LBC Cross Talk
The crosstalk (or XTalk) typically occurs during the simultaneous readout of multiple amplifiers. When multiple amplifiers are readout, such as occurs when using multiple amplifiers in a single CCD or multiple CCDs in a mosaic, the controller electronics relate the signals from pairs of extensions in multiextension format (typically pairs of CCDs in raw mosaic exposures, called "victim" and "killer", respectively).
The crosstalk causes pixel values produced by one amplifier to be affected by the signal in another amplifier. For this reason, the victim and killer pixels must be matched in "amplifier coordinates" (given by the ATM keywords).
Using a simple model for the crosstalk, the signal for a pixel in one amplifier, which we call the "killer", adds or subtracts a small amount to the pixel value read at the same time in another amplifier, called the "victim". A correction is obtained by multiplying the pixel value of the killer image by a crosstalk coefficient and adding or subtracting it from the matching pixel in the victim image.
Note that it is possible that a killer may also be a victim and that a victim may be affected by multiple killers. In our simple model each pair of killer and victim are treated independently and the killer pixel values used to correct a victim are treated as unaffected by other amplifiers.
The crosstalk correction should be performed before any other operation. The simple model of the crosstalk is that the raw data from the amplifier readout is used. Therefore the correction should generally be applied only to the raw data.
Correcting the Cross Talk
The victim pixels of each extension must be corrected taking into account the contribution of all the corresponding pixels in the killer extensions:
corrected = victim - xtcoeff * (killer1 + ... + killerN)
where the arithmetic is done on each pixel in the victim image extension with the matching pixels in the killer image extensions and the scale is the numerical coefficient xtcoeff. For LBC the xtcoeff is -1.83E-4.
Alternatively, LBC users can correct their frames with IRAF.
In this case, the task is:
cl> xtalkcor input output bpmasks xtalkfile
where bpmasks is the Bad Pixel Mask (BPM) image to be used and xtalkfile is the crosstalk coefficient file or the header keyword containing the xtalk file name.