hello,friends, i need to do rayleigh correct for L8 OLI,its parameters following: ROLL_ANGLE = -0.001 SUN_AZIMUTH = 161.83335842 SUN_ELEVATION = 35.29565303 what is the roll_angle? the sun_azimuth and sun_elevation is the sun_azimuth and sun_elevation of image center point? do i need to calculate sun_azimuth and sun_elevation of every pixel? how do i acquire those parameters by using IDL language? i am not able to use IDL acquire specify parameter in TXT fie?