Step-by-Step ETC Guide for MIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk

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A walk-through of the JWST Exposure Time Calculator for the "NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris DiskExample Science Program is provided, demonstrating how to select exposure parameters for this observing program. 

Introduction

Main article: NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris DiskJWST Exposure Time Calculator Overview
See also: JWST High-Contrast ImagingMIRI Coronagraphic Imaging 

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 which are used by the ETC to run calculations for the requested observing mode. Following the instructions in Stage 5 of the HCI Roadmap, we focus on selecting the exposure parameters to detect the Beta Pictoris debris disk at the desired signal-to-noise ratio (SNR) for the MIRI Coronagraphic Imaging and NIRCam Coronagraphic Imaging observing modes. An accompanying ETC workbook for this tutorial can be downloaded as a sample workbook from the ETC user interface and used for reference.

NB: This sample workbook is based on work prepared by the #1411 GTO Program coordinators.

The ETC workbook associated with this Example Science Program is called "#35: NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris Disk" and can be selected from the Get a Copy of an Example Science Program dropdown on the ETC Workbooks page to get the read only version. The nomenclature and reported SNR values in this article are based on ETC v. 1.5. There may be subtle differences if using an alternate version of ETC.

After determining the exposure specifications required to meet the science goals (e.g., numbers of groups and integrations), these will be input into the Astronomer's Proposal Tool (APT), which is used to specify an observing program and submit proposals. For instructions on how to implement this example science program into the APT, see the Step-by-Step APT Guide for NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris Disk.



 Define Sources and Scenes in ETC

Main article: JWST ETC Scenes and Sources Page Overview
See also: JWST ETC Defining a New SceneJWST ETC Defining a New Source

In order to perform our intended calculations, we must first build a library of relevant sources and scenes. In the ETC, scenes are idealized representations of spatial (two angular coordinates) and spectral brightness distributions, before being observed by a telescope. For MIRI and NIRCam Coronagraphic Imaging modes, scenes are composed of relatively small areas (8.19" and 3.14"/6.36", respectively) centered on the coronagraphic masks. When defined, a scene should contain the source targets of an observation, as well as all other nearby sources that could contribute to the observed target and background fluxes within that area.

The filter choice for this program includes the NIRCam F182M, F210M, F250M, F300M, F335M and F444W filters, as well as the MIRI's F1550C and F2300C coronagraphic filters (see parent article for scientific justification). In order to model the disk at these wavelengths we use a surface brightness model based on the results of Ballering et al. 2016. To create the model, we used the 1.16 μm scattered light surface brightness (shown in Fig. 3 of Ballering et al. 2016) scaled to other wavelengths by the flux of a stellar model of the Beta Pic star, and the 24 μm thermal emission surface brightness (shown in Fig. 5 of Ballering et al.) scaled to other wavelengths based on the provided disk SED (shown in Fig. 14 of Ballering et al. 2016). We then combined the scattered light model and the thermal emission model to get the total surface brightness model at all wavelengths.

Now that we are able to model the disk at a given wavelength, we define a different scene for each of the observed filters—referred to as our "science" scenes—in addition to a "reference" scene for performing our TA calculations for the reference PSF source. For each science scene, the target source (Beta Pic) will be placed a the center of the scene, with the reference PSF source (Alpha Pic) placed at a significant offset, e.g. 15 arcsec). For the reference scene, the central source—moreover the only source—will be the reference PSF source (Alpha Pic).

For our Beta Pic source (i.e. source ID #1 in Example Science Program workbook), we assume an A5I star (i.e. the "A5I 8500 2.0Phoenix Stellar Model) normalized to a known Vega magnitude of V = 3.9 in the Johnson filter. For Alpha Pic (i.e. source ID #2 in Example Science Program workbook) we assume a F0I spectral type star ("F0I 7750 2.0Phoenix Stellar Model) normalized to a Vega magnitude of V = 3.3 in the Johnson filter. Because the ETC allow users to define sources that are persistent and reusable, we need only define each source once and then associate it with each of the relevant scenes. 

