The figure below shows the results from several through-put measurements of flux standards. The Y-axis is the apparent AB magnitude of star observed at zenith which gives one detected photon per second per Angstrom. In each case the lowest resolution grating was used (R158V) except for RED+2 (R300V was used) with a wide slit (10 arcsec). The conditions were photometric in all cases, with negligible dust levels present. This shows the response of the whole IDS system (i.e. INT primary + secondary + IDS optics + detector response) from ~3200 to ~8000Å.