Step-by-Step ETC Guide for MIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk
Example Science Program #35 ETC Guide
Dated material
This example was created pre-launch, and the ETC has been updated since its creation. You may see differences in the details of the results from the ETC, the information provided, or the appearance of the ETC GUI from what is shown herein.
Please refer to JWST Example Science Programs for more information.
See also: NIRCam and MIRI Coronagraphy of the Beta Pictoris Debris Disk, JWST Exposure Time Calculator Overview, JWST High-Contrast Imaging, MIRI 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 JWST High-Contrast Imaging Roadmap, 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 Example Science Program Workbooks dropdown tab on the ETC Workbooks page. 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
See also: JWST ETC Scenes and Sources Page Overview, JWST ETC Defining a New Scene, JWST ETC Defining a New Source
In order to perform the intended calculations, you 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 use a surface brightness model based on the results of Ballering et al. 2016. To create the model, use 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). Then, combine the scattered light model and the thermal emission model to get the total surface brightness model at all wavelengths.
Now that you are able to model the disk at a given wavelength, define a different scene for each of the observed filters—referred to as the "science" scenes—in addition to a "reference" scene for performing the 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).
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
MIRI coronagraphic imaging Scenes and Sources
For the MIRI Coronagraphic observations, create two 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, 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)
First, define the source representing the 15.5 µm disk at a separation of 2", which you'll identify as "Disk 15.5mic, 2" sep" (i.e., source ID #3 in Example Science Program workbook). In the Shape tab, 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 SurfaceBrightnesspersquarearcsec as the Normalizationchoice—this will automatically change the units in the Renorm tab to read perarcsec^2. In the Continuum tab, define the source to have a FlatContinuum in fnu units and renormalize to its "15.5" µm surface brightness of "15" mJyperarcsec^2 in the Renorm tab. Lastly, ensuring that the source has been appropriately associated with the "MIRI F1550C science scene", 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 10arcsec from the target star) will share many of the same properties, you can save some time by simply duplicating the "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 Sourcespage—four times. Appropriately renaming the sources ("Disk 15.5mic, 4" sep", "Disk 15.5mic, 6" sep" etc) in the ID tab, you 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).
Define the remaining four sources (duplicates of the 2" sep source) by making the following alterations in the Configuration pane:
Source ID # in the Ex.Sci.Prog. Wb. | Alterations | ||
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#2: MIRI F2300C science Scene
(i.e., Scene ID #2 in Example Science Program Workbook)
As with the previous science scene, create the 5 extended sources (uniform brightness, 1" × 1") 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:
- 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").
- Renormalize the "Disk 23mic, 4" sep" source (i.e., ID #9) to a "23" µm surface brightness of "42" mJy per arcsec^2.
- Renormalize the "Disk 23mic, 6" sep" source (i.e., ID #10) to a "23" µm surface brightness of "22" mJy per arcsec^2.
- Renormalize the "Disk 23mic, 8" sep" source (i.e., ID #11) to a "23" µm surface brightness of "14" mJy per arcsec^2.
- 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
The NIRCam Coronagraphic Imaging observations 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". 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), identify the source as "Disk 1.82mic, 1" sep" in the ID tab. 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, 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", place the source at an X Offset of "1" arcsec in the Offset Tab. For the remaining four sources, duplicate the "1.82 mic disk, 1" sep" source and associate them with the 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, you cannot place the sources at the desired 4, 6 and 8". Instead, position the sources at positions 2" away from the target source, such that you 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 | ||
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ID Tab: | Renorm Tab: | Offset: | |
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#4: NIRCam F210M science Scene
The NIRCam F210M science scene is set up in much the same way as the NIRCam F182 Science Scene: create five extended sources sharing the same Shape, Continuum 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, 1.414) arcsec, respectively—however, with different ID and Renorm properties, as below:
Source ID # (including in the Ex.Sci.Prog. Wb.) | Alterations | |
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ID Tab: | Renorm Tab: | |
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#4: NIRCam F250M Science Scene
Given that you will be using the F250M filter with the MASK335R (rather than MASK210R, as previously), the ETC will allow you 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), change the following properties in the ID, Renorm and Offset tabs:
Source ID # (including in the Ex.Sci.Prog. Wb.) | Alterations | ||
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ID Tab: | Renorm Tab: | Offset Tab | |
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#3–5: NIRCam F300M–F444W science scenes
The NIRCam F300M, F335M and F444W science scenes are set up in much the same way as the 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 | |
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ID Tab: | Renorm Tab: | ||
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#9: Reference scene
The 7th and final scene will be your reference scene, which will be used to perform the 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
See also: JWST ETC Creating a New Calculation, JWST ETC Calculations Page Overview, Proposal Planning Video Tutorials
The Calculations page in the JWST ETC is where you 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
See also: JWST ETC Creating a New Calculation
In order to setup a science calculation for each of the observations, 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 you will specify the parameters that control the exposure time and photon-collecting duration.
