Step-by-Step ETC Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids

This article provides a walk-through of the construction of an ETC workbook for an example moving target program to observe near-Earth asteroids (NEAs) and should be read after NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids. The Step-by-Step APT Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids should be read after this article.

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Using the ETC

The ETC workbook associated with this Example Science Program is called "#34: NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids" 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 1.5. There may be subtle differences if using a different version of the ETC.

Step 1: Define Sources and Scenes in the ETC

Main article:Moving Target ETC InstructionsJWST ETC Scenes and Sources Overview
See also: JWST ETC User Supplied Spectra

There are no moving target-specific options in the ETC. Spectral energy distributions (SEDs) for your targets must either be created using the built-in options in the ETC or computed offline and uploaded to the ETC. There is currently no functionality in the ETC to include absorption features; only emission features can be included at this time.

It is highly recommended that you compute your own SEDs to ensure the right scaling between the reflected light and thermal emission components, as well as to account for any absorption features of interest. For NEAs, it is even more important to compute your own SEDs due to phase angle effects, which cannot be accounted for in the ETC using the template spectra alone.

(3908) Nyx Scene

On the Scenes and Sources tab, select "Scene 1," the default scene, in the Select a Scene pane. The default source already in the scene is called "default source from default source/scene wb." On the ID tab in the Source Editor pane, rename the default scene to "(3908) Nyx Scene" and the default source to "(3908) Nyx Reflected." Add a new source to this scene by clicking the New button in the Select a Source pane, then highlight the scene and source and click the Add Source button in the Select a Scene pane. Select the "(3908) Nyx" scene and the new source, called "Source 2", and name the source "(3908) Nyx Thermal" (the scene only needs to be named once). In the associated ETC workbook, this is scene 1 and sources 1 and 2.

(3908) Nyx Reflected Source

  • Go to the Continuum tab and select the Phoenix Stellar Model option as the Spectral Energy Distribution.
  • Select G2V 5750 4.5 from the dropdown list of stellar types. This is the stellar model that best represents the Sun.
  • Next, on the Renorm tab, click the button to select the Normalize in bandpass option.
  • The normalization units are set to flam by default; change these to vegamag units from the dropdown.
  • Select the sub-option Other and choose Johnson and V from their respective dropdown menus. Based on the values from JPL/Horizons during Cycle 1, (3908) Nyx should be normalized to V = 22.0.
  • No changes to other tabs are needed, and the SED can be viewed in the Source Spectrum Plots panel.

(3908) Nyx Thermal Source

  • Go to the Continuum tab and select a Blackbody Spectrum as the Spectral Energy Distribution.
  • We take the brightness temperature (Tb) to be equal to the equilibrium temperature at the distance of the NEA in Cycle 1. For (3908) Nyx, we calculate the equilibrium temperature to be 155 K (assuming a geometric albedo of 0.23).
  • On the Renorm tab, we will choose the Normalize at wavelength option this time. Based on thermal modeling code, we will normalize the blackbody to 0.34 mJy at 30 µm. (Improper normalization of the thermal component can significantly alter the results. Hence the recommendation above to create your own SEDs and upload them, given that you will have to go through most of the same steps just to normalize the thermal component correctly anyway.)
  • The two SEDs, reflected and thermal, can be viewed together in the same plot by checking the boxes next to each source.

(1221) Amor Scene

On the Scenes and Sources tab, create a new scene using the New button in the Select a Scene pane. Add a new source to this scene by clicking the New button in the Select a Source pane, then highlight the scene and source and click the Add Source button in the Select a Scene pane. On the ID tab in the Source Editor pane, name the scene "(1221) Amor Scene" and the source "(1221) Amor Reflected." Repeat this process to add a second source to the "(1221) Amor Scene" and name the source "(1221) Amor Thermal" (the scene only needs to be named once). In the associated ETC workbook, this is scene 2 and sources 3 and 4.

(1221) Amor Reflected Source

To create the "(1221) Amor Reflected" source, follow the same process as described above for the "(3908) Nyx Reflected" source. Normalize the SED to V=22.0.

(1221) Amor Thermal Source

To create the "(1221) Amor Thermal" source, follow the same process as described above for the "(3908) Nyx Thermal" source. The equilibrium temperature of (1221) Amor is 170 K and it has a geometric albedo of 0.15. The flux density at 30 µm is modeled at 1.55 mJy.

(1915) Quetzalcoatl Scene

On the Scenes and Sources tab, create a new scene using the New button in the Select a Scene pane. Add a new source to this scene by clicking the New button in the Select a Source pane, then highlight the scene and source and click the Add Source button in the Select a Scene pane. On the ID tab in the Source Editor pane, name the scene "(1915) Quetzalcoatl Scene" and the source "(1915) Quetzalcoatl Reflected." Repeat this process to add a second source to the "(1915) Quetzalcoatl Scene" and name the source "(1915) Quetzalcoatl Thermal" (the scene only needs to be named once). In the associated ETC workbook, this is scene 3 and sources 5 and 6.

(1915) Quetzalcoatl Reflected Source

Follow the same process as described above for the "(3908) Nyx Reflected" source. Normalize the SED to V = 24.0.

(1915) Quetzalcoatl Thermal Source

To create the "(1915) Quetzalcoatl Thermal" source, follow the same process as described above for the "(3908) Nyx Thermal" source. The equilibrium temperature of (1915) Quetzalcoatl is 150 K and it has a geometric albedo of 0.21. The flux density at 30 µm is modeled at 0.28 mJy.



Step 2: Select NIRSpec Fixed Slit Calculation

Main article: JWST ETC Calculations Page OverviewJWST ETC Images and Plots 
See also: NIRSpec Fixed Slits Spectroscopy

In a real proposal, the SNR requirement and the required spectral resolution may be set by the properties of specific absorption features of interest. For the purpose of this example proposal, we will adopt an SNR requirement of >20 over as large a portion of the spectrum as possible. Due to instrument throughputs, it is prohibitively expensive to obtain SNR > 20 at every wavelength. We will let this requirement guide our choice of grating/filter combination as well.

(3908) Nyx

  • On the Calculations tab, click on the NIRSpec dropdown and create a Fixed Slit/BOTS calculation (Calculation 1).
  • Highlight Calculation 1 and click on the Scene tab.
  • In the Scene for Calculation dropdown, assign Scene 1, "(3908) Nyx Scene," to Calculation 1.
  • Don't change anything in the Sources in that Scene dropdown (this is only used if you would like to change properties of a source on the Calculations tab rather than the Sources and Scenes page).
  • Change the Background configuration in the Backgrounds tab to High. Observations along the ecliptic are more likely to be subject to high backgrounds due to zodiacal light.
  • Move to the Instrument Setup tab.
  • Select S200A1 (0.2" x 3.3") (FS only) for the Slit.
  • Select Prism/CLEAR for the Grating/Filter Pair. This grating/filter combination provides the lowest spectral resolution (R ~ 100) but also allows spectra to be obtained across the entire 0.6–5.3 µm range in one setting. We start with this grating/filter combination in order to see how easy it is to reach our desired SNR. If the SNR with the Prism is very high (e.g., >100) in a reasonable amount of time then it would likely be safe to move to the medium-resolution gratings (G140M/F070LP, etc.). There is a trial-and-error aspect to determining the best combination of settings on the Instrument Setup and Detector Setup tabs.
  • Move to the Detector Setup tab.
  • There are three Subarray options available for the S200A1 slit: SUBS200A1ALLSLITS, and FULL. The SUBS200A1 subarray only reads out the S200A1 detector region, the ALLSLITS option reads out all the fixed slit regions, and FULL reads out the entire NIRSpec detector area. According to the NIRSpec Detector Recommended Strategies, the optimal Readout pattern is NRSIRS2RAPID, and that pattern can only be used with the FULL subarray.

At the present time, the fixed slit calculations for (3908) Nyx and (1221) Amor in the ETC workbook both use the NRSRAPID readout pattern. This does not align with the best practices, and we suggest you read the NIRSpec Detector Recommended Strategies to learn more. The rest of this article uses the optimal parameters.

  • Set the Subarray to FULL.
  • Set the Readout pattern to NRSIRS2RAPID.

(1221) Amor

  • On the Calculations tab, click on the NIRSpec dropdown and create a Fixed Slit/BOTS calculation (Calculation 2).
  • Highlight Calculation 2 and click on the Scene tab.
  • In the Scene for Calculation dropdown, assign Scene 2, "(1221) Amor Scene," to Calculation 2.
  • Don't change anything in the Sources in that Scene dropdown.
  • Change the Background configuration in the Backgrounds tab to High.
  • Move to the Instrument Setup tab.
  • Select S200A1 (0.2" × 3.3") (FS only) for the Slit.
  • Select Prism/CLEAR for the Grating/Filter Pair.
  • Set the Subarray to FULL.
  • Set the Readout pattern to NRSIRS2RAPID.



Step 3: Select NIRSpec IFU Calculation

Main article: NIRSpec IFU Spectroscopy

As in Step 2 above, we adopt an SNR requirement of >20 over as large a portion of the spectrum as possible for the NIRSpec IFU calculation.

(1915) Quetzalcoatl

  • On the Calculations tab, click on the NIRSpec dropdown and create an IFU calculation (Calculation 3).
  • Highlight Calculation 3 and click on the Scene tab.
  • In the Scene for Calculation dropdown, assign Scene 3, "(1915) Quetzalcoatl Scene," Calculation 3.
  • Don't change anything in the Sources in that Scene dropdown.
  • Change the Background configuration in the Backgrounds tab to High.
  • Move to the Instrument Setup tab.
  • Select Prism/CLEAR for the Grating/Filter Pair.
  • Move to the Detector Setup tab.
  • FULL is the only Subarray option for IFU calculations, so no changes to the Subarray need to be made.
  • The optimal Readout pattern is NRSIRS2RAPID.



Step 4: Select Instrument Parameters

The ETC does not support dithering, but the number placed in the Exposures per specification box is used as a stand-in for dither positions. In other words, the noise calculated will decrease in accordance with the number of "dither positions" specified in this box.

