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Ghost with the NB_4.07 filter.

This filter has a ghost around (x,y)=(530,330). It is probably caused by imperfections with the 4.07 micron NB filter, as it is not seen in other long wavelength filters. In most cases, the ghost is removed by nodding, but some residuals can be seen when paralactic angle is changing quickly. This occurs when the telescope crosses the meridian near Zenith. Flats taken with the 4.07 micron NB filter will contain this ghost, so it may be better to flat field with one of the other NB filters.

Christopher Lidman