Response function

The response function represents the relative fiber throughputs for the IFU unit. It is derived from the flat field.

A twilight can be used to ensure the flatness of the field of view, however that twilight needs to be obtained at the same position and elevation as the target to match the flexure. That is rarely the case for the twilight obtained as part of the Gemini Baseline Calibration.

Based on experience from several Gemini staff, the twilight has not been found useful at all. The GCAL flat is sufficiently flat within the small IFU field-of-view anyway.

Calculating the response curve is simple and quick. Note that we do not use the default value for order and func. Setting order=45 and func='spline3' seem to work better.

for flat in iraf.type('flat.lis', Stdout=1):
    flat = flat.strip()
    iraf.imdelete(flat+'_resp')
    iraf.gfresponse('eqbrg'+flat, outimage=flat+'_resp', sky='', \
                    order=45, func='spline3', sample='*', \
                    fl_fit='yes', fl_inter='no')

Let us have a look at it.

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

- Press <spacebar> to plot the spectrum under the cursor.

The response spectra should always hover around 1 because of the normalization across all the fibers.