Step-by-Step ETC Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A

A walk-through of the JWST ETC for the Example Science Program MIRI MRS and NIRSpec IFU Observations of Cassiopeia A is provided, demonstrating how to select exposure parameters for this observing program.

On this page

Main article: MIRI MRS and NIRSpec IFU Observations of Cassiopeia A, JWST ETC Exposure Time Calculator Overview
See also: Video Tutorials

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 MRS and NIRSpec IFU Observations of the Cassiopeia A 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 a knot in the Cas A supernova remnant.  We show how to run ETC calculations to achieve the desired SNR for MIRI MRS and the NIRSpec IFU. 

The optimal exposure specifications (e.g., the number of groups and integrations) are the input needed for the Astronomer's Proposal Tool (APT) IFU observation template, which is used to specify an observing program and submit proposals.

The ETC workbook associated with this Example Science Program is called "#26: MIRI MRS and NIRSpec IFU Observations of Cassiopeia A" and can be selected from the Get a Copy of an Example Science Program dropdown on the ETC Workbooks page. The nomenclature and reported SNR values are based on ETC 1.4. There may be subtle differences if using a different version of ETC.


Define Sources for the NIRSpec IFU

We set up three scenes with three extended sources. Here, we selected "No Continuum" in the "Renorm" tab, since the 2MASS images show little continuum emission at near-infrared wavelengths. We included emission lines in the "Lines" tab, with line strengths given by Hurford & Fesen (1996) and velocity widths set to 4,000 km/s, which corresponds to the largest line widths of the lines observed by Spitzer-IRS (Ennis et al 2006, ApJ, 652, 376; Rho et al 2008, ApJ, 673, 271) and slightly exceeds the velocity widths reported in Table 2 of Kilpatrick et al (2014, ApJ, 796, 144):


Table 2. Description of Sources used for NIRSpec IFU Calculations

Source NameScene NameLine Name

Line Center

(μm)

Line Strength

(10-14 erg/cm2/s)

FMK 1, near-IRNIRSpec IFU Scene 1

ArIII

ArIII

CaII

CI

CIII

OI

OI

OII

SII

SII

SIII

SIII

OI

0.71379029

0.77530669

0.72929443

0.98396232

0.85812877

0.630168

0.63656971

0.73269533

0.67267933

1.0331254

0.90714184

0.95335416

0.77760731

2.905

0.996

0.6225

2.739

1.3695

3.3615

1.079

21.995

8.0095

46.065

13.695

41.5

0.3735

FMK 2, near-IRNIRSpec IFU Scene 2

CI

FeII a4F9/2-a4D7/2

FeII a6D(/2-a4D7/2

PII 3P1-1D2

PII 3P2-1D2

Si VI

Si x 2P1/2-2P3/2

SI 3P2-1D2

SII

SIII

SIII

OI 3P0, 1, 2, -3D1,2,3

0.97848639

1.639

1.251

1.143

1.184

1.955

1.425

1.078

1.0273759

0.90209343

0.94804857

1.135

3.993

2.7

3.7

2

5

6.9

3.5

5.4

95.59

52.03

151.25

3.3

FMK 3, near-IRNIRSpec IFU Scene 3

CI

SII

SIII

SIII

0.97773222

1.026584

0.90139814

0.94731786

3.672

33.32

18.02

53.72

The extended source parameters are the same as those defined for the MIRI MRS knots described in Table 1. 


