JWST ETC NIRCam Coronagraphic Imaging

The JWST Exposure Time Calculator (ETC) offers a NIRCam coronagraphic imaging calculation with expanded capabilities to support simultaneous short- and long-wavelength coronagraphy.

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Starting with Cycle 2, simultaneous shortwave (SW) and longwave (LW) coronagraphic imaging is offered with NIRCam. This enables both wavelength channels to be used to increase the science gain and the efficiency of observations. However, the use of both channels simultaneously comes with complications. Specifically, the "primary" channel determines the location of the target in the "secondary" channel with respect to the chosen coronagraphic mask, which is the same for both channels. Users must therefore be careful to choose the correct primary channel based on their science goals.

Setting up the ETC calculation

Instrument Setup

The screenshot below shows the Instrument Setup tab for a default NIRCam Coronagraphic Imaging calculation.

Figure 1. Instrument Setup tab for the default calculation

This default calculation, using the MASK210R round coronagraphic mask, presents 3 drop-downs:

  • Coronagraph: Choice of coronagraphic mask
    • Round mask options: MASK210R, MAS335R, MASK430R
    • Bar mask options: MASKSWB, MASKLWB

  • Channel: Choice of primary channel
    • Options: Short Wavelength (SW), Long Wavelength (LW)

  • SW Filter: Choice of short wavelength filter
    • Options: F182M, F187N, F200W, F210M, F212N

Because the choice of mask matches the choice of primary channel in the above example, this particular calculation is a "primary" calculation. As shown in Figure 2, changing the Channel to Long Wavelength (LW), while leaving the mask the same, changes the third drop-down to a LW Filter selection. This is a "secondary" calculation, with the following options:

  • LW Filter: Choice of long-wavelength filter
    • Options: F250M, F277W, F300M, F322W2, F335M, F356W, F360M, F410M, F430M, F444W, F460M, F480M

Figure 2. Instrument Setup tab for a secondary calculation using a round mask

The situation is more complicated for the "secondary" calculation for the bar masks. The "primary" calculation has the same 3 drop-downs as shown in Figure 1, but the "secondary" calculation presents a 4th dropdown, as shown below.

Figure 3. Instrument setup tab for a secondary calculation using a bar mask

The combination of MASKSWB and Long Wavelength (LW) or MASKLWB and Short Wavelength (SW) will introduce a 4th drop-down menu titled SW Filter (Paired) or LW Filter (Paired). This is because the combination of coronagraphic mask and secondary filter determines the position of the PSF with respect to the bar.

Disallowed combinations

The following combinations are disallowed and produce an error in the ETC, preventing the calculation from being run:

  • MASK201R + Long Wavelength (LW) + F277W
  • MASK335R + Long Wavelength (LW) + F277W
  • MASK430R + Long Wavelength (LW) + F277W
  • MASKSWB + Long Wavelength (LW) + F322W2 + Any SW Filter (Paired)
  • MASKLWB + Long Wavelength (LW) + F322W2
  • MASKLWB + Short Wavelength (LW) + Any SW Filter + F322W2

Detector setup

The following Coronagraph/Subarray combinations are available (the FULL subarray is valid for all coronagraphic masks):


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