MIRI Coronagraphic Imaging Scenes and Sources 

For our MIRI Coronagraphic observations we create 2 science scenes: a MIRI F1550C science scene and a MIRI F2300C science scene. Both scenes include Beta Pic (source #1) and Alpha Pic (source #2). For each scene, we define 5 extended sources with uniform brightness. Each source is placed at a different location and assigned a surface brightness according to the model at seperations of 2, 4, 6, 8 and 10 arcsec. The properties of these scenes and sources are defined as follows:

 #1: MIRI F1550C Science Scene

(i.e. Scene ID #1 in Example Science Program workbook)

We first define the source representing the 15.5 µm disk at a separation of 2", which we identify as "Disk 15.5mic, 2" sep" (i.e. source ID #3 in Example Science Program workbook). In the Shaptab, we change the default point source to an Extended source with a Flat profile defined by Semi-Major and Semi-Minor axes of "1" arcsec (i.e. a circular source), and select the Surface Brightness per square arcsec as the Normalization choice—this this will automatically change the units in the Renorm tab to read per arcsec^2. In the Continuum tab, we then define the source to have a Flat Continuum in fnu units and renormalize to its "15.5" µm surface brightness of "15" mJy per arcsec^2 in the Renorm tab. Lastly, ensuring that the source has been appropriately associated with the "MIRI F1550C science scene", we position the source at 2" from the target star by setting the X Offset to "2" arcsec.

Because the remaining four sources (used to model the 15.5 µm disk at 4, 6, 8 and 10 arcsec from the target star) will share many of the same properties, we can save some time by simply duplicating our "Disk 15.5mic, 2" sep" source—using the Copy Source option available under the drop-down menu of the Edit button at the top of the Scenes and Sources page—four times. Appropriately renaming the sources ("Disk 15.5mic, 4" sep", "Disk 15.5mic, 6" sep" etc) in the ID tab, we can then reposition each source in the scene in the Offset tab and assign it a 15.5 µm surface brightness according to the model at that location (in the Renorm tab). 

Note, because the ETC only allows for MIRI Coronagraphic Imaging calculations to be performed over a maximum scene size of 8.19", we cannot place our 6", 8" and 10" sources at the desired locations. Instead, we will position these sources at unique positions 4" from the target source in order to make an approximation of the achievable SNRs of the disk at these locations, based on their modeled surface brightnesses.

We define the remaining four sources (duplicates of our 2" sep source) by making the following alterations in the Configuration pane:

Source ID # in the Ex.Sci.Prog. Wb.Alterations
ID Tab:Renorm Tab:Offset:
4
  • Change name to "Disk 15.5mic, 4" sep" in the Source Identity Information field.  
  • re-normalize the source to its "15.5" µm surface brightness of "15" mJy per arcsec^2
  • Reposition the source at an X Offset of "2.8" arcsec
  • Reposition the source at a Y Offset of "-2.8" arcsec.
5
  • Change name to "Disk 15.5mic, 6" sep" in the Source Identity Information field. 
  • re-normalize the source to its "15.5" µm surface brightness of "8" mJy per arcsec^2
  • Reposition the source at an X Offset of "0" arcsec.
  • Reposition the source at a Y Offset of "-4" arcsec.
6
  • Change name to "Disk 15.5mic, 8" sep" in the Source Identity Information field. 
  • re-normalize the source to its "15.5" µm surface brightness of "5" mJy per arcsec^2
  • Reposition the source at an X Offset of "-4" arcsec.
  • Reposition the source at a Y Offset of "0" arcsec.
7
  • Change name to "Disk 15.5mic, 10" sep" in the Source Identity Information field. 
  • re-normalize the source to its "15.5" µm surface brightness of "3" mJy per arcsec^2
  • Reposition the source at an X Offset of "0" arcsec.
  • Reposition the source at a Y Offset of "4" arcsec.

Figure 1: Scene Sketch of "MIRI F1550C Science Scene"

 #2: MIRI F2300C Science Scene

(i.e. Scene ID #2 in Example Science Program Workbook)

As with the previous science scene, we create the five extended sources (uniform brightness, 1x1") to model the disk at locations 2–10" from the star. The 2, 4, 6, 8 and 10 arcsec sources are positioned in the same configuration as the previous scene ("MIRI F1550C science scene"), and are assigned the following values in the Renorm tab:

  • We renormalize the "Disk 23mic, 2" sep" source (i.e. source ID #8 in the Example Science Program workbook) to its "23" µm surface brightness of "42" mJy per arcsec^2 (according to the model at 2").
  • We renormalize the "Disk 23mic, 4" sep" source (i.e. ID #9) to a "23" µm surface brightness of "42" mJy per arcsec^2.
  • We renormalize the "Disk 23mic, 6" sep" source (i.e. ID #10) to a "23" µm surface brightness of "22" mJy per arcsec^2.
  • We renormalize the "Disk 23mic, 8" sep" source (i.e. ID #11) to a "23" µm surface brightness of "14" mJy per arcsec^2.
  • We renormalize the "Disk 23mic, 10" sep" source (i.e. ID #12) to a "23" µm surface brightness of "8" mJy per arcsec^2.

