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There may be low level background light with some of the gratings. Figure 10 shows the typical pattern of scattered light for various fibers when using the email@example.com grating.
Figure 10: Scattered light images for the firstname.lastname@example.org grating. UPDATED FIGURE NOT YET AVAILABLE
Keep in mind that scattered light is additive according to the illumination level of each fiber. With all fibers equally illuminated (as in a flat field), the scattered light contamination may be as large as 5% in some regions. On the target fields, faint objects will suffer from contamination when brighter objects are observed at the same time. To minimize the effects of scattered light contamination on the faint spectra to less than 5%, the brightness range of the targets should not exceed about 4 to 5 magnitudes. A magnitude range of 3 has proven to be ideal under most circumstances.
User verification of the scattered light can be carried out by observing a bright star down one fiber or by taking a strong exposure of the quartz lamp in the Simultaneous Comparison Source.
There are fiducial gaps in both fiber cables for the monitoring the background contamination.
The dohydra routine in iraf contains the option of fitting the scattered light as part of its process (an equivalent to the iraf apscatter routine).