Define Sources for MIRI MRS

We set up three scenes with three extended sources to represent the three knots.  We define the following extended sources in ETC, which are based on spectra measured from the CUBISM software (Smith et al. 2007) and are uploaded as user supplied spectra in ETC:

Table 1. Description of Sources used for MIRI MRS Calculations

Source NameScene NameUser Supplied SpectrumExtended Source Parameters
FMK 1, mid-IRMIRI-MRS Scene 1fmk1_cubism_spec_mod.txt

Flux distribution: 2D Gaussian

Normalization: Integrated Flux

σx = 2.29"

σy = 1.66"

FMK 2, mid-IRMIRI-MRS Scene 2fmk2_cubism_spec_mod.txt

Flux distribution: 2D Gaussian

Normalization: Integrated Flux

σx = 2.421"

σy = 2.166"

FMK 3, mid-IRMIRI-MRS Scene 3fmk3_cubism_spec_mod.txt

Flux distribution: 2D Gaussian

Normalization: Integrated Flux

σx = 1.465"

σy = 1.104"

The values of σx and σy are determined by measuring the full width at half maximum (FWHM) of the knots in Spitzer Space Telescope IRAC images from AOR 10737152 in program ID 3310 (PI: L. Rudnick). In all cases, we selected "Do not renormalize" in the Renorm tab.

We defined new scenes for each of these sources and assigned sources to the scenes by clicking "Add Source" in the "Select a Scene" pane. We renamed the scenes in the "ID" tab of the "Source Editor" pane. In all cases, we applied no offsets to the sources in the scenes, meaning the source is in the center of each scene. Note that by checking the checkbox in the "Plot" column in the "Select a Source" pane, the SED of the selected source can be plotted.


Define Sources for MIRI Simultaneous Imaging

We assume most stars in the MIRI imager field of view will be late M stars, so under the "Continuum" tab of the "Source Editor" pane, we choose "Phoenix Stellar Models" and "M5V 3500K 5.0"; redshift and extinction are set to zero.  Under the "Renorm" tab, we select "Normalize in Bandpass", and normalize to Johnson K-band magnitude of 10.5 (Vega).  Under the Shape tab, we choose Point, since we are simulating a field star.
This source is named "Sim. Imaging Star" and is assigned to the "MIRI Simul. Imaging" scene.


Run ETC calculation for IFU

Main articles: JWST ETC Calculations Page Overview, JWST ETC Creating a New Calculation, JWST ETC IFU Nod in Scene and IFU Nod off Scene StrategyJWST ETC Images and Plots

In general, we wish to obtain IFU observations such that every pixel within the spatial FWHM range of the knots over the spectral FWHM range of the lines detects the knot at SNR > 5, to give confidence in the detection of the signal at that wavelength at that spatial position.  The knot is assumed brighter in the center than at the periphery.  So if pixels are observed at SNR = 5 at the FWHM periphery of the knot at a half-power wavelength in the emission line, the peak pixel will have a factor 20.5 higher SNR at the peak pixel of the knot at that wavelength.  Further, the peak pixel at the peak wavelength of the line will have another factor 20.5 higher SNR than the peak pixel at the half-power wavelength in the emission line.  This means the peak pixel at the peak wavelength of the line has a factor of (20.5)2 = 2 higher SNR than the pixel at the FWHM periphery of the knot at a half-power wavelength of the line, or SNR = 5x2 = 10.  So the desired SNR at peak pixel at peak wavelength in all lines of interest is 10.

Select MIRI Medium Resolution Spectroscopy (MRS) Calculation

MIRI MRS observations are taken in each of three grating settings: SHORT, MEDIUM, and LONG.  For the observation at each MRS grating setting, all four channels are observed simultaneously.  By obtaining the observations at each of the three 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.  We selected "Medium" for "Background configuration", which corresponds to the 50th percentile of the sky background.  The positions entered for the FMKs are the following:

  • FMK1 - 23:23:36.1120 +58:50:10.81 ;
  • FMK2 - 23:23:29.1050 +58:50:26.71;
  • FMK3 - 23:23:27.3400 +58:50:35.67.

Select Instrument Parameters

Because of the SNR requirements we stated above, we run ETC calculations for MIRI MRS spectroscopy for every band in which a desired emission line exists, in order to determine the MIRI MRS exposure parameters we need to achieve this SNR threshold. All four channels for a particular grating setting have the same exposure time. In ETC, there is a single calculation for each of the 12 MRS bands, and in each calculation, the exposure parameters are set such that the desired SNR is achieved: a peak SNR of 10 at the peak pixel (spatially in the IFU slice image) at the peak wavelength (spectrally) of the line (or, for a dust emission feature, SNR = 10 at the peak pixel of the peak wavelength of the dust feature).  The exposure parameters for a given MRS grating are equal to the exposure parameters giving rise to the greatest exposure time for any ETC calculation in that grating setting.  That way, the band requiring the greatest exposure time achieves the desired SNR, while lines or dust features in other bands achieve even greater SNR.

For all three knots, we determined the number of groups per integration needed to achieve the desired SNR via trial and error. It was found that the required exposure times for FMK2 and FMK3 were lower than for FMK1, as FMK2 and FMK3 have greater surface brightness than FMK1.  Because all three knots are observed in the same target group, they must have the same exposure parameters.  So FMK1 determines the exposure parameters for the target group.

Throughout, we chose the following parameters for all MIRI MRS calculations:

  • "Detector Setup" tab:
    • subarray is set to FULLin the subarraymenu 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. In addition, the FAST readout pattern requires a slight lower total exposure time to achieve the same SNR as the SLOWreadout pattern.
    • the number of "groups per integration" are listed in Table 3 for each calculation, along with the corresponding exposure time (before overhead) for reference. We determined the number of groups by trial and error to achieve the desired SNR for relevant emission line, as indicated by the 2-D SNR image which shows the SNR per pixel in the 2-D scene.
    • number of "integrations per exposure" was kept at the default value of 1;
    • number of "exposures per specification" is set to 4 because the Nod off Scene option chosen under the Strategy tab does not assume a dithering pattern, and we assume a 4-point dithering pattern.
  • "Strategy" tab:
    • We selected the "IFU Nod Off Scene" option, as this is the option corresponding to using a dedicated sky observation, which is what we choose for our MIRI MRS target group.
    • Aperture location was set to "Specify position in scene", with X = Y = 0, which is the same as centered on the source.
    • Aperture radius was set to 1", which is the default value for the "IFU Nod Off Scene" option in ETC.


Table 3. ETC Calculations for MIRI MRS Observations to obtain a peak SNR = 10 for emission lines covered by channel/grating combination

Calculation #Source NameChannelWavelength rangeGroups Exposure time (s)
5FMK 1, mid-IR2 (MRS_SHORT)

Medium (B)

8.65 - 10.14 μm

50555.01
6FMK 1, mid-IR2 (MRS_SHORT)

Long (C)

9.99 - 11.71 μm

54599.41
7FMK 1, mid-IR3 (MRS_LONG)

Short (A)

11.53 - 13.48 μm

32355.21
8FMK 1, mid-IR4 (MRS_LONG)

Short (A)

17.66 - 20.92 μm

33366.31
9FMK 1, mid-IR4 (MRS_LONG)

Long (C)

23.95 - 28.45 μm

73810.31
10FMK 2, mid-IR2 (MRS_SHORT)

Medium (B)

8.65 - 10.14 μm

26288.6
11FMK 2, mid-IR2 (MRS_SHORT)

Long (C)

9.99 - 11.71 μm

20222.0
12FMK 2, mid-IR4 (MRS_LONG)

Long (C)

23.95 - 28.45 μm

20222.0
13FMK 2, mid-IR3 (MRS_LONG)

Short (A)

11.53 - 13.48 μm

18199.8
14FMK 2, mid-IR4 (MRS_LONG)

Short (A)

17.66 - 20.92 μm

19210.9
15FMK 3, mid-IR1 (MRS_SHORT)

Long (C)

6.52 - 7.66 μm

888.8
16FMK 3, mid-IR4 (MRS_LONG)

Long (C)

23.95 - 28.45 μm

12133.2
17FMK 3, mid-IR2 (MRS_SHORT)

Medium (B)

8.65 - 10.14 μm

15166.5
18FMK 3, mid-IR2 (MRS_SHORT)

Long (C)

9.99 - 11.71 μm

14155.4
19FMK 3, mid-IR3 (MRS_LONG)

Short (A)

11.53 - 13.48 μm

13144.3
20FMK 3, mid-IR4 (MRS_LONG)

Short (A)

17.66 - 20.92 μm

15166.5

Adjust Exposure Parameters to Achieve Desired SNR

As stated above, all four channels for a given grating setting have the same exposure time. Above, we determined the minimum number of groups that give us a peak SNR of 10 for emission lines and dust features covered by the various MRS channels. As an example, if we focus on the Long (C) grating for all three knots (Calculations #6, 9 11, 12, 15, 16, 18), we see that the greatest exposure time is Calculation 9 (Channel 4), with 73 groups per integration (810.31 s). We will thus choose 73 groups for all Long (C) grating observations for this program, where we estimate a peak SNR of 10 for the  OIV line at 25.9 μm (Channel 4C) in FMK 1, with higher peak SNR values for the OIV lines in FMK 2 and 3, and higher peak SNR values for the SIV line at 10.5 μm (Channel 2C) in all three knots.

Similarly, we find the optimal number of groups for the Medium (B) grating to be 50, where the ArIII line at 9.0 μm in FMK1 will have a peak pixel SNR of 10 (Channel 2; Calculation #5), with higher peak SNR values for the other knots. The number of groups we choose for the Short (A) grating is 33, so that the peak dust emission feature at 20.6 μm has a SNR of 10 (Calculation #8; Channel 4) and the peak pixel SNR of the NeII line at 12.8 μm (Channel 3) exceeds 10 (Calculation #7).

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 F1130W filter, as this allows observing the brightest stars without saturation, since most stars will not have significant excess emission above that from the stellar photosphere.

Similar to the MIRI MRS calculations, we chose Medium for Background Configuration and entered the coordinates for each knot.

Select Instrument Parameters

Here we assess the SNR in calculation #21:

  • "Instrument Setup" tab - Filter is set to F1130W
  • "Detector Setup" tab -
    • subarray is set to Full
    • we choose the FAST readout pattern because it needs 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 73 for the Long grating setting, 50 for the Medium grating setting, and 33 for the Short grating setting, the same as for the MRS observations.
  • "Strategy" tab -
    • The 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 = Y = 0
    • "Aperture radius" is set to 0.3", 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", respectively

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 11.0 magnitudes.  So K = 11 is the saturation limit for the simultaneous imaging obtained for the Long grating setting.  Even brighter stars could therefore be observed in simultaneous imaging without saturation for the Medium and Short grating settings, for which the "groups per integration" is less.

Select NIRSpec IFU Calculation

NIRSpec IFU observations are taken using the G140H grating with the F100LP filter, as discussed in the main article.  Our goal is to detect as many emission lines as possible.  With G140H/F100LP, this includes [CI] at 0.9824 and 0.9850 μm, [SII] at 1.0287–1.0370 μm, and various lines longward of the [SII] line out to ~ 1.9 μm as reported by Gerardy and Fesen (2001).  We run ETC calculations for NIRSpec IFU spectroscopy with this grating and filter combination to determine the exposure parameters we need to achieve the desired SNR of >10 at the peak pixel at the peak wavelength on the [SII] line at ~ 1.03285 microns.

Similar to the MIRI MRS calculations, we chose Medium for Background Configuration and entered the coordinates for each knot.

