The ISP data from late 2006 / early 2007 makes a good demo case for building and testing detrend images.

Some notes from the analysis:

• the shutter correction can be measured and applied, but there are some limitations:
  • the input images with excessive saturated pixels should be excluded - this is especially an issue for the ISP since it seems to saturate with a severe non-linear regime (>60k).
  • the correction does not work below ~0.05 sec. The correction image ranges from -4 msec to +45ms.
  • the correction appears to have some variations as a function of time:

the residual images include a term which appears to be the difference between the shutter correction at two small offsets. I conclude that the shutter, detector, and optics are not remaining co-aligned. this effect appears at below the 1% level.

• flat-field residual images (verify mode)
  • g,r,i images are easy to explain
    • short exposures (<0.05) show the effect of the shutter correction hitting its limits
    • some images show large scale gradients, perhaps clouds or sky illumination pattern
    • some images show the effect of non-linearity / saturation

• z band images have some peculiarities
  • the show all the expected features above (not peculiar)
  • images from 2007/1/18 seem to be very poor most of these are 240 sec exposures - bad dark correction?? some of these have stars (ie, night, not twilight) - IR sky glow?

• y band images show similar issues
  • problem from 2007/1/18

looking in detail, it is clear that the dark correction is poor for long exposures