NIRCam Coronagraphic Imaging Scenes and Sources

For our NIRCam Coronagraphic Imaging observations we require 5 science scenes: "NIRCam F182M science scene"; "NIRCam F210M science scene"; "NIRCam F250M science scene"; "NIRCam F335M science scene" and "NIRCam F444W science scene". We define 5 extended sources with uniform brightness, defined by Semi-major and Semi-minor axes of "0.5" arcsec. Each source is placed at a different location and assigned a surface brightness according to the model at seperations of 1, 2, 4, 6 and 8 arcsec. The properties of these scenes and sources are defined as follows:

 

  #3: NIRCam F182M Science Scene

(Scene ID #3 in Example Science Program Workbook)

Starting with the source located at 1" from the target star (i.e. source ID #13 in the example science program workbook), we identify the source as "Disk 1.82mic, 1" sep" in the ID tab. We then define the Shape of source as Extended, the Flux distribution to be Flat, the Semi-Major and Semi-Minor axes as "0.5" arcsec, and the Normalization choice to be Surface Brightness per Square arcsec. In the Continuum tab, we select a Flat Continuum in fnu units and renormalize the source to its "1.82" µm surface brightness of "16" mJy per arcsec^2. To position the source at a separation of 1", we then place the source at an X Offset of "1" arcsec in the Offset Tab. For the remaining four sources, we duplicate our "1.82 mic disk, 1" sep" source and associate them with our NIRCam F182M science scene, adjusting their properties as follows:
Note that because the ETC only allows for NIRCam Coronagraphic calculations to be performed over a maximum scene size of 3.14" square, we cannot place our sources at the desired 4, 6 and 8". Instead, we position the sources at positions 2" away from our target source, such that we can at least make an approximation of the achievable SNRs of the disk at these locations, based on their modeled surface brightnesses.


Source ID #(including in the Ex.Sci.Prog. Wb.)

Alterations
ID Tab:Renorm Tab:Offset:
  • Change name to "1.82 mic disk, 1" sep" in the Source Identity Information field.   
  • re-normalize the source to its "1.82" µm surface brightness of "16" mJy per arcsec^2
  • Reposition the source at an X Offset of "1" arcsec.
  • Reposition the source at a Y Offset of "0" arcsec.
14
  • Change name to "1.82 mic disk, 2" sep" in the Source Identity Information field.      
  • re-normalize the source to its "1.82" µm surface brightness of "5" mJy per arcsec^2
  • Reposition the source at an X Offset of "1.414" arcsec.
  • Reposition the source at a Y Offset of "-1.414" arcsec.
15
  • Change name to "1.82 mic disk, 4" sep" in the Source Identity Information field.      
  • re-normalize the source to its "1.82" µm surface brightness of "1.5" mJy per arcsec^2
  • Reposition the source at an X Offset of "0" arcsec[2].
  • Reposition the source at a Y Offset of "-1.5" arcsec.
16
  • Change name to "1.82 mic disk, 6" sep" in the Source Identity Information field.      
  • re-normalize the source to its "1.82" µm surface brightness of "0.6" mJy per arcsec^2
  • Reposition the source at an X Offset of "-1.414" arcsec[2].
  • Reposition the source at a Y Offset of "-1.414" arcsec.
17
  • Change name to "1.82 mic disk, 8" sep" in the Source Identity Information field.      
  • re-normalize the source to its "1.82" µm surface brightness of "0.21" mJy per arcsec^2
  • Reposition the source at an X Offset of "-1.414" arcsec[2].
  • Reposition the source at a Y Offset of "1.414" arcsec.

  #4: NIRCam F210M Science Scene

Our NIRCam F210M science scene is set up in much the same way as the NIRCam F182 Science Scene: we create five extended sources sharing the same ShapeContinuum and Offset properties as the five extended sources above—i.e. ID #13, #14, #15 and #16, located at (1, 0), (1.414, -1.414), (0, -1.5), (-1.414, -1.414) and (-1.414, 1.414arcsec, respectively—however, with different ID and Renorm properties, as below:

Source ID #
(including in the Ex.Sci.Prog. Wb.)
Alterations
ID Tab:Renorm Tab:
18
  • Change name to "2.1 mic disk, 1" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.1" µm surface brightness of "15" mJy per arcsec^2. 
19
  • Change name to "2.1 mic disk, 2" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.1" µm surface brightness of "4.5" mJy per arcsec^2. 
20
  • Change name to "2.1 mic disk, 4" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.1" µm surface brightness of "1.2" mJy per arcsec^2. 
21
  • Change name to "2.1 mic disk, 6" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.1" µm surface brightness of "0.5" mJy per arcsec^2. 
22
  • Change name to "2.1 mic disk, 8" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.1" µm surface brightness of "0.2" mJy per arcsec^2. 