Select Instrument Parameters

Here we assess the SNR.  For FMK1, we focus on Calculation 1 (for FMK2 and FMK3, we would focus on Calculations 3 and 4):

  • "Instrument Setup" tab - Grating/Filter Pair is set to G140H/F100LP
  • "Detector Setup" tab -
    • subarray is set to "Full" (only Full frame readout is supported for the NIRSpec IFU);
    • we chose the NRSIRS2 Readout Pattern 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
    • number of "integrations per exposure" is set to 1
    • number of "groups per integration" is set to 11, 7, and 7 for FMK 1, FMK 2, and FMK 3, respectively, in Calculations 1, 3, and 4, respectively, in order to achieve the desired SNR (SNR > 10 at the peak pixel at the peak wavelength) for all emission lines of interest from these FMKs.  The observations for the three FMKs are all separate, and they are not combined into a target group (unlike the MRS observations), so they do not require the same exposure parameters.
    • running Calculation 1 for FMK 1 with these parameters gives a SNR of ~ 10 at the peak wavelength of the [SII] feature between 1.0287–1.037 μm wavelength (see Hurford and Fesen 1996).  For FMK 2 and FMK 3, the SNR values at the peak pixel at the peak wavelength is also ~ 10 for this line.  These SNR values are provided graphically in the plot displayed under the 2-D SNR tab in the Images pane at the lower-left of the screen when the Calculations tab of the workbook is selected.
  • "Strategy" tab -
    • We selected the "IFU Nod Off Scene" option, as this is the option corresponding to using a dedicated sky observation.
    • Aperture location was set to "Specify position in scene", with X = Y = 0
    • Aperture radius was set to 1" for all bands

Determine if a Dedicated Background is Needed for NIRSpec IFU Observations

After downloading the .tgz file for each of the NIRSpec IFU calculations (Calculations 1, 3, and 4) and expanding the file, within the /cube/model/ folder are files with a naming convention of cube_flux_N.fits and cube_flux_plus_background_N.fits, where N is between 0 and 29, inclusive.  These are the y-axis slices of the scene cube assumed by ETC.  We added the cubes from all the cube_flux_N.fits files together, and this gives the total scene cube assumed by ETC containing just the science target (i.e., the FMK) emission.  We did the same for the cube_flux_plus_background_N.fits files to get the total scene cube assumed by ETC containing both science target (FMK) emission and background emission.  From this exercise, it was determined that, over the spectral FWHM span of the 1.03 μm [SII] line for all knots, at the edge of the 3" × 3" field probed by the ETC (which is within the spatial FWHM range for FMK1 and comparable to the spatial FWHM range for FMK2 and FMK3), the background surface brightness is ~10% of that of the total (background plus FMK) surface brightness.  This is a sufficiently high level of background emission relative to science target emission that, to be safe, it was decided to add dedicated background observations for the NIRSpec IFU observations, in order to subtract the contaminating background emission.

For more on strategies to deal with NIRSpec backgrounds, please read this article.

With the exposure parameters now determined for this program, we can populate the observation template in APT.  See the Step-by-Step APT Guide for MIRI MRS and NIRSpec IFU Observations of Cassiopeia A to complete the proposal preparation for this example science program.



References

Ennis, J. A., et al., 2006, ApJ, 652, 376
Spitzer IRAC Images and Sample Spectra of Cassiopeia A's Explosion

Gerardy, C. L., & Fesen, R. A., 2001, AJ, 121, 27812781
Near-Infrared Spectroscopy of the Cassiopeia A and Kepler Supernova Remnants

Hurford, A. P., & Fesen, R. A., 1996, ApJ, 469, 246
Reddening Measurements and Physical Conditions for Cassiopeia A from Optical and Near-Infrared Spectra

Kilpatrick, C. D., et al., 2014, ApJ, 796, 144
Interaction between Cassiopeia A and Nearby Molecular Clouds

Rho, J., et al., 2008, ApJ, 673, 271
Freshly Formed Dust in the Cassiopeia A Supernova Remnant as Revealed by the Spitzer Space Telescope

Smith, J. D. T., et al., 2007, PASP, 119, 1133
Spectral Mapping Reconstruction of Extended Sources




Published

  

Latest updates