The JWST Exposure Time Calculator (ETC) calculates the signal-to-noise ratio for imaging modes of all JWST instruments, and for the NIRISS aperture masking interferometry mode, using the Imaging Aperture Photometry strategy. The user specifies the apertures for extracting the source flux and background flux.

The Imaging Aperture Photometry strategy presents users with various options to define the size, shape, and location for the photometry aperture used for extracting the source flux. The background flux is determined from the Sky annulus whose Inner radius/Inner Width and Outer radius/Outer width are also provided as input parameters.The following parameters are to be provided for Imaging Aperture Photometry:

• Aperture location: (1) The aperture may be centered on any desired source within the scene used for the calculation. The drop-down menu is populated with a list of the sources available in the scene from which the selection can be made. (2) The aperture may be chosen to be at any desired location within the scene (that is, not necessarily centered on a source) by specifying the X- and Y-offsets in arcsec from the scene center.
  
  
• Aperture: This parameter is "Radius" for the "circle/arcsec" Shape/Angular units option and "Width" for the "square/pixel" option. The "Radius" is the radius of the circular aperture in arcseconds, while the "Width" is the full-width of the square aperture in pixels.
  
  
• Shape/Angular units: The 2 options include "circle/arcsec" and "square/pixel." The option chosen affects the behavior of the Aperture and Sky annulus parameters.
  
  
• Perform Background Subtraction Using: There is an option to perform background subtraction using the background estimated from a background region specified by the user, or using a noiseless sky background. The background region is defined by specifying either the Inner radius and Outer radius parameters for a circular aperture or the Inner Width and Outer Width parameters for a square aperture for the Sky annulus.
  
  Note that when a Sky Annulus is specified, it includes contamination from all sources within the scene that contribute to the annulus in addition to the sky, e.g., when overlapping sources are present, or extended profile wings of the source itself contributes to the background extraction aperture. When the background subtraction is performed, the contamination and sky contributions are subtracted. The estimated contamination from sources within the scene over the sky background is reported in the ETC output Reports pane as "Fraction of total background due to signal from the scene." 
  
  If the option to use the noiseless sky background is selected, then the count rate corresponding to the sky background chosen in the Backgrounds tab is used for background subtraction. When the noiseless sky background is opted for background subtraction, the contamination from within the scene is not subtracted from the extracted source flux. The users may want to consider using a noiseless sky background in some cases, for example, when performing photometry of a source that is located on a bright extended background with significant gradient within the background annulus.  Also, a noiseless sky background may be preferred when performing point source photometry with a constant aperture radius and doing a batch expansion over filters, to account for the larger PSF at longer wavelengths.
  
  
• Sky annulus: For the "circle/arcsec" Shape/Angular units option, the Inner radius and Outer radius options are supplied, corresponding to the inner and outer radius of the circular sky annulus, measured from the center of the extraction aperture. For the "square/pixel" Shape/Angular units option, the Inner width and Outer width options are supplied, corresponding to the full-widths of the inner and outer squares used to define the background region, both of which are centered on the center of the square extraction aperture.

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