  #4: NIRCam F250M Science Scene

Given that we will be using the F250M filter with the MASK335R (rather than MASK210R, as previously), the ETC will allow us to perform calculations over a larger scene size. While the five extended sources will share the Shape and Continuum properties as previous sources 5 sources (in the F210M scene), we change the following properties in the ID, Renorm and Offset tabs: 

Source ID #
(including in the Ex.Sci.Prog. Wb.)
Alterations
ID Tab:Renorm Tab:Offset Tab
23
  • Change name to "2.5 mic disk, 1" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.5" µm surface brightness of "10" mJy per arcsec^2. 
  • Reposition the source at a X Offset of "1" arcsec
  • Reposition the source at a Y offset of "0" arcsec
24
  • Change name to "2.5 mic disk, 2" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.5" µm surface brightness of "3" mJy per arcsec^2. 
  • Reposition the source at a X Offset of "1.414" arcsec
  • Reposition the source at a Y offset of "-1.414" arcsec
25
  • Change name to "2.5 mic disk, 4" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.5" µm surface brightness of "0.9" mJy per arcsec^2. 
  • Reposition the source at a X Offset of "-2.8" arcsec
  • Reposition the source at a Y offset of "-2.8" arcsec
26
  • Change name to "2.5 mic disk, 6" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.5" µm surface brightness of "0.4" mJy per arcsec^2. 
  • Reposition the source at a X Offset of "-3" arcsec
  • Reposition the source at a Y offset of "3" arcsec
27
  • Change name to "2.5 mic disk, 8" sep" in the Source Identity Information field.      
  • Re-normalize the source to its "2.5" µm surface brightness of "0.15" mJy per arcsec^2. 
  • Reposition the source at a X Offset of "3" arcsec
  • Reposition the source at a Y offset of "3" arcsec

   #3–5: NIRCam F300M–F444W science scenes

Our NIRCam F300M, F335M and F444W science scenes are set up in much the same way as our F250M science scene: containing create five extended sources with the same Shape, Continuum and Offset properties as the five extended sources above—i.e. located at (1, 0), (1.414, -1.414), (-2.8, -2.8), (-3, 3) and (3, 3) arcsec, respectively—however, with different ID and Renorm properties, as below:

ID # / Scene Name(including in the Ex.Sci.Prog. Wb.)Source ID #
(including in the Ex.Sci.Prog. Wb.)
Alterations
ID Tab:Renorm Tab:




#6: NIRCam F300M science scene



28
  • Change name to "3 mic disk, 1" sep" in the Source Identity Information field.     
  • Re-normalize the source to its "3" µm surface brightness of "8" mJy per arcsec^2. 
29
  • Change name to "3 mic disk, 2" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3" µm surface brightness of "2.5" mJy per arcsec^2. 
30
  • Change name to "3 mic disk, 4" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3" µm surface brightness of "0.7" mJy per arcsec^2. 
31
  • Change name to "3 mic disk, 6" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3" µm surface brightness of "0.3" mJy per arcsec^2. 
32
  • Change name to  "3 mic disk, 8" sep" in the Source Identity Information field.
  • Re-normalize the source to its "3" µm surface brightness of "0.1" mJy per arcsec^2. 




#7: NIRCam F335M science scene

33
  • Change name to "3.35 mic disk, 1" sep" in the Source Identity Information field.     
  • Re-normalize the source to its "3.35" µm surface brightness of "6" mJy per arcsec^2. 
34
  • Change name to "3.35 mic disk, 2" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3.35" µm surface brightness of "2" mJy per arcsec^2. 
35
  • Change name to "3.35 mic disk, 4" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3.35" µm surface brightness of "0.55" mJy per arcsec^2. 
36
  • Change name to "3.35 mic disk, 6" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "3.35" µm surface brightness of "0.25" mJy per arcsec^2. 
37
  • Change name to "3.35 mic disk, 8" sep" in the Source Identity Information field.
  • Re-normalize the source to its "3.35" µm surface brightness of "0.09" mJy per arcsec^2. 




#8: NIRCam F444W science scene



38
  • Change name to "4.4 mic disk, 1" sep" in the Source Identity Information field.     
  • Re-normalize the source to its "4.4" µm surface brightness of "4" mJy per arcsec^2. 
39
  • Change name to "4.4 mic disk, 2" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "4.4" µm surface brightness of "1.2" mJy per arcsec^2. 
40
  • Change name to "4.4 mic disk, 4" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "4.4" µm surface brightness of "0.35" mJy per arcsec^2. 
41
  • Change name to "4.4 mic disk, 6" sep" in the Source Identity Information field.    
  • Re-normalize the source to its "4.4" µm surface brightness of "0.15" mJy per arcsec^2. 
42
  • Change name to  "4.4 mic disk, 8" sep" in the Source Identity Information field.
  • Re-normalize the source to its "4.4" µm surface brightness of "0.05" mJy per arcsec^2. 

