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## The ARF (ancillary response file)

Once an arf has been created (see the CIAO threads on how to generate an arf) we can load the arf and create a fluxed spectrum.

`isis> load_arf("Capella_MEG_garf.fits");`

    *** Encountered ARF uncertainties <= 0;  Reset those to 1 ***

    Current ARF List:
     id grating detector   m prt src   nbins  exp(ksec)  target
      1    HETG     ACIS   1  2   0     8192    88.88  CAPELLA

Which tells us we have loaded a MEG arf for order +1.

Use the assign_arf command to associate the arf we just read in to spectrum 10.

`isis> assign_arf(1,10);`

Next we need to create a fluxed spectrum - that is a spectrum that has been divided by the arf and the exposure time. This results in a spectrum that is an approximation to the source spectrum blurred by the instrumental line spread function (lsf). Note that this calculation assumes that the effective area function is slowly varying so that it can be moved in front of the integral over wavelengths contributing to each histogram bin; this approximation may break down near instrumental edges and other sharp features in the effective-area function.

`isis> flux_corr(10,1);`

This assigns the arf to spectrum 10. The second argument applies a S/N threshold so that data in low count bins do not dominate the plotted spectrum.