Step Arguments

The background image subtraction step has several optional arguments. The first two are used only when the step is applied to non-WFSS exposures. They are used in the process of creating an average background image, to control the sigma clipping, and are passed as arguments to astropy.stats.sigma_clip():

--sigma

When combining multiple background images, the number of standard deviations to use for the clipping limit. Defaults to 3.

--maxiters

When combining multiple background images, the number of clipping iterations to perform, or None to clip until convergence is achieved. Defaults to None.

--save_combined_background

Saves the combined background image used for background subtraction. Defaults to False.

--soss_source_percentile

The threshold flux percentile, above which values are deemed to be source or contaminated. Pixels with flux below this percentile will be added to the background mask. The default value is 35.0.

--soss_bkg_percentile

This pair of percentile values describes the range of flux percentiles in the background mask to use for reference template scaling. The default is [25.0, 50.0].

--wfss_mmag_extract

Only applies to Wide Field Slitless Spectroscopy (WFSS) exposures. Sets the minimum (faintest) magnitude limit to use when selecting sources from the WFSS source catalog, based on the value of isophotal_abmag in the source catalog. Defaults to None.

--wfss_mask

Only applies to Wide Field Slitless Spectroscopy (WFSS) exposures. Provides a custom user mask to use for background scaling, overriding the source-catalog-derived mask. The argument should point to a FITS or ASDF file openable as ImageModel containing a 2D array of integers in its .mask attribute (FITS MASK extension) with pixels to be used as background set to 1 and other pixels set to 0. To use the mask as-is without any additional outlier rejection, the wfss_maxiter argument should be set to 0. The .data attribute is not used. Defaults to None.

--wfss_maxiter

Only applies to Wide Field Slitless Spectroscopy (WFSS) exposures. Sets the maximum number of iterations allowed for iterative outlier rejection during determination of the reference background scaling factor. If set to 0, no outlier rejection will be applied. Defaults to 5.

--wfss_rms_stop

Only applies to Wide Field Slitless Spectroscopy (WFSS) exposures. If the percentage difference in the RMS of the background-subtracted image between iterations is smaller than this value, stop the iterative outlier rejection process. Defaults to 0, i.e., do all iterations up to wfss_maxiter.

--wfss_outlier_percent

Only applies to Wide Field Slitless Spectroscopy (WFSS) exposures. Sets the percentile of outliers in the data to reject on both the low and high end per iteration during determination of the reference background scaling factor Defaults to 1, i.e., keep the middle 98 percent of the data each iteration.

--bkg_list

Provides a list of background files to combine and use for subtraction. It can have one or more files separated by a comma with no spaces. This argument will be ignored for WFSS or if an association file is provided as input for the step.