(3908) Nyx

  • Move to the Strategy tab for Calculation 1.
  • We will change the Aperture Half-Height because the default value does not provide the highest SNR. This parameter determines the size of the aperture used to extract signal from the source.
  • Leaving the parameters on the Detector Setup tab unchanged and varying the aperture value, we find that the optimal value that results in the highest SNR, while still effectively sampling the PSF, is 0.10" for the Aperture Half-Height; any value between 0.10" and 0.15" is a reasonable choice.
  • Move to the Detector Setup tab.
  • As per the NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids, set the Exposures per specification to 3.
  • We are now ready to experiment with different values for the Groups per integration and Integrations per exposure. The general strategy here is to increase the number of groups until saturation is reached, then increase the number of integrations until the desired SNR is reached. The SNR increases at a faster rate by increasing the number of groups.
  • As noted by the Maximum Number of Groups Before Saturation value in the Reports pane, saturation will not be reached for (3908) Nyx so the Integrations per exposure should remain set to 1 for all calculations.
  • After experimentation, we find that a Groups per integration value of 40 results in an SNR > 20 from ~0.6–4.0 µm. This results in ~30 minutes of time on (3908) Nyx.

(1221) Amor

  • Move to the Strategy tab for Calculation 2.
  • Leaving the parameters on the Detector Setup tab unchanged and varying the aperture value, we find that the optimal value that results in the highest SNR, while still effectively sampling the PSF, is 0.10" for the Aperture Half-Height; any value between 0.10" and 0.15" is a reasonable choice.
  • Move to the Detector Setup tab.
  • As per the NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids, set the Exposures per specification to 3.
  • We are now ready to experiment with different values for the Groups per integration and Integrations per exposure.
  • For (1221) Amor, saturation will not be reached so the Integrations per exposure should remain set to 1 for all calculations.
  • After experimentation, we find that a Groups per integration value of 40 results in an SNR > 20 from ~0.6–4.0 µm. This results in ~30 minutes of time on (1221) Amor.

(1915) Quetzalcoatl

  • Move to the Strategy tab for Calculation 3.
  • Change the strategy from IFU Nod In Scene to IFU Nod Off Scene. The first option is the equivalent of AB subtraction for long slit observations and is a valid option for point sources. The second option mimics the subtraction of a dedicated background and is the best option for extended sources. However, for practical reasons, the IFU Nod Off Scene strategy is selected to eliminate the need to determine the optimal offset distance between the A and B positions for the IFU Nod In Scene strategy. Additionally, IFU Nod Off Scene is closer to the aperture photometry method that would most likely be used to analyze IFU data (think photometry on each image to build up a full spectrum without subtracting another science observation or a dedicated background). See JWST ETC IFU Nod in Scene and IFU Nod off Scene Strategy for more details on the two currently available extraction strategies for IFU observations.
  • Leaving the parameters on the Detector Setup tab unchanged and varying these aperture values, we find that the optimal value that results in the highest SNR, while still effectively sampling the PSF, is 0.25" for the Aperture radius; any value between 0.25" and 0.30" is a reasonable choice.
  • Move to the Detector Setup tab.
  • As per the NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids, set the Exposures per specification to 4.
  • We are now ready to experiment with different values for the Groups per integration and Integrations per exposure.
  • For (1915) Quetzalcoatl, saturation will not be reached so the Integrations per exposure should remain set to 1 for all calculations.
  • After experimentation, we find that a Groups per integration value of 123 results in an SNR > 20 from 0.6–2.5 µm (and SNR > 10 from 0.6–3.5). This results in 2 hours of time on (1915) Quetzalcoatl. This target is 2 magnitudes fainter than (3908) Nyx and (1221) Amor and the IFU has lower throughput than the fixed slits, hence why the SNR is so much lower for so much additional time.



Step 5: Target Acquisition

Main article: JWST ETC Target Acquisition

We will be performing target acquisition (TA) for both the (3908) Nyx and (1221) Amor fixed slit observations. These targets are normalized to the same magnitude (V = 22) in the near-infrared, so the calculation described below will apply to both targets. To determine the parameters for a successful TA:

  • Create a 4th calculation by selecting Target Acquisition from the NIRSpec dropdown menu.
  • On the Scene tab, select the "(1221) Amor Scene" as the Scene for Calculation.
  • Select a High background.
  • On the Instrument Setup tab, the only Acq Mode possible for moving targets is WATA and the CLEAR Filter will provide the highest throughput for a relatively faint target.
  • No changes need to be made on the Strategy tab.
  • The unique nature of NIRSpec TA prevents the user from changing the number of GroupsIntegrations, or Exposures. These parameters are fixed at 3, 1, and 1, respectively.
  • The only parameters that can be altered on the Detector Setup tab are the Subarray and Readout  pattern. The SNR must be at least 20 to ensure that TA will not fail, and experimentation with the parameters shows that the SUB2048 Subarray and the NRSRAPIDD6 Readout pattern result in an SNR of >80 in only ~12 s. This is more than sufficient for TA.

Now it is time to put everything together in an Astronomers Proposal Tool (APT) file. Please see the Step-by-Step APT Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids article to continue with this step.



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