure time, as this results in SNR > 20 short-ward of 26.0 μm for all 12 MRS bands. In ETC, there is a single calculation for each of the 12 MRS bands (Calculations #6–#17).
We use the 4-point extended dither pattern for the MRS optimized for all channels, since we care about the data for all 12 bands of the MRS wavelength range. Because of the IFU Nod Off Scene option we choose, this means we must set the number of Exposures per specification under the Detector Setup tab to 4, in order to simulate the full set of dithers.
We chose the following parameters for the MIRI MRS calculations in the calculations pane:
- Detector Setup tab:
- Subarray is set to FULL since only full frame readout is supported for MRS
- we chose the FAST readout pattern since the MIRI MRS Recommended Strategies indicates that this readout pattern should be used if the default readout pattern of SLOW requires under 10 groups (see also here). In addition, the FAST readout pattern requires a slightly lower total exposure time to achieve the same SNR as the SLOW readout pattern.
- the number of Groups per integration are:
- 282 for MEDIUM and LONG grating settings (Calculations #7-8, #10-11, #13-14, #16-#17)
- 255 for the SHORT grating setting (Calculations #6, #9, #12, #15)
- number of Integrations per exposure was set to 1
- number of Exposures per specification is set to 4 because we chose the IFU Nod Off Scene option, and we have chosen a 4-point dither pattern for the MRS.
- Strategy tab:
- We selected the IFU Nod Off Scene option, as the "IFU Nod On Scene" option would be inappropriate because our target is extended.
- Aperture location was set to Centered on source and then ring seg. #1 (MIR) to see the SNR for a ring segment
- Aperture radius was set to 0.1 (arcsec)
Select MIRI simultaneous imaging Calculation
Along with the MIRI MRS observations, we include simultaneous imaging with the MIRI imager. We wish to determine the brightest star that could be observed with the longest exposure time setting of the imager without saturation. We use the F560W, F770W, and F1000W filters in order to observe stars at the shortest wavelengths with MIRI.
Similar to the MIRI MRS calculations, under the Backgrounds tab, we chose Medium for Background Configuration.
Select instrument parameters
Here we assess the SNR in calculation #23. We also perform in ETC the same type of calculations (#21 and #22) for simultaneous imaging using the F770W and F1000W filters, though the principles are the same:
- Instrument Setup tab, Filter is set to F560W
- Detector Setup tab:
- Subarray is set to FULL
- we choose Readout pattern as FAST to be the same readout pattern as the MRS observations
- number of Exposures per specification is set to 1 because we are only interested in determining the magnitude of the field star that saturates for the same exposure time as the MRS observation
- number of Integrations per exposure is set to 1
- number of Groups per integration is set to 255, the same as for the MRS SHORT grating setting observation.
- Strategy tab:
- Imaging Aperture Photometry is the only option at the top, so it is chosen.
- Under Aperture location, Specify offsets in scene is chosen, with X and Y set to 0
- Aperture radius is set to 0.3 (arcsec), which is the default value for MIRI imaging in ETC.
- For Sky annulus, inner radius and outer radius are set to 0.45 and 0.7 (arcsec), respectively, as these are the default values
The magnitude under the Renorm tab of the Source Editor is then varied until the calculation returns a saturation warning. This happens for Johnson K magnitude less than 14.5 magnitudes for the F1000W observation and for K < 16.0 for the F770W and F560W observations. So K = 14.5 and 16.0 are the saturation limits for the F1000W and F770W/F560W simultaneous imaging observations, respectively.
Select NIRSpec IFU Calculation
NIRSpec IFU observations are taken using the G140M grating with the F100LP filter, the G235M grating with the F170LP filter, and the G395M grating with the F290LP, as discussed in the main article. Our goal is to detect as many emission lines as possible with the highest SNR possible (including the He I line at 2.058 microns; see Larsson et al. 2016), and to detect the continuum with as much SNR as possible. We run ETC calculations for NIRSpec IFU spectroscopy with these grating and filter combinations to determine the exposure parameters we need.
We chose Medium for Background configuration, which corresponds to the 50th percentile of the sky background. We add the coordinates 5:35:27.968 -69:16:11.09 to compute the background at these coordinates.
Select instrument parameters
Here we assess the SNR. We focus on the G235M/F170LP calculation for the NIRSpec IFU, which is Calculation #19:
- Instrument Setup tab: Grating/Filter Pair is set to G235M/F170LP
- Detector Setup tab:
- Subarray is set to FULL (only Full frame readout is supported for the NIRSpec IFU);
- we chose the Readout pattern as NRSIRS2RAPID because this readout pattern is an IRS2 readout pattern, which gives lower correlated noise
- number of Exposures per specification is set to 4 because the IFU Nod Off Scene option was selected under the Strategy tab, and we are using a 4 dither cycling pattern for the NIRSpec IFU. IFU Nod In Scene would have been inappropriate, as it would have simulated subtracting one nod from another, which must be avoided for an extended source such as ours. However, please note that the APT for this program does not include a dedicated background observation for the NIRSpec IFU. As noted in this program's example science page, a nearby field that could be used to obtain a dedicated background for the NIRSpec IFU would likely have ISM structure, so it would perhaps not be ideal for measuring background.
- number of Integrations per exposure is set to 1, as it is advised here to keep the number of integrations as low as possible.
- number of Groups per integration is set to 29
- Strategy tab:
- We selected the IFU Nod Off Scene option for the reason given above
- Aperture location was set to Centered on source, choosing ring seg. #1 (NIR)
- Aperture radius was set to 0.1 (arcsec) for all bands
Please note that a number of warnings are returned for this calculation in ETC pertaining to the wavelength range of the source spectrum not covering the entire wavelength range of the instrument configuration used in the ETC calculation. This does not affect the calculations, and these ETC warnings can be ignored.
Select NIRSpec WATA Calculation
The SN 1987A 2" diameter ring fills up most of the region of the sky common to all 4 dithers, which will be smaller than the 3" × 3" NIRSpec IFU field of view; therefore, it is important that the ring be positioned accurately for the observation by the telescope. For this reason, we perform a NIRSpec WATA observation. We cannot perform WATA on the ring itself due to its extended nature; however, we can perform WATA on an offset star, and we choose to do so using the nearby star [LSB2000] Star 2.
We chose Medium for the Background configuration. We add the coordinates 5:35:27.968 -69:16:11.09 to compute the background at these coordinates.
Select instrument parameters
Here we assess the SNR in calculation #5:
- Instrument Setup tab: Acq Mode is set to WATA, and the Filter is set to CLEAR. CLEAR was chosen to give the greatest SNR.
- Detector Setup tab:
- Subarray is set to SUB32, as the other subarray options (SUB2048 and FULL) resulted in saturation regardless of the Readout pattern chosen
- we chose the Readout Pattern as NRSRAPID, though any of the other available readout patterns could have been chosen
- For WATA, it is not possible to change the number of Exposures per specification, Integrations per exposure, or Groups per integration away from 1, 1, and 3, respectively, so these are the values used
- Strategy tab:
- We selected the Target Acquisition option at top, as this was the only option available within its drop-down options
- For Aperture centered on source, we chose Star 2, as this was the only option available within its drop-down options
We find a SNR > 60, which assures us that the TA will be performed successfully.
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