DARK:

spd = dark( spd )

dark determines the dark currents and subtracts them from the data:

• dark current subtraction is done per detector and scan
• dark value is the median of the valid points of a dark block
• if there are dark measurements directly before and after a scan with the same gain setting and reset interval, the dark is interpolated linearly and subtracted from the scan data
• if one of these two darks is missing or contains no valid data, the remining one is used and subtracted (early mission!)
• if neither of these two darks can be used, the average of all darks with same gain and reset for the entire SPD is subtracted
• if there is no valid dark with the same gain setting and reset interval at all, the scan is flagged as NODATA
• for photometric checks allways the preceding dark is subtracted
  
• dark currents are dark subtracted themselves
• dark subtraction is critical for low fluxes
• antimem has to precede

• cal21_x: dark current/noise for x=1, 2, 4 (or 8) sec reset interval

Alternative: use dark_inter for a highly interactive dark subtraction