The wavelengths falling on slits S2, S3, and S4 may change because of slow deformation of
the spectrophotometer main frame casting, compression of the gasket between the base and lid, or
a shift of some of the optical components. To determine whether the Q-setting table in use is
applicable (i.e., that ozone observations are being made on correct wavelengths), mercury lamp
tests are performed. A mercury lamp supplied with the instrument can be fixed above the inlet
window to illuminate slit S1. For routine checks it is sufficient to measure the value of Q1 when the
effective mercury wavelength 3129 A.U. falls centrally on slit S2. Tests should be conducted at
different temperatures in order to check on the temperature dependence of the Q-lever settings.
The test procedure is as follows