  Using the atmosphere as a detector volume, Imaging Air Cherenkov Telescopes
(IACTs) depend highly on the properties and the condition of the air mass above
the telescope. On the Canary Island of La Palma, where the Major Atmospheric
Gamma-ray Imaging Cherenkov telescope (MAGIC) is situated, the Saharan Air
Layer (SAL) can cause strong atmospheric absorption affecting the data quality
and resulting in a reduced gamma flux. To correlate IACT data with other
measurements, e.g. long-term monitoring or Multi-Wavelength (MWL) studies, an
accurate flux determination is mandatory. Therefore, a method to correct the
data for the effect of the SAL is needed. Three different measurements of the
atmospheric absorption are performed on La Palma. From the determined
transmission, a correction factor is calculated and applied to the MAGIC data.
The different transmission measurements from optical and IACT data provide
comparable results. MAGIC data of PG 1553+113, taken during a MWL campaign in
July 2006, have been analyzed using the presented method, providing a corrected
flux measurement for the study of the spectral energy distribution of the
source.
