HOW-TO Subtract sky background

Overview

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:

  • RegriddedFrame: see text above
  • ReducedScienceFrame: when/if an IlluminationCorrectionFrame is applied, a background image is created and the illumination correction is applied to everything but the background. Afterwards the background is added again.
  • SourceList: when SExtractor is run, default behaviour of SExtractor is to subtract a background.
  • In SWarp. Default behaviour of SWarp is to make and subtract a background image. In Astro-WISE this default is changed to no background subtraction.
  • In addition, background images are determined in the algorithms for HotPixelMap, ColdPixelMap, CosmicMap and SatelliteMap.

Configuring background subtraction

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:

  • 0: Leave background subtraction to SWarp
  • 1: Create a background image outside SWarp, which gives better results than SWarp
  • 2: Subtract a constant as background, which is determined by iteratively clipping around the median pixel value

See HOW-TO Configure process parameters 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.

The background image

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:

  • Determine all pixels which are part of sources. This is done by running SExtractor on the image and using the “segmentation” check image.
  • Exclude those pixels and calculate the median of the other pixels.
  • Replace the pixels attributed to sources with the median calculated in the previous step.
  • Run SExtractor to obtain the “background” check image. This is the background image that will be subtracted.