Having defined:
where slit is the slit width in arcsec, seeing is the effective seeing in arcsec, T_{x} is the slit transmission along the direction x and L_{x}=1T_{x} is the slit loss along the direction x. Being the slit width set directly from the user on the input form, it is more appropriate to calculate this correction term on the fly. ^{2.2}
From tabulated data one could for example find:

The ETC reports the total efficiency comprehensive of the loss due to the slit. For an extended source the slit trasmission is one.