2012-10-06

New Photo Preprocessing

This year St. Václav's day and long weekend I dedicated to develop a new approach to a photometric preprocessing in Munipack. My body had treated from flu and cold so I spend a long time at terminal.

As the photometric preprocessing I means photometric corrections:
  • correction for dark current (dark frame),
  • removing of preamplifier offset (bias),
  • correct a variable light response of (CCD) detector together with whole optical system (flat-field).
Munipack's older approach on the corrections has roots in '90, when we acquired dark frames for every exposure time. The bias is included in both scientific and dark exposure and the bias correction is naturally included in boths. The same approach is valid for both scientific and flat frames.

New approach is modeled as following the widely reccomended method in today. A photometrically corrected image Icij is computed for every input image Iij as
  Icij = (Iij - t Dij - Bij) / fij,
where i,j is an index of a pixel, Dij is a dark frame t is ratio of exposure times  of I and  D. Bij is the bias and fij is normalized flat-field frame (to save absolute photometric fluxes):
  fij = Fij / 〈Fij.
〈Fij means averaged level determined by robust mean.

I think the change does not cause any controversy. More over:
  • The older approach is still available when bias is treated as zero.
  • From previous point of view, the approach is just primary a new façade for users. More easy user's usage.
  • User is not confused. The method is exactly the same as textbooks describes.
  • Important parts of algorithms are the same, changes in functional parts are minor.
  • Observation time save. A long-exposure dark would be prepared and applied to all shortest exposures (also on longer ones, but the noise can devalue frames).
  • Bias exposures are very short, so acquisition of a lot of biases it is not too time consuming.
  • Processing tools has improvements in use of previous data products and no temporary reduction of frames are created.
This is practically a complete new implementation of preprocessing routines. Of course, all my previous experiences are also included. The work is currently uploaded to Mercurial repository and will be available in a first issued public release.
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