The relative detector pixel response is calibrated by dividing by a scaled "flat field" calibration image. For LBC, flat field images are obtained by exposures of the twilight sky.
For LBC images an important effect must be kept into account: the sky concentration due to the optical distorsion. Due to the optical design of LBC, this effect is particularly evident, of the order of 5-6% across the FoV of the instrument. The light distribution follows a pin-cushion pattern, enhancing photon counts on the center of the focal plane and decreasing the counts on the borders. This effect depends on the filter used for data aquisition. This multiplicative effect may be removed using a correction image of optical distorion which can be obtained or by theoretical distorsion maps or estimating the effective pixel area using the astrometric solution.
Currently, LBC masterflats show an unexpected residual radial dependence of the response. This effect is under investigation but it is expected to be mainly caused by vignetting.
Given a set of flat field files, each image has to be debiased as described in Section masterbias. Then, the effective area of all pixels of each debiased flat image is corrected using the correction map, according to the given filter, as previously described. For each corrected frame, the global median over the four chips is computed and this value is used to rescale the four chips in order to equalize the response. Finally, the masterflat is obtained combining the equalized frames.
Bad Pixel Masks (BPM)
Bad Pixel Masks are also extracted from flat field
images. These corresponds to hot pixels and columns in the four
chips. The images provided below contain 0 in the good pixels and
1 in the bad pixels. The stability of BPM has not been
analyzed. We therefore provide the individual BPM obtained in the
Download Calib DATA
It is possible to download all calibration fields in the LBC Survey Centre at this address: