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| Joint Astronomy Centre |
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Anomalous patterns/structures in UFTI imagesBelow we show examples of some unusual image structures that have been seen in UFTI frames. In some cases, the fix seems to be to take a sequence of short dark exposers (use the "flush array" sequence in the ORAC-OT template library). These rare structures often result from leaving the array exposed to background signal (i.e. leaving the shutter open with a filter in place) or from observing very bright sources. Fringes may appear if bright stars are present just outside the target field-of-view.
1The difference of two spatially-offset (by a few arcseconds) frames; hence fringe seen in positive and (offset) negative. 2Mosaic from a 5-point jitter with fringe at same loaction in each frame. 3Mosaic from a 5-point jitter with ghosts (one per image, shifted relative to bright star in each image) appearing in the resulting mosaic after registration and averaging. 4Horizontal "banding" from electronic pickup (10-20 rows wide; amplitude ~10 counts), carried through into the dark-subtracted and (self)-flat-fielded object frame. 5Mosaic from a nine-point jitter. The 1-0S(1) filter vignettes the bottom few rows of each image, so when the nine frames are registered and averaged horizontal strips of noise appear in the mosaic. Vary N-S jitter offsets and MEDIAN average the data (this can be done with ORAC). 6Sudden change, from clean images to this after only 2-3 exposures; power-cycling the electronics fixed this (at least temporarily). There is also available a short document (postscript file) written by Sandy Leggett when she investigated the appearance of "fringes" in IRCAM data (from 1997). The fringes seem to be caused by reflections or scattered light from IR-bright stars that are situated just outside the array field-of-view.
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