Because you are interested in the disk at a range of separations, including regions where the stellar residuals dominate, you have a large dynamic range to consider and must avoid saturation for precise PSF subtraction. Thus you 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 the MIRI Coronagraphic Imaging observation in the F1500C filter, set up the calculation as follows (i.e., Calc ID #1 in Example Science Program workbook):
- First, select the "1: MIRI F1550C scence scene" as your Scene for Calculation in the Scene tab.
- In the Instrument Setup tab, select the 4QPM/F1550C Coron Mask/ Filter combination.
- In the Detector Setup tab, because of the brightness of the targets, keep the default Readout pattern of FAST—FAST1 mode provides short MULTIACCUM exposures to maximize dynamic range and minimize noise in a background-dominated regime.
- In the Strategy tab, click on the Observation tab and select "2: Alpha Pic" as the PSF Subtraction Source, keeping the Optimal (PSF Autoscaling)/ default PSF subtraction method, allowing for the reference star to brightness to be re-scaled to that of the science target for optimal PSF subtraction.
- 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 the MIRI coronagraphic observation in the F2300C filter, adopt the same detector setup and observation strategy, however, make the following changes to the calculation ((i.e., Calc ID #2 in Example Science Program workbook:
- Select the "2: MIRI F2300C science scene" in the Scene tab.
- Select the LYOT/F2300C Mask/Filter combination for the instrument setup.
- For the Extraction Strategy, select the "12: Disk 23mic, 10" sep" as the extraction source and set the Aperture Radius to "0.35" arcsec
For the MIRI science calculations, set the extraction radius to 0.25" for F1550 (l/D = 0.5"), 0.35" for F2300 (l/D = .7")
Once setup, 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 the F1550C/4QPM observation (i.e., Calc ID #1), you'll find that by setting the Groups per Integration to "100" and Integrations per Exposure to "50", you 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., Calc ID #2), you'll find the same exposure parameters (Groups per Integration = "100" and Integrations per Exposure = "50") allow you 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
This example was created pre-launch. Starting in Cycle 2, NIRCam is able to obtain both the short and long wavelength coronagraphic data simultaneously. Users should perform ETC calculations for a given mask in both a longwave filter and a shortwave filter.
For NIRCam observations, you 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, 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, you will a select a Readout Pattern for each calculation that allows you 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 the observation in the F182M filter, set up the calculations as follows:
NIRCam F182M calculation (i.e., Calc ID #5 in Example Science Program workbook):
- Under the Scene ★ tab, select the "3: NIRCam F182M science scene" as the Scene for calculation.
- In the Instrument Setup tab, select the MASK210R Coronagraph and the F182M Filter.
- In the Detector Setup tab, select the SUB640 Subarray, allowing for the 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).
- For the Observation Strategy, select "2: Alpha Pic" as the PSF Subtraction Source and keep the default PSF Subtraction Method as Optimal.
- For the Extraction Strategy, 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, in exploring the exposure parameters, you'll find that using the "RAPID" Readout pattern, "4" Groups per integration and "90" Integrations per exposure, you 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, you can 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 the F210M calculation, adopt the same general settings as the F182M calculation, however with the following differences:
- Select "4: NIRCam F210M science scene" as the Scene for calculation.
- In the Instrument Setup tab, select the MASK210R Coronagraph and F210M Filter.
- In the Extraction/Strategy tab, set the SNR source as "22: Disk 2.1mic, 8" sep" and Aperture Radius to "0.033" arcsec.
Exploring exposure parameters, you will find that with the RAPID Readout pattern, "4" Groups per integration and "90" Integrations per exposure, you 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):
Again, adopt the same general settings as the F182M calculation, however with the following differences:
- Select "5: NIRCam F250M science scene" as your Scene for calculation.
- In the Instrument Setup tab, select the MASK335R Coronagraph and F250M Filter.
- In the Detector Setup tab, select the SUB320 Subarray (again, because using the subarray will allow for shorter exposures).
- In the Extraction/Strategy tab, set the SNR source as "27: Disk 2.5mic, 8" sep" and Aperture Radius to "0.04" arcsec.
For the F250M calculation, you will find that by using the BRIGHT2 Readout Pattern, "10" Groups per integration and "80" Integrations per exposure, you 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, Copy the F250M calculation, but change the following parameters:
- Select "6: NIRCam F300M science scene" as your Scene for calculation.
- In the Instrument Setup tab, select the F300M Filter.
- In the Detector Setup tab, select the SUB320 Subarray.