Figure 2: Scene Sketch of the NIRCam F444W science scene with the star in the center, sources 38 to 42 around and the PSF reference star located in the top right corner.

Figure 3: On the left, an image of the beta Pictoris disk imaged by ESO's 3.6m telescope (ADONIS instrument, scattered light imaging in J-band ~1.25 µm, Mouillet et al. 1997) superimposed with 3.8µm VLT/NACO images of the planet b located in 2003 and 2009 on either side of the star. The labeled grey circles  correspond to five 0.5" zones of the disk that our program is simulating and on the right is shown the corresponding ETC sources that have been created in the NIRCam LW calculation field of view (6.36"). This is a zoom in on the same Scene Sketch of the NIRCam F444W science scene as in Figure 2.

   #9: Reference scene

Our 7th and final scene will be our reference scene, which we will use to perform our TA and science calculations on the PSF reference source. This scene contains only the PSF reference source (Alpha Pic), which is placed at the scene center. 



Define Calculations

Main article: JWST ETC Creating a New Calculation
See also: JWST ETC Calculations Page OverviewVideo Tutorials

The Calculations page in the JWST ETC is where we specify the instrument and mode, background, instrument and detector configuration, observing setup and extraction strategy for a given calculation. The aim of these calculations is to make a comparative analysis of locations in the scene and determine the exposure parameters required to obtain the desired signal-to-noise ratio (SNR). 

Science Calculations

Main article: JWST ETC Creating a New Calculation

In order to setup a science calculation for each of our observations, we select the appropriate Scene for Calculation in the Scene  tab and Coronagraphic Mask and Filter combination in the Instrument Setup tab. For the Coronagraphic Imaging calculations, the Strategy tab is split into two tabs: (1) Observation, where the parameters for the PSF calibration are set, and (2) Extraction, where the details of the extraction aperture and background subtraction are set. The Detector Setup tab is where we will specify the parameters that control the exposure time and photon-collecting duration.

Because we are interested in the disk at a range of separations, including regions where the stellar residuals dominate, we have a large dynamic range to consider and must avoid saturation for precise PSF subtraction. Thus we will avoid full saturation of the target star's speckles, but will allow for a few dozen pixels to be partially saturated, under the assumption that the ramps for these pixels can be recovered by the pipeline.

MIRI Science Calculations

Starting with our MIRI Coronagraphic Imaging observation in the F1500C filter, we set up our calculation as follows (i.e. Calc ID #1 in Example Science Program workbook)

  1. First, we select the "1: MIRI F1550C scence scene" as our Scene for Calculation in the Scene tab.
  2. In the Instrument Setup tab, we then select the 4QPM/F1550C Coron Mask/ Filter combination. 
  3. In the Detector Setup tab, because of the brightness of our targets, we keep the default Readout pattern of FASTFAST1 mode provides short MULTIACCUM exposures to maximize dynamic range and minimize noise in a background-dominated regime. 
  4. In the Strategy tab, we click on the Observation tab and select "2: Alpha Pic" as the PSF Subtraction Source, keeping the Optimal (PSF Autoscaling)/ default PSF subtraction methodallowing for the reference star to brightness to be re-scaled to that of the science target for optimal PSF subtraction. 
  5. We click on the Extraction tab (also located in the Strategy tab) and set the SNR source as "7: Disk 15.5mic, 10" Sep" (i.e. the source representing the faintest region of the disk), the Aperture Radius to "0.25" arcsec; the Sky annulus Inner radius to "0.45" arcsec and the Sky annulus Outer radius to "0.7" arcsec.

For our MIRI coronagraphic observation in the F2300C filter, we adopt the same detector setup and observation strategy, however we make the following changes to the calculation ((i.e. Calc ID #2 in Example Science Program workbook:

  1. We select the "2: MIRI F2300C science scene" in the Scene tab.
  2. We select the LYOT/F2300C Mask/Filter combination for our instrument setup.
  3. For our Extraction Strategy we select the "12: Disk 23mic, 10" sep" as the extraction source and set the Aperture Radius to "0.35" arcsec.

