The data were reduced using the CIA[^6] (CAM Interactive Analysis) software package. First, glitches due to cosmic rays were removed using a spatial and temporal filtering algorithm (Starck et al. 1999), then a dark current image was subtracted from all the frames. Because of the time lag in the response of the detectors, transient effects had to be corrected for. For this purpose, we used the algorithm developed by Abergel et al. (1996). Finally, the images were divided by a flatfield image to correct for the non-uniform response of the detectors over the array. Most images were made up of over 500 frames that covered a large surface area. This enabled us to use a median image of all the frames as a flatfield image. Flatfielding is here a critical operation as we are looking for intensity variations in dark objects and any flux mismatch between two contiguous raster images appears as a step in an intensity profile. Additional pixels had to be masked where the trails of glitches still affected the response of the detectors or induced a downward step in the flux level.

### Noise estimation

In order to estimate the rms noise levels in our final images, 