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Fig. 1

image

Example of chop-nod observation footprints (top), and observed position projected on the sky (bottom). In the first nod, the detector alternates between two optical paths to observe the object and the background. These paths have slightly different telescope background signals. In the second nod, the optical paths are inverted after slewing the telescope. By adding the two object-background differences, it is possible to remove the effects of the different telescope background levels.

This figure is made of several images, please see below:

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