For our MIRI science calculations we have set the extraction radius set to 0.25" for F1550 (l/D = 0.5"), 0.35" for F2300 (l/D = .7")


Once setup, we then explore the exposure parameters in the Detector Setup tab, with the aim of determining the exposure time required to obtain a high SNR on the faintest portion of the disk (10" separation, which is at (0,+4) in this scene), whilst also avoiding saturation in the brighter regions, and maintaining some balance with the resulting observatory overheads (science time to overhead time ratio is roughly 1:2). 

For our F1550C/4QPM observation (i.e. CalID #1), we find that setting the Groups per Integration to "100" and Integrations per Exposure to "50", we are able to obtain an extracted SNR of 30.16 on the disk at a ~10" separation, in a Total exposure time of 1198.4  s and with no saturation issues. For the F2300C/Lyot observation (i.e. CalID #2), we find the same exposure parameters (Groups per Integration = "100" and Integrations per Exposure = "50") allow us to achieve a SNR of 22.67  on the 10" portion of the disk, without saturation, in an estimated exposure time of 1620s.

NIRCam Science Calculations

For NIRCam our observations, we will observe with the 210R coronagraphic mask (MASK210R) in two filters (F182M and F210M), and the 335R mask (MASK335R) in 4 filters (F250M, F300M, F333M, F444W). For each calculation we will determine the optimal exposure parameters, with the aim of determining the exposure time required to obtain a sufficient SNR on the faintest portion of the disk (in this case at the 8" separation) while maintaining balance with observatory overheads (for NIRCam coronagraphic imaging, the science-to-overhead-time ratio varies from 1:1 to 2:1). Furthermore, we will a select a Readout Pattern for each calculation that allows us to maintain a large number of Groups per integration (around 10) while also avoiding saturation as much as possible (i.e. no more than a few partially saturated pixels). 

Starting with our observation in the F182M filter, we set up our calculations as follows:

NIRCam F182M calculation (i.e. Calc ID #5 in Example Science Program workbook):

  1. Under the Scene tab, we select the "3: NIRCam F182M science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the MASK210R Coronagraph and the F182M Filter.
  3. In the Detector Setup tab, we select the SUB640 Subarray, allowing for our bright target to be observed in shorter exposures (subarrays are read out more quickly than the full detector) to avoid saturation (see NIRCam Coronagraphic Imaging Recommended Strategies).
  4. For our Observation Strategy, we select "2: Alpha Pic" as our PSF Subtraction Source and keep the default PSF Subtraction Method as Optimal.
  5. For the Extraction Strategy, we set "17: Disk 1.82mic, 8" Sep" as the SNR source; define a "0.029" arcsec Aperture Radius; set the Sky Annulus Inner Radius to "0.5" arcsec and Outer Radius to
    "0.7" arcsec.

As before, we explore our exposure parameters, finding that using the "RAPIDReadout pattern, "4Groups per integration and "90Integrations per exposure, we are able to achieve a SNR on the faintest portion of the disk (at 8" separation) of approximately 1.6 in an exposure time of 1885.47 s. Because the target star is very bright (Kmag=3.5), even when reading out the coronagraphic Subarray in RAPID, a small number of pixels may fully saturate. The ETC reports 1 fully saturated pixel and 51 partially saturated pixels in the Warnings tab of the Reports pane; however by selecting the Saturation tab in the Images pane, we are determine that this saturation occurs in the vicinity of the coronagraph, and not at the position of the source of interest, which is acceptable.

NIRCam F210M calculation
(i.e. Calc ID #6 in Example Science Program workbook):

For our F210M calculation, we adopt the same general settings as the F182M calculation, however with the following differences:

  1. We select "4: NIRCam F210M science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the MASK210R Coronagraph and F210M Filter.
  3. In the Extraction/Strategy tab, we set the SNR source as "22: Disk 2.1mic, 8" sep" and Aperture Radius to "0.033arcsec.

Exploring exposure parameters, we find that with the RAPID Readout pattern, "4Groups per integration and "90Integrations per exposure, we are able to achieve a SNR of 1.5 on the faintest portion of the disk (at 8" separation) in a Total exposure time of 1885.47s, with only 15 pixels partially saturated.


NIRCam F250M calculation
(i.e. Calc ID #7 in Example Science Program workbook):

We again adopt the same general settings as the F182M calculation, however with the following differences:

  1. We select "5: NIRCam F250M science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the MASK335R Coronagraph and F250M Filter.
  3. In the Detector Setup tab, we select the SUB320 Subarray (again, because using the subarray will allow for shorter exposures)
  4. In the Extraction/Strategy tab, we set the SNR source as "27: Disk 2.5mic, 8" sep" and Aperture Radius to "0.04arcsec.

