Section 4: Low Level Flats

A sequence of 10 full-frame flat fields were obtained with an exposure time of 0.1 sec (files 349803o.fits to 349820o.fits, see Data Collected). A set of 10 full-frame bias images were also obtained, with the LED connected to its power supply, but not used (files 349868b.fits to 349886b.fits, see Data Collected). Median stacks of the low-level flats and of the bias images were obtained, the difference image is shown below:

Low Level Flat Image

The mean counts within the image are ~130 ADU (~ 500 electrons) and so this image will show all significant traps at this level. The circular pattern arises from non-uniform illumination of the CCD by the LED. There is one major trap which destroys a swathe 4 columns wide upwards from pixel (860,793). There is one hot column, starting at pixel (1817,43) (but see also Biases). There is one significant dark spot at pixel (1251,459) which has the appearance of a small defect in the CCD coating since it is surrounded by a faint halo. There are numerous small dark spots scattered over the CCD, presumably resulting from dust motes on the CCD surface. The lower edge of the image appears to be slightly warm (possibly resulting from overflow of the serial register during the CCD-flush routine applied before an exposure). There is some minor flat-field pattern visible in the image.