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Estimating the Sky Background

The sky is measured in two regions usually located below and above the object spectrum and which pixel boundary limits are stored in the keywords \fbox{LOWSKY} and \fbox{UPPSKY}. The algorithm removes the cosmic rays from the sky spectrum before interpolation, using the CCD detector parameters \fbox{RON} for the read-out-noise, \fbox{GAIN} for the gain, \fbox{SIGMA} as threshold of the kappa-sigma clipping and 2* \fbox{RADIUS}+1 as size of the rejection window. The sky is fitted along the columns of the spectrum by a polynomial of degree \fbox{SKYORD}. Unique or independent spatial profiles are computed depending of the value of the parameter \fbox{SKYMOD}. Sky estimation parameters are displayed together with extraction parameters by:

Midas...> SHOW/LONG e.

The commands:

Midas...> SET/LONG LOWSKY=50,180 UPPSKY=220,350
Midas...> SKYFIT/LONG ccd0056 sky56

generate an image sky56.bdf of which size is identical to the input image ccd0056.bdf and corresponding to the interpolated sky background. Sky subtraction is performed with:

Midas...> COMPUTE/IMAGE corr56 = ccd0056 - sky56

Petra Nass