JWST ETC MSA Aperture Photometry Strategy

The MSA Aperture Photometry strategy is used for the NIRSpec MOS Verification imaging mode in the JWST ETC.

The NIRSpec MOS Verification Imaging mode is the ETC equivalent of the VERIFY_ONLY target acquisition option in the JWST Astronomer's Proposal Tool (APT). This is an imaging mode that allows aperture photometry through the ALLOPEN MSA configuration after the science observations are executed and uses the target acquisition filter set. It is designed to allow the target to be offset within the source shutter and perform aperture photometry including signal from all open shutters and excluding the signal blocked by the MSA bars. A detector readout pattern is selected from among those available to target acquisition mode. An exposure time is specified through the number of Groups per integration, which allows for only a single-ramp observation.

Figure 1. Layout of the Strategy Tab for MSA Aperture Photometry

The following  parameters are available for the MSA Aperture Photometry strategy:

  • Centered on source - From the drop-down menu in the Scene tab, select a source within the scene to use for the calculation. "Centered" refers to the center of an MSA shutter.
  • Source offset from shutter center – The extraction region may be specified by providing the X- and Y-offset from the shutter center. This moves the selected source off the shutter center. Throughput is dependent on the exact location in the shutter and will affect the signal. The number of "fractional shutters" that an X- or Y-offset corresponds to is reported below each input box. Note that "1.00 fractional shutters" equates to one shutter half-width or half-height; the source is now on the edge of the shutter. Equivalently, "2.00 fractional shutters" equates to a full shutter width or height; the source is now in the center of an adjacent shutter.
  • Aperture radius – The aperture used for imaging aperture photometry is circular, and this parameter specifies the radius in arcsec of the circular aperture used for extraction.
  • 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 the inner radius and outer radius parameters for the Sky annulus. 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 – 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." 
  • Shape - The imaging aperture photometry uses a circular aperture.




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