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 
and 
. The algorithm
removes the cosmic rays from the sky spectrum before interpolation,
using the CCD detector parameters 
for the read-out-noise,
for the gain, 
as threshold of the
kappa-sigma clipping and 2*
+1 as size of the
rejection window. The sky is fitted along the columns of the spectrum by
a polynomial of degree 
. Unique or independent spatial profiles
are computed depending of the value of the parameter 
.
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