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Fringe correction in the PREREPIX package

Fringe correction in the PREREPIX package . M. Radovich (OAC, Naples) Y. Mellier, E. Bertin, G. Missonnier (IAP). Prereduction. Implementation: PERL + PDL [+ pTK] Status of the pipeline: all the prereduction steps are implemented Main problems: fringe correction and masking

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Fringe correction in the PREREPIX package

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  1. Fringe correction in the PREREPIX package M. Radovich (OAC, Naples) Y. Mellier, E. Bertin, G. Missonnier (IAP)

  2. Prereduction • Implementation: PERL + PDL [+ pTK] • Status of the pipeline: all the prereduction steps are implemented • Main problems: fringe correction and masking • To be done: quality assessment (e.g. fringe correction); the generation of masks should be improved

  3. The Preprocessing steps Generation of master frames: • BIAS • DARK • DOME & TWILIGHT FLATS, • FRINGE • SUPERFLAT Science frames: preprocessing + generation of • Weight maps • Previews

  4. Fringe correction • The method: for each science frame we assume that • A smoothed background is subtracted from each image, the so obtained frames are combined to build a master fringe frame • Objects cannot be masked since “detections” may include fringes • For each science frame, the scaling factor is computed using a robust fitting method • >5 frames and large dither patterns are required !! • Is the fringe pattern stable during one night/run ? PCA analysis could be used. • To be done: evaluate fringe residuals

  5. Rescaling of the master fringe frame: counts in the master fringe and the image frames are displayed. The red line shows the results of the robust fit

  6. Example of fringe subtraction with and without rescaling K=0.83 K=1.0 Raw image (section)

  7. Fringe frame built from a run with: narrow dither sequence, only two separate fields Raw image (first field) Corrected image, first field Corrected image, second field

  8. Superflat • One master frame for each run, obtained from the combination of fringe corrected science frames. • To minimize the impact of bright stars, before combination science frames are “smoothed” using SExtractor

  9. Speed Example: processing of 300 files for each step

  10. Conclusions • The empirical method for fringe subtraction (combination of frames + computation of the scaling factor by a robust fit) works for CFH12K data even in non optimal conditions (narrow dither, few different pointings, one fringe frame per run ) • The algorithm is still slow • Need to evaluate residuals (how much the correction is good ?)

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