The Hawaii array suffers from 50Hz pickup, which appears as lines that are almost aligned with detector rows. The strength of the pickup depends on how the array is readout and is a function of time. It is strongest in DCR and for some DITs in NDR. For NDR we have come up with a recommended list of DITs (See the ISAAC web page) and users are encouraged to use these DITs. As an example, before April 2001 DITs of 60 and 180 seconds were strongly affected by the pickup. After this date, we tuned the number of reads so that the pickup with these DITs is weak.
If most cases, the 50 Hz signal will not be a problem, but if you find that it is, a very simple methyod to remove it was recently developed by Fernando Comeron. It turns out that the periodicity of the 50Hz signal is very close to 6.5 pixels in the vertical direction. To see how this is useful, take an image where the 50Hz signal is strong, shift it by 13 pixels in the vertical direction and subtract it from itself. The 50 Hz disappears. This simple procedure does not account for objects so a more sophisticated procedure is required. Such a procedure may work as follows.
This method is probably only applicable to images which already have been sky subtracted and special attention has to be paid when the row is near the quadrant borders.
For data taken after 24/7/2001, when the detector readout speed was changed, the periodicity of the 50 Hz pickup is about 3.25 pixels. The procedure to remove it remains unchanged.