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.

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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) 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 "#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).

Define Sources for MIRI Simultaneous Imaging

We assume the same type of star for simultaneous imaging as for the MRS target, as late M stars are common stars.  The only difference is that, under the "Renorm" tab, under "Normalize in Bandpass", we set Johnson K-band magnitude to 11.5 (Vega).  This source is named "Imager Star" and is assigned to the "Simultaneous Imaging" scene.



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 1 (Calculation # 8, 10, and 12); (1.5", 1.5") for Channel 2 (Calculation # 7, 9, and 11); (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 Channels 1, 2, and 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.  Attaining this minimal SNR for Channels 3 and 4 first, we should obtain even greater SNR for Channels 1 and 2 because of the shape of the stellar photosphere spectrum.  The number of integrations and groups for channels 1 and 2 of the MRS do not have to be the same as those for MRS channels 3 and 4.  The only requirement on this is that the total exposure time should be the same for Channels 1/2 versus Channels 3/4.  This is useful in this example science program because the star is so much brighter at shorter wavelengths in the MRS than longer wavelengths.  The ETC shows that, when the same number of groups and integrations are used for all 4 MRS channels, pixels begin to saturate in MRS Channel 1 when the number of groups is set so that SNR=25 at 26.0 microns wavelength in Channel 4 is achieved.  By using more integrations and less groups for Channels 1/2 than for Channels 3/4 of MRS, saturation at shorter wavelengths can be avoided.
    • We wanted to avoid saturation warnings in the ETC, though we note that, for MIRI, as long as saturation does not happen within 5 groups in an integration, the data is still usable even if the groups past the 5th group are saturated (for more, please see the MIRI Cross-Mode Recommended Strategies article).  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
      • For a given grating setting, if we could set the "groups per integration" for all 4 MRS channels to achieve the desired S/N for the "integrations per exposure" equal to 1 without the ETC returning a saturation warning (though the end of the ramp being saturated does not mean the data is unusable; please see above), then we used these "groups per integration" and "integrations per exposure" values.  If not, then, if the saturation happened in the shorter-wavelength channels, we increased the number of "integrations per exposure" for Channels 1/2 by 1 and tried again (leaving the "integrations per exposure" and "groups per integration" for Channels 3/4 alone).
      • This iterative process found that, by setting the number of "integrations per exposure" to 1, the number of "groups per integration" needed to be 27 for the SHORT grating setting (Calculation # 3, 4, 11, and 12) and 59 for the MEDIUM grating setting (Calculation # 2, 5, 9, and 10) to achieve the desired S/N.  For the LONG grating setting (Calculation # 1, 6, 7, and 8), by setting the "integrations per exposure" to 2 and the "groups per integration" to 49 for Channels 1/2 and "integrations per exposure" to 1 and "groups per integration" to 98 for Channels 3/4, we were able to avoid saturation warnings and achieve the desired S/N.

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


Band

exposures per

specification

integrations

per exposure

groups per

integration

1A2127
1B2159
1C2249
2A2127
2B2159
2C2249
3A2127
3B2159
3C2198
4A2127
4B2159
4C2198

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 choose Medium for Background Configuration.


Select Instrument Parameters

Here we assess the SNR in calculation #14:

  • "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 (matching the MRS Channel 3/4 value for its Long grating setting)
    • number of "groups per integration" is set to 98 (again, matching the MRS Channel 3/4 value for its Long grating setting)
  • "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 11.5 magnitudes.  So K = 11.5 is the saturation limit for the simultaneous imaging obtained for the Long grating setting.  Somewhat 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.


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;
  • "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.  We set the number of groups to the relatively low value of 6;
  • "Strategy" tab - the only field under this tab for target acquisition is "Aperture centered on source", which for our case is set to 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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