Trace reference

The flat field is the only exposure where all the fibers are well illuminated. It is uses to define the position of the light on the detector. This information is needed to extract the arc spectra. A reference flat is also needed identify the position of the bundle gaps.

For the science reduction, the flat needs to have been corrected for differences in quantum efficiency between the chips. This is wavelength dependent. So, we need the flat for the wavelength calibration but we need to the wavelength calibration to produce the science flat.

This circular requirement forces us to first reduce the flat without the QE correction and use that as reference to extract the arc and calculate the wavelength solution. Then we throw it away and reduce it again, this time with the wavelength solution and a QE correction.

Let us call this first flat a “trace reference”. It will not be used to flat field anything, but it is needed nonetheless.

As we have done before, let us first set some key variables.

mdf = 'gsifu_slitr_mdf.fits'
bias = '../calibrations/S20060314S0091_bias.fits'
rawdir = '../tutorial_data/'

Then we call gfreduce to do the bias and overscan correction, and the extraction (fl_extract='yes'). We use our modified MDF. There is no need for the interactive mode. This step takes a few minutes to run.

imdelete('g@flat.lis')
imdelete('rg@flat.lis')
imdelete('erg@flat.lis')

gfreduce(flat, rawpath=rawdir, fl_extract='yes', bias=bias, \
         fl_over='yes', fl_trim='yes', mdffile=mdf, mdfdir='./', \
         slits='red', fl_fluxcal='no', fl_gscrrej='no', \
         fl_wavtran='no', fl_skysub='no', fl_inter='no', fl_vardq='yes')

We can check the extracted spectra with gfdisplay. Once the reconstructed image shows up, put the cursor on any hexagon and press the <space> bar, a spectrum plot will show up. Notice the big jump around 5100 Angstrom. That location is where two CCDs meet. That jump is due to difference in quantum efficiency. Clearly, we would not want to use this flat as a flat field. It will do fine as a trace reference however.

for flat in iraf.type('flat.lis', Stdout=1):
    flat = flat.strip()
    iraf.gfdisplay('erg'+flat, 1, version='1')

- Hit <space> with the pointer on an hexagon.