WFS Pipeline Processing

• De-biassing and trimming

  stacked bias frames using the default clear (flash) and readout (slow) speeds show some low level repeatable structure therefore full 2-D bias removal was necessary. For debiassing and trimming the data and bias sections as specified in the FITS header were used.

• Bad pixel replacement

  bad pixels and dud partial columns have been flagged and interpolated over. The bad pixel mask files are available by clicking here:... CCD#1 ... CCD#2 ... CCD#3 ... CCD#4 ... note that CCD#2 has zero entries in the bad pixel file.

• Non-linearity correction

  linearity tests using sequences of dome flats revealed CCD#2 and CCD#4 to have significant non-linearities over the whole dynamic range. CCD#1 and CCD#3 are essentially linear to <1% over the full range. A non-linearity correction has been applied via a Look Up Table (LUT) to all data. CCDs #2 & #4 share one ADC and #1 & #3 another. We believe most of the non-linearity arises in the ADC and should therefore be stable with time
  (see known WFC foibles for more details).

• Flatfielding

  for each observing run a sequence of master sky flats was constructed. Because of readout overheads it is impossible to generate complete sky flats in UBVRIZ each night, hence flats from several nights were combined to produce the master flats. Some low level fringing <1% is still visible on the master I and Z sky flats but for the current processing we have ignored this problem (see the section on flatfield frames for examples).

• Defringing

  both I and Z-band data frames suffer from significant sky fringes. R-band images also show weak fringing due to the red tail of the Harris glass filter. We are currently investigating and developing algorithms for automatically removing the sky fringes and are updating the reduced data accordingly when we are satisfied with the results (see the section on fringe frames for examples).

• Gain correction

  although processed independently a mean flatfield sky level was used to place all the CCDs on the same response zeropoint system, ie. sky in deep frames should be the same for all CCDs to better than 0.5%, the residual non-linearity level.

• Astrometry

  an astrometric solution based on GSC objects visible in the data frames has been used to define a 1st pass astrometric solution. World Coordinate System (WCS) information is embedded in the reduced images FITS headers. Currently the external accuracy of this astrometry is limited by the GSC accuracy to about 1 arcsec. Further improvements of this are planned. Note that most extant image display programs do not recognise the RA--ZPX and DEC--ZPX keywords even though it is the new WCS standard (see astrometric distortions for more information).

• Photometry

  approximate photometry at the 10% level can be obtained by using the following default CCD zero-points: U = 22.4, B = 24.5, V = 24.4, R = 24.4, I = 23.7, Z'= 22.2 ie. that magnitude star which gives 1 count/s total. Note that these zero-points are in counts and the Z' system denotes natural WFC filter +CCD response approximately normalised to the Vega system.

  The renormalised CCD gain is approximately 3.12 so to convert to photons/s magnitude zero points add 1.24 magnitudes. Full colour equations and accurate photometric zero points and extinction correction for each survey night are in the process of begin constructed (see the section on photometric calibration for more information).

• Object Catalogues

  a standard APM-style object detection and parameterisation algorithm is being run on all data frames to provide a first pass image catalogue for the data. The output format for the catalogues is a separate FITS binary table.

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