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Twilight Flats

ISAAC imaging data from the SW imaging arm are flat fielded with twilight flats. The flats are derived by imaging a region of the sky relatively free of stars. Between 10 to 25 exposures with constant DIT and NDIT are taken for each filter and a robust linear fit between the flux in each pixel and the median flux of all pixels is used to produce the flat field.

UT1 does not have a flat field screen, so it is not possible to take dome flats.


Christopher Lidman