Step-by-Step ETC Guide for MIRI MRS Spectroscopy of a Late M Star

A walk-through of the JWST Exposure Time Calculator (ETC) for the MIRI-MRS Spectroscopy of a Late M Star example science program is provided, demonstrating how to select exposure parameters for this observing program.

On this page

Main article: MIRI MRS Spectroscopy of a Late M Star, JWST ETC Exposure Time Calculator Overview
See also: Video Tutorials

The JWST Exposure Time Calculator (ETC) 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 IFU Spectroscopy of a Late M Star" Example Science Program, we focus on selecting exposure parameters for MIRI-MRS.  We start by defining a scene with a point source that is a late M star.  We show how to run ETC calculations to achieve the desired SNR for MIRI-MRS. 

The optimal exposure specifications (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 "#28: MIRI MRS Spectroscopy of a Late M Star" 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 and Scene in the ETC

Main articles: JWST ETC Defining a New Source, JWST ETC Source Spectral Energy Distribution

We first set up a scene with a single late M star.  We define the following source in ETC:

  • Star: a point source star with a spectrum described by a PHOENIX Stellar Model for a M5V star of effective temperature 3,500 K and log(g) = 5.0.  The star has zero redshift and zero extinction, and it is normalized to be 100 mJy at 10.0 μm wavelength.

Note that the position of the source in the scene can be viewed in the lower left "Scene Sketch" pane.  By checking the checkbox in the "Plot" column in the "Select a Source" pane, the SED of the selected source can be plotted (note: it may be helpful to limit the wavelength axis to the range relevant to the MIRI-MRS mode; i.e., 4.9–28.8 µm).

Select MIRI Medium Resolution Spectroscopy (MRS) Calculation

Main articles: JWST ETC Creating a New CalculationJWST ETC Backgrounds

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.

Our goal is to detect the late M star at a SNR > 25 for all wavelengths shortward of 26.0 μm, so we run ETC calculations for MIRI MRS Spectroscopy for all three grating settings of all four MRS channels to determine the exposure parameters we need to achieve this SNR.

Since the JWST background is position-dependent, fully specifying background parameters is important for the most accurate SNR calculation.  We therefore selected "Medium" for "Background configuration", which corresponds to the 50th percentile of the sky background.  Though we don't provide RA and Dec for a specific star, we provide generic RA and Dec coordinates of 04:30:00 and +25:00:00, respectively.

Select Instrument Parameters

Main article: JWST ETC IFU Nod in Scene and IFU Nod off Scene Strategy

Here we set up the ETC workbook for MRS calculations:

  • "Instrument Setup" tab - Calculation #1 shows our initial calculation to assess the SNR for Band 3C.  Here, the channel is set to Channel 3 (MRS_Long), and Wavelength Range is set to Long (C) for Band 3C (and would be sent to other Channels and Wavelength Ranges for other Bands).
  • "Detector Setup" tab -
    • Subarray is set to "Full" (only full frame readout is supported for the MRS).
    • Number of "exposures per specification" is set to 2 because the Nod In Scene option chosen under the Strategy tab assumes a 2-point dither.  By so doing, we simulate a 4-point dither in ETC.
    • We chose the Fast Readout Pattern.
  • "Strategy" tab-
    • We selected the "IFU Nod in Scene" option.  This allows for background subtraction using the other dither in the 2-point dither (assumed by ETC for "IFU Nod in Scene").  The source is detected in both dithers, and the "IFU Nod in Scene" option assumes the source detected in both dithers is coadded.  This makes it easier to simulate the 4-point dither pattern (i.e., by setting "exposures per specification" to 2, as noted above).
    • Aperture location was set to "Centered on source", with the only choice in the drop-down menu being "default source from default source/scene wb".
    • Aperture radius was set to 0.5" for Channels 1 (Calculation # 8, 10, and 12) and 2 (Calculation # 7, 9, and 11) and 0.75" for Channels 3 (Calculation # 1, 2, and 4) and 4 (Calculation # 3, 5, and 6).  The radius is larger for the longer-wavelength channels because the PSF grows larger at longer wavelengths.
    • Nod Position in Scene was set to (1", 1") for Channel 2 (Calculation # 7, 9, and 11); (1.5", 1.5") for Channel 1 (Calculation # 8, 10, and 12); (1.75", 1.75") for Channel 3 (Calculation # 1, 2, and 4); and (2.25", 2.25") for Channel 4 (Calculation # 3, 5, and 6).  As with the aperture radius, we increase the x and y coordinates of the nod position in scene with wavelength, in order to account for the larger PSF at longer wavelengths.

