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Run Sky-Subtraction on the Science Exposures

As usual, we take a look at the parameter file first:
editpar subsky

type q to end

minlambda           4800.000000       min wavelength of extraction
maxlambda           7800.000000       max wavelength of extraction
exclude                      5       half width of exclusion region
deltaknot             1.000000       dispersion knot interval, in pixels
splineorder                  3       order of dispersion spline fit to sky
2d_spline                    n       Perform 2-d spline fit to spectra?
medbox                      15       boxsize for running median CR rejection
siglimit              8.000000       CR rejection threshold in sigmas
diag_0              5520.000000       lower limit of diagnostic range
diag_1              5620.000000       upper limit of diagnostic range
edge                         1       excluded edge of spectrum
edge2                        1       excluded edge of other side
objshift              0.000000       number of pixels to shift object position
noise                 4.100000       CCD read noise
gain                  0.470000       CCD gain
Since these are relatively short slits, we do not need to fit a 2-D spline to the spectra. The parameters deltaknot, medbox, exclude, and edge/edge2 are important for getting an optimal sky fit. Once these look fine, we can run subsky on the science frames.
subsky -m ift0012 -f ift0009_f
subsky -m ift0012 -f ift0010_f
These commands generate ift0009_s_cX.fits and ift0010_s_cX.fits, respectively. Figure 9 shows a section of one of the sky-subtracted frames.

Figure 9: Section of sky-subtracted science exposure generated by subsky.
Image subsky-sampleIMACS

Next: Extracting 2-D spectra Up: Reducing the Spectra Previous: Flat-field the Science Exposures   Contents

Edward Villanueva 2014-08-27