Sky background subtraction is a factor in several places in the reduction process. In most cases SExtractor/SWarp is involved in the algorithm to determine the background image.
Sky background subtraction may be applied first and foremost in RegriddedFrame. This is the last opportunity in the data-reduction process before images are coadded into a CoaddedRegriddedFrame. But here is a complete overview of the places where background subtraction plays a role:
At this point it is not possible to configure the background subtraction in great detail, but it is possible to choose from several options in RegriddedFrame.
awe> pars = Pars(RegriddedFrame) awe> pars.show() RegriddedFrame | +--process_params | | | +--BACKGROUND_SUBTRACTION_TYPE: 0 | +--MAXIMUM_PSF_DIFFERENCE: 0.25 | +--swarpconf | | | +--BACK_DEFAULT: 0.0 | +--BACK_FILTERSIZE: 3 | +--BACK_SIZE: 128 | +--BACK_TYPE: AUTO etc.
Note in particular the process parameter BACKGROUND_SUBTRACTION_TYPE. The possible values for this parameter are:
for details about how to configure process parameters.
Note that a check is performed to prevent subtracting background both outside and inside SWarp; in other words, when setting the BACKGROUND_SUBTRACTION_TYPE parameter to something other than 0, the RegriddedFrame.swarpconf.SUBTRACT_BACK option has to be 'N', which is the default.
When option 1 is chosen for the BACKGROUND_SUBTRACTION_TYPE, a background image is created. How is this image created? The process consists of these steps: