Electrical ghosts

The HAWAII detector generates 'electrical ghosts' which seem to have a reproducible behavior. The effect basically consists of an additional signal, which, on one row, is proportional to the sum of the intensity along this row and the row 512 rows away.

This effect is mostly an issue in spectroscopy of relatively bright sources (e.g. standard stars), as it might affect the level of the continuum, and e.g. the line equivalent widths.

The procedure to remove the electrical ghost is to use a dedicated eclipse recipe called ghost. The simplest way is probably to use it as:

% isaacp ghost *.fits

On some Unix systems, *.fits refers to too many file names to be handled correctly on the command-line. The solution is to use a loop, depending on the shell you use. In csh/tcsh this would be:

% foreach i (*.fits)
% foreach? isaac ghost $i
% foreach? end

The ghost recipe writes into any processed file header two keywords: GHOSTREM is a flag indicating that the process has been applied to the file, and GHOSTVER indicating what the algorithm version ID is. This way, the algorithm cannot be applied twice to the same file (unless you use the -force option).

The implemented algorithm is the following:

• Create a 1d signal in which each element contains the sum of all pixels along a line of the input image. In other words, collapse the 2d image along the x axis.
• The signal in [1..512] is added to the signal in [513..1024] and visa-versa.
• Multiply by 1.35e-5.
• Subtract element N of this 1d signal from line N in the input image.