For our F250M calculation, we find that using the BRIGHT2 Readout Pattern, "10" Groups per integration and "80" Integrations per exposure, we can obtain a a SNR of 7.64 on the faintest region of the disk (at 8" separation) in a Total exposure time of 1797.63s, with only 8 partially saturated pixels.  


NIRCam F300M calculation
(i.e. Calc ID #8 in Example Science Program workbook):

For this calculation, we Copy our F250M calculation, but change the following parameters:

  1. We select "6: NIRCam F300M science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the F300M Filter.
  3. In the Detector Setup tab, we select the SUB320 Subarray. 
  4. In the Extraction/Strategy tab, we set the SNR source as "32: Disk 3mic, 8" sep" and Aperture Radius to "0.047arcsec.

For our F250M calculation, we find that using the BRIGHT2 Readout Pattern, "10" Groups per integration and "80" Integrations per exposure, we can obtain a a SNR of 5 on the faintest region of the disk (at 8" separation) in a Total exposure time of 1797.63s, with only 1 partially saturated pixels.  


NIRCam F335M calculation
(i.e. Calc ID #9 in Example Science Program workbook):

Duplicating the previous calculation, we change the following parameters:

  1. We select "7: NIRCam F335M science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the F335M Filter.
  3. In the Extraction/Strategy tab, we set the SNR source as "37: Disk 3.35mic, 8" sep" and Aperture Radius to "0.053arcsec.

We find that with the SHALLOW4 Readout pattern, "10" Groups per integration and "35" Integrations per exposure, the ETC reports an extracted SNR of 7.06 in a Total exposure time of 1871.54 s, with 40 partially saturated pixel.


NIRCam F444W calculation
(i.e. Calc ID #10 in Example Science Program workbook):

Again, we duplicate the previous calculation and then:

  1. We select "9: NIRCam F444W science scene" as our Scene for calculation.
  2. In the Instrument Setup tab, we select the F444W Filter.
  3. In the Strategy/Extraction tab, we set the SNR source as "42: Disk 4.44mic, 8" sep" and Aperture Radius to "0.07arcsec.

For this calculation, we find that using the SHALLOW4 Readout pattern, "10Groups per integration and "35" Integrations per exposure we are able to obtain a SNR of 8.54 at on the disk 8" from the star, in a Total exposure time of 1871.54s and resulting in 28 partially saturated pixels

Warnings


The ETC calculation field of view is limited to a 3.14", 6.36" or 8.91"  square box for NIRCam SW, NIRCam LW and MIRI, respectively. A field of view of ~20" would be necessary to fit the whole beta Pictoris disk. This ETC workbook is thus a workaround where various parts of the edge-on disk have been simulated at the four edges of our calculation boxes (see the NIRCam F444W example in Figure 3.).  The sky background is estimated in a Sky annulus which can only be centered around the extraction SNR source of interest. In our case, some of our sources are 0.5" or 1" and and already grazing the edges of the calculation field of view. As a result, there is the following warning in the Reports : "Background estimation region partially outside of the field of view.” For instance, for Calc ID #10, the source #42 is 0.5" in diameter with a Sky annulus of 0.5" to 0.7" around it and provokes this warning.

Reducing the size of the sources (also scaling with angular distance) or moving the sources closer to the calculation window centers might solve this issue but can introduce biases due to the presence of residual stellar signal around the occulters. For training purposes, we leave and acknowledge this warning to emphasize on the limitations of the ETC which nevertheless, allows a decent workaround to prepare such a complex program.


Target Acquisition 

Main article: JWST ETC Target Acquisition
See also: MIRI Coronagraphic Imaging Target Acquisition, NIRCam Coronagraphic Imaging Target AcquisitionJWST ETC MIRI Target AcquisitionJWST ETC NIRCam Target Acquisition

All coronagraphic observations require a science instrument assisted target acquisition (TA) procedure, with the goal of accurately aligning a bright astronomical source (the "host") at the location of maximum attenuation on the coronagraphic mask (occulter). For each of our observations, we will use the ETC to determine the exposure time required to obtain a sufficient signal-to-noise for the TA procedure to achieve the desired centroid accuracy. For MIRI a SNR  20 is required in order to obtain an absolute centroid accuracy of ≤ 10 and 22.5 mas for the 4QPM and Lyot coronagraphs, respectively; for NIRCam TA, the minimum recommended integrated SNR is ≥ 30, which is required to obtain a centroid accuracy better than 0.1 pixels. Saturation can also affect the accuracy of the centroiding procedure, and should be avoided. 

