This is a brief description of the processing done on the DISR raw images to produce the images seen in this directory ('UnSmoothed_Images').  The smoothing steps were not performed on these images in order to preserve high frequency features. 

In general, the on-board processing steps were reversed (square rooting, compression, flat fielding), and the instrument induced artifacts (shutter effect, dark current, bleed-through, bad pixels, etc) removed.  

The specific processing steps follow:



Processing of DISR images       Nov 13, 2005        Erich Karkoschka


1)  I estimated the compressor threshold for each image based on the
    relative frequency for the three lowest amplitude bins.
    I tested the estimates with the compressor simulation with three
    images.  The difference between actual and estimated threshold were
    0, 0.125, and 0.25, which is good enough.

2)  I changed the square-root lookup table into a real function.

3)  For adaptive schemes, I changed the slope of the function in both
    outer section to a new slope, which is a quarter of the original
    slope plus three quarters of the slope in the middle section.  This
    way, regions outside the limits are not as noisy as in the original
    images, but still noisier than in the middle region.

4)  I applied the reverse of the square-rooting to the data.

5)  I multiplied the data by the on-board flatfield.

6)  I created a images according to Andy's equation for each image and
    subtracted it from the data.

7)  I subtracted a constant data number from the whole image:  one
    quarter of the dark current number (which peaks at 34 DN).

8)  I subtracted the expected contributions of light during the vertical
    transfer of data in the CCD.  This is based on the assumption that
    each pixel sees the light at the actual position for the exposure
    time and the light of each pixel below (lower row number) for
    2 microseconds each.

9)  I subtracted the constant of 0.9 DN from the whole image.

10) For images taking after landing, I subtracted the following data
    numbers  14.5 - 11.5 * SIND(360.*((228-J)/165.-((228-J)/300.)**2))),
    where J is the row number from 1 to 256 (in the SLI row 1 is above
    the horizon).

11) I shifted the on-board flatfields by cubic interpolation for
    resampling.  The assumed shift of flatfield features with CCD
    temperature T(K) in micropixels is:
    SLI:  X = 1485 (T-254.6)
          Y = -569 (T-223.9)
    MRI:  X = 1678 (T-257.0)
          Y = -563 (T-221.9)
    HRI:  X =  819 (T-260.8)
          Y = -704 (T-228.0).
    I modified the flatfields according to obvious flatfield artifacts
    seen in high-altitude images, typically by about 1 percent or a few
    percent.  Most of them were adjustments next to the edge of the
    field of view.  For the HRI, however, there was an adjustment of
    about 1 percent across the whole field of view.  For the MRI, there
    was an adjustment of 2 percent over the top 50 rows.  The latter
    two corrections are probably due to non-constant intensity of the
    integrating sphere.
    I then divided the image by the adjusted flatfield.