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The transmission curves of the Swift/UVOT UVW1 and UVW2 filters have extended red tails. This means that the flux measured with these filters will be a combination of the ultraviolet flux from the source and a contribution from the source's optical flux. The degree to which the optical flux affects the observed flux in the filter will depend on the spectrum of the source. For a source with a predominantly optical spectrum, the percentage of light that leaks into an ultraviolet filter could be significant compared with the actual ultraviolet flux. Figure 1 shows the effective area curves of the Swift/UVOT UVW1 (red) and UVW2 (blue) filters.

Figure 1: The effective area curves of the Swift/UVOT UVW1 (red) and UVW2 (blue) filters showing their red leaks.

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