Define scenes and sources in the ETC

See also: JWST ETC Defining a New Source, JWST ETC Defining a New Scene, JWST ETC User Supplied Spectra, JWST ETC Defining an Extended Source

Define sources for MIRI MRS

Set up 3 scenes with 3 extended sources to represent the 3 knots. Define the following extended sources in the 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 2. Description of sources used for MIRI MRS Calculations

The values of σ_x and σ_y 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, select Do not renormalize under the Renorm tab in the Source Editor.

Define sources for MIRI simultaneous imaging

Assume that most stars in the MIRI imager field of view will be late M stars, so under the Continuum tab of the Source Editor pane, choose Phoenix Stellar Models and M5V 3500K 5.0; redshift and extinction are set to 0.  Under the Renorm tab, select Normalize in Bandpass, and normalize to Johnson K-band magnitude of 10.5 (Vegamag). Under the Shape tab, choose Point, since you will be 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

See also: JWST ETC Calculations Page Overview, JWST ETC Creating a New Calculation, JWST ETC IFU Strategies, JWST ETC Images and Plots

In general, you will want to obtain IFU observations such that every pixel detecting radiation from the knot does so with SNR > 5 to give confidence in the detection.  So, in every IFU cube wavelength slice within the FWHM (spectrally speaking) of each line of interest, each pixel within the spatial FWHM of the knot should detect radiation from the knot at SNR > 5. 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 2^0.5 higher SNR at the peak pixel of the knot at that wavelength. Furthermore, the peak pixel at the peak wavelength of the line will have another factor 2^0.5 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 (2^0.5)² = 2 higher SNR than the pixel at the FWHM periphery of the knot at a half-power wavelength of the line, or SNR = 5 × 2 = 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 3 grating settings: SHORT (A), MEDIUM (B), and LONG (C). 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. Select Medium for Background configuration, which corresponds to the 50th percentile of the sky background. The positions entered for the FMKs are the following:

• FMK1 23:23:36.0700 +58:50:08.16
• 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 stated above, 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 needed to achieve this SNR threshold. 

All 4 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 3 knots, determine 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 3 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, choose the following parameters for all MIRI MRS Calculations:

• Detector Setup tab:
  • Subarray is set to FULL in the subarray menu since only full frame readout is supported for MRS.
  • Choose the FASTR1 readout pattern, since MIRI MRS Recommended Strategies indicates that this readout pattern is the default readout pattern.  In addition, the FASTR1 readout pattern requires a slightly lower total exposure time to achieve the same SNR as the SLOWR1 readout 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. Determine 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 is kept at the default value of 1;
  • Number of Total Dithers is set to 4 because a 4-point dithering pattern is assumed.
    
    
• Strategy tab:
  • Select the IFU On-Target + Off-Target Pointing option, as this is the option corresponding to using a dedicated sky observation, which is chosen for the MIRI MRS target group.
  • Aperture location is set to Specify position in scene, with X and Y set to 0, which is the same as centered on the source.
  • Aperture radius is set to 1 (arcsec).

Table 3. ETC Calculations for MIRI MRS observations to obtain a peak SNR = 10 for emission lines covered by channel/grating combination.  The calculations assume FASTR1 readout pattern unless otherwise indicated.

Adjust exposure parameters to achieve desired SNR

As stated above, all 4 channels for a given grating setting have the same exposure time. The minimum number of groups that give a peak SNR of 10 for emission lines and dust features covered by the various MRS channels was determined above. 

As an example, if you focus on the Long (C) grating for all 3 knots (Calculations #6, 9, 11, 12, 15, 16, 18), you'll see that the greatest exposure time is Calculation 9 (channel 4), with 245 Groups per integration (2719.54 s). You will thus choose 245 groups for all Long (C) grating observations for this program, where you 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.

• Detector Setup tab -
  • Subarray is set to FULL
  • Choose the FASTR1 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 you 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 245 for the Long grating setting, 48 for the Medium grating setting, and 31 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 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 (in arcsec), 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 13.5 magnitudes.  So K = 13.5 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.

Split MRS observations to avoid Data Volume issues

When the APT is designed with these MRS observations and Smart Accounting is run with the Visit Planner, there is an error in the MRS observation of the science targets concerning data volume.  The large number of Groups per integration for the Long grating causes the error, so this requires setting the readout pattern to SLOWR1 in ETC for all the calculations involving the Long grating setting.  When this is done, the calculation requiring the longest integration is still Calculation 9, with 30 Groups per integration (2866.79s).  After determining this, the exercise with the simultaneous imaging is repeated, and it is still found that K=13.5 is the saturation limit for the simultaneous imaging obtained along with the Long grating setting.

Select NIRSpec IFU Calculation

NIRSpec IFU observations are taken using the G140H grating with the F100LP filter, as discussed in the main article. The 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). Run ETC Calculations for NIRSpec IFU spectroscopy with this grating and filter combination to determine the exposure parameters needed to achieve the desired SNR of >10 at the peak pixel at the peak wavelength on the [SII] line at ~ 1.03285 μm.

Similar to the MIRI MRS Calculations, choose Medium for Background Configuration and enter the coordinates for each knot.

Select Instrument parameters

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

• 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);
  • Chosoe the NRSIRS2 Readout pattern because this readout pattern is an IRS2 readout pattern, which gives lower correlated noise
  • Number of Total Dithers is set to 4 because the IFU On-Target + Off-Target Pointing option was selected under the Strategy tab
  • Number of Integrations per exposure is set to 1
  • Number of Groups per integration is set to 8, 6, and 5 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. Unlike the MRS observations, the observations of the FMKs are obtained separately, not in a target group.
  • 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 -
  • Select the IFU On-Target + Off-Target Pointing option, as this is the option corresponding to using a dedicated sky observation.
  • Aperture location is set to Specify position in scene" with X and Y set to 0
  • Aperture radius is set to 1 (arcsec) 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.  Add 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. Do 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 is 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 is 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 NIRSpec Background Recommended Strategies.