- In the Extraction/Strategy tab, set the SNR source as "32: Disk 3mic, 8" sep" and Aperture Radius to "0.047" arcsec.
For the F250M calculation, you will find that by using the BRIGHT2 Readout Pattern, "10" Groups per integration and "80" Integrations per exposure, you 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, change the following parameters:
- Select "7: NIRCam F335M science scene" as your Scene for calculation.
- In the Instrument Setup tab, select the F335M Filter.
- In the Extraction/Strategy tab, set the SNR source as "37: Disk 3.35mic, 8" sep" and Aperture Radius to "0.053" arcsec.
You will 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, duplicate the previous calculation and then:
- Select "9: NIRCam F444W science scene" as your Scene for calculation.
- In the Instrument Setup tab, select the F444W Filter.
- In the Strategy/Extraction tab, set the SNR source as "42: Disk 4.44mic, 8" sep" and Aperture Radius to "0.07" arcsec.
For this calculation, you will find that by using the SHALLOW4 Readout pattern, "10" Groups per integration and "35" Integrations per exposure you 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 the 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 this case, some of the 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, leave and acknowledge this warning to emphasize the limitations of the ETC which nevertheless, allows a decent workaround to prepare such a complex program.
Target acquisition
See also: JWST ETC Target Acquisition, MIRI Coronagraphic Imaging Target Acquisition, NIRCam Coronagraphic Target Acquisition, JWST ETC MIRI Target Acquisition, JWST 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 the observations, 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 the F1550C science observation, initialize a MIRI TA calculation and specify the following in the Configuration pane:
- In the Scene ★ tab, select "1: MIRI F1550C" as the Scene for Calculation.
- In the Instrument Setup tab, 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 the bright target star (see MIRI Coronagraphic Recommended Strategies).
- In the Strategy tab, specify that the extraction Aperture be centered on "1: Beta Pic".
- In the Detector Setup tab, keep the default FAST Readout Pattern and the number of Groups as "4".
In running this initial calculation (Calc ID #3 in Ex.Sci.Prg.Wb), you will find that "4" Groups 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", you will find the same is true for the Lyot TA calculation.
Because TA exposures are so short compared to the rest of the program (i.e., low cost), you can push for a number of Groups of "6", allowing you 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 you have plenty of dynamic range with TA left, 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, you need only perform a TA calculation for the two coronagraphic masks that you plan to use: MASK210R and MASK335R. Starting with MASK210R, create a new NIRCam TA calculation (Calc ID #11 in Ex.Sci.Prg.Wb) and set it up as follows:
- In the Scene ★ tab, select "3: NIRCam F210M" as the Scene for Calculation.
- In the Instrument Setup tab, 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, specify that the extraction Aperture be centered on "1: Beta Pic".
- In the Detector Setup tab, 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 the MIRI calculations—choose to 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 issues. For 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, 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, you will find that a minimum number of "9" Groups is required to achieve an extracted SNR > 30. As before, choose to push for a higher SNR, and select "33" Groups for an extracted SNR of 118.83in 1.71s. For 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.
References
Golimowski, D. A., et al. 2006, AJ, 131, 3109
Hubble Space Telescope ACS Multiband Coronagraphic Imaging of the Debris Disk around β Pictoris*
Kammerer J., Lawson K., Perrin M., et al. 2024 AJ
Accepted for publication (Paper #2, published using the GTO 1411 Cycle 1 Program that inspired this example science program)
JWST-TST High Contrast: JWST/NIRCam observations of the young giant planet β Pic b
Lagrange, A.-M., Boccaletti, A., Milli, J. et al 2012, A&A 542, A40
The position of β Pictoris b position relative to the debris disk
Mouillet, D., Larwood, J.-D., Papaloizou et al. 1997, MNRAS, 292, 896
A planet on an inclined orbit as an explanation of the warp in the β Pictoris disc
Pantin, E., Lagage, P. O., Artymowicz, P. 1997, A&A, 327, 1123
Mid-infrared images and models of the beta Pictoris dust disk
Rebollido I., Stark C. C., Kammerer J., et al. 2024, AJ, 167, 2
(Paper #1, published using the GTO 1411 Cycle 1 Program that inspired this example science program)
JWST-TST High Contrast: Asymmetries, Dust Populations, and Hints of a Collision in the β Pictoris Disk with NIRCam and MIRI
Smith, B. A. & Terrile, R. J. 1984, Science, 226, 1421
A Circumstellar Disk Around β Pictoris
Wyatt, M. C., et al. 1999, ApJ, 527, 918
How Observations of Circumstellar Disk Asymmetries Can Reveal Hidden Planets: Pericenter Glow and its Application to the HR 4796 Disk
Links
"Coronagraphy of the Debris Disk Archetype Beta Pictoris" GTO Program