MIRI Target Acquisition 

Because MIRI coronagraphic imaging filters are directly associated with each coronagraph and are not interchangeable, each filter change requires a new TA procedure. Starting with our F1550C science observation, we initialize a MIRI TA calculation and specify the following in the Configuration pane:

  • In the Scene  tab, we select "1: MIRI F1550C" as the Scene for Calculation.
  • In the Instrument Setup tab, we specify the Acq Mode as TA for 4QPM/1550 and select the FND (Neutral Density) Filter (which provides the strongest flux attenuation) in order to avoid saturation and persistence while performing TA on our bright target star (see MIRI Coronagraphic Recommended Strategies).
  • In the Strategy tab, we specify that the extraction Aperture be centered on "1: Beta Pic".
  • In the Detector Setup tab, we keep the default FAST Readout Pattern and the number of Groups as "4".  

Running this initial calculation (Calc ID #3 in Ex.Sci.Prg.Wb), we find that "4Groups is indeed sufficient to obtain an integrated SNR above the SNR threshold for MIRI Coronagraphic Imaging TA. Changing the Acq Mode to TA for LYOT/2300 and the Scene for Calculation to "2: MIRI F2300C", we find the same is true for our Lyot TA calculation. 

Because TA exposures are so short compared to the rest of the program (i.e. low cost), we decide to push for a number of Groups of "6", allowing us to obtain an extracted SNR of ~182 and ~237 in 1.44 s for the 4QPM/F1500C and Lyot/F2300C calculations, respectively. Because Alpha Pic is only a factor of 2 brighter, and we have plenty of dynamic range with TA left, we adopt the same exposure settings for the reference PSF target in each filter. Changing the Scene for Calculation to "37: Reference Scene" and running the same exposure settings, the ETC estimates an extracted SNR of 301.93 and 372.87 for the 4QPM/F1550C (Calc ID #3) and Lyot/F2300C (Calc ID #4) reference target TA calculations, respectively. 

NIRCam Target Acquisition

Unlike MIRI, NIRCam can take multiple filters on the same coronagraph after one target acquisition. For this reason, we need only perform a TA calculation for the two coronagraphic masks that we plan to use: MASK210R and MASK335R. Starting with MASK210R, we create a new NIRCam TA calculation (Calc ID #11 in Ex.Sci.Prg.Wb) and set it up as follows:

  • In the Scene  tab, we select "3: NIRCam F210M" as the Scene for Calculation.
  • In the Instrument Setup tab, we specify the Acq Mode as Coronagraphy MASK210R and because Beta Pic is a bright target, select the F210M+ND Square (Bright) Filter to avoid saturation.
  • In the Strategy tab, we specify that the extraction Aperture be centered on "1: Beta Pic".
  • In the Detector Setup tab, we keep the default RAPID Readout Pattern and the number of Groups as "3"

Running this calculation, the ETC indicates that 3 Groups is sufficient to meet the recommended threshold of 30 for the extracted SNR, achieving a SNR of 35.23. However—as with our MIRI calculations—we choose to push push for higher SNR, and set the number of Groups to "17", achieving a SNR of 181.72 in 3.28 seconds with no saturation issuesFor the Reference PSF observation (changing the Scene for calculation to "9: Reference scene"), these exposure specifications reach a SNR of 290.32 for the TA procedure on Alpha Pic.

For the NIRCam Coronagraphy MASK335R Acq mode, we select the F335M+ND square (Bright) as the TA Filter and change the Scene for Calculation to "7: NIRCam F335W". Setting the Readout Pattern to RAPID, we find that a minimum number of "9" Groups is required to achieve an extracted SNR > 30. As before, we choose to push for a higher SNR, and select "33" Groups for an extracted SNR of 118.83in 1.71sFor the Reference PSF observation (again, setting the Scene for calculation to "9: Reference scene"), these exposure specifications reach a SNR of 199.56.




See the Step-by-Step APT guide to complete the proposal preparation for this example science program, where we will input the exposure parameters we derived here.



Links 

"Coronagraphy of the Debris Disk Archetype Beta Pictoris" GTO Program



References

Golimowski, D. A., et al. 2006, AJ, 131, 3109

Lagrange, A.-M., Boccaletti, A., Milli, J. et al  2012, A&A 542, A40

Mouillet, D., Larwood, J.-D., Papaloizou et al. 1997, MNRAS, 292, 896

Pantin, E., Lagage, P. O., & Artymowicz, P. 1997, A&A, 327, 1123

Smith, B. A. & Terrile, R. J. 1984, Science, 226, 1421 

Wyatt, M. C., Dermott, S. F., Telesco, C. M., Fisher, R. S., Grogan, K., Holmes, E. K., & Pi˜na, R. K. 1999, ApJ, 527, 918


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