Adjust Exposure Parameters to Obtain Desired Signal-to-Noise Ratio

Main article: JWST ETC Batch Expansions

Here we assess the SNR:

  • "Detector Setup" tab -
    • We chose the Fast Readout Pattern because the same SNR could be achieved with slightly less total exposure time than for the SLOW readout pattern.  We tested this by fixing the number of exposures and groups, keeping the same subarray (Full), and finding the number of groups needed for each of SLOW and FAST readout pattern to achieve the same SNR at the longest wavelength of the band.
    • Because the stellar photosphere spectrum monotonically decreases with wavelength, in general, we begin our ETC experiment by determining exposure parameters for each grating setting of Channels 3 and 4, since we want SNR > 25 for all 3 grating settings of Channel 3 and the Short and Medium grating settings of Channel 4, and SNR > 25 for all wavelengths < 26 μm in Channel 4 with the Long grating setting.  The exposure parameters must be the same for a single grating setting for all 4 channels of MRS because data is obtained for all 4 MRS channels in a single observation.  Then we check the ETC calculations for Channels 1 and 2 to make sure there are no saturation warnings.
    • We wanted to avoid setting the number of "groups per integration" to a number that was too close to when ETC would give a saturation warning.  So we took the following approach:
      • First, we set the "integrations per exposure" to 1, to see if the desired SNR could be achieved by increasing the "groups per integration" but without returning any saturation warnings in ETC
      • If the desired SNR could be achieved thusly, we continued increasing the "groups per integration" until ETC returned a saturation warning.
      • We wanted to avoid a value of "groups per integration" too close to the value that returns ETC saturation warnings, to be safe, so we set a goal that the value of "groups per integration" should be about half the value that causes ETC saturation warnings.  If the value of "groups per integration" that achieved the desired SNR for "integrations per exposure" = 1 was about half the value of "groups per integration" that caused ETC saturation warnings, then this was the value of "groups per integration" used for the given grating setting.  If not, we increased the "integrations per exposure" by 1 and determined a new value of "groups per integration" to achieve the desired SNR.  If this new value of "groups per integration" was about half the value of "groups per integration" that caused ETC saturation warnings, then this was the value of "groups per integration" used for the given grating setting.  If not, "integrations per exposure" was increased by 1, and the "groups per integration" test was performed again.  This process was repeated until a value of "integrations per exposure" was found that resulted in a value of "groups per integration" that was about half the value of "groups per integration" that caused ETC saturation warnings.
      • This iterative process found that the number of "groups per integration" needed to be 27 for the SHORT grating setting (Calculation # 3, 4, 11, and 12), 37 for the MEDIUM grating setting (Calculation # 2, 5, 9, and 10), and 32 for the LONG grating setting (Calculation # 1, 6, 7, and 8).
      • This process also found the number of "integrations per exposure" to be 1 for the SHORT grating setting (Calculation # 3, 4, 11, and 12), 3 for the MEDIUM grating setting (Calculation # 2, 5, 9, and 10), and 5 for the LONG grating setting (Calculation # 1, 6, 7, and 8).

Here is a table of the exposure parameters for the Late M Star program found from the above-described experimentation in ETC:


exposures per



per exposure

groups per



Target Acquisition Calculation

Main articles: MIRI MRS Target Acquisition, JWST ETC MIRI Target Acquisition

A target aquisition (TA) is required in order to position precisely the target on the detector.  TA for the MRS improves pointing precision to 90 mas.

Calculation #13, where we selected "Target Acquisition" under the MIRI pull-down menu, shows our initial calculation to determine which parameters to specify for TA for the target star.

  • "Backgrounds" tab - we entered the coordinates for the generic star (04:30:00 +25:00:00) and selected "Medium" for "Background configuration";
  • "Instrument Setup" tab - we updated the selection of "Acq Mode" to "TA for LRS-Slit, MRS and LRS-Slitless".  For Filter, we chose FND (Neutral Density) filter and set the number of groups to the relatively low value of 6;
  • "Detector Setup" tab - the only option for subarray for MRS TA is Full, so we choose Full.  We choose the FAST Readout pattern, as the FASTGRPAVG pattern results in longer exposure times.  The only option for Integrations and Exposures are 1 for each;
  • "Strategy" tab - the only permissible option for target acquisition is "Aperture centered on source", which for our case is the Late M Star

From this calculation, we see the SNR is ~175.

Run ETC Calculation

Running the calculations with these parameters gives SNR of 25 at the longest wavelengths of Bands 4A and 4B and at 26.0 μm in Band 4C.  At wavelengths short-ward of these wavelengths in Bands 4A, 4B, and 4C, the SNR is greater than 25, and at all wavelengths in Bands 1A, 1B, 1C, 2A, 2B, 2C, 3A, 3B, and 3C, the SNR is much greater than 25.  These SNR values are provided graphically in the plot displayed under the 2D SNR tab in the Images pane at the lower-left of the screen when the Calculations tab of the workbook is selected.

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 Spectroscopy of a Late M Star to complete the proposal preparation for this example science program.



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