Description

The background subtraction step performs image-from-image subtraction in order to accomplish subtraction of background signal. The step takes as input one target exposure, to which the subtraction will be applied, and a list of one or more background exposures.

Multiple background averaging

If more than one background exposure is provided, they will be averaged together before being subtracted from the target exposure. Iterative sigma clipping is applied during the averaging process, to reject sources or other outliers. The clipping is accomplished using the function astropy.stats.sigma_clip. The background step allows users to supply values for the sigma_clip parameters sigma and maxiters (see Step Arguments), in order to control the clipping operation.

The average background image is produced as follows:

  • Clip the combined SCI arrays of all background exposures

  • Compute the mean of the unclipped SCI values

  • Sum in quadrature the ERR arrays of all background exposures, clipping the same input values as determined for the SCI arrays, and convert the result to an uncertainty in the mean

  • Combine the DQ arrays of all background exposures using a bitwise-OR operation

The average background exposure is then subtracted from the target exposure. The subtraction consists of the following operations:

  • The SCI array of the average background is subtracted from the SCI array of the target exposure

  • The ERR array of the target exposure is currently unchanged, until full error propagation is implemented in the entire pipeline

  • The DQ arrays of the average background and the target exposure are combined using a bitwise-OR operation

If the target exposure is a simple ImageModel, the background image is subtracted from it. If the target exposure is in the form of a 3-D CubeModel (e.g. the result of a time series exposure), the background image is subtracted from each plane of the CubeModel.

The output results are always returned in a new data model, leaving the original input model unchanged.

Upon successful completion of the step, the S_BKDSUB keyword will be set to ‘COMPLETE’ in the output product.