#
RONCHICAL

###
Purpose: Ronchical is used to do the preliminary processing of ronchi images.

## What does it do?

Ronchical applies the camera corrections (dark, clamp and smear) and
averages all 6 bands. It then uses the average calibration image
((east + west)/2) and the diffuser image to generate a flat-field that
is divided into the ronchi image to remove high spatial frequency camera
artifacts.

At this point, if one plots a line across the
image, we see a periodic signal, but it more closely
resembles a sin**2 function than it does simple sine function.

(Slice through dark, clamp
and flat-field corrected ronchi image)
Therefore, we take the square root, giving:

(Square root of slice through dark,
clamp and flat-field corrected ronchi image)
Which is much more "sine wave like".

Now we need to compute a constant that, when subtracted from the ronchi
image, will produce a sine wave that is symmetric about zero. Since the
distribution of data values in the ronchi images is essentially bi-modal, a
simple mean or median subtraction does not work. Instead, we compute
a histogram of all of data values within a specified distance from the center
of the image. Using this we find the "central-value".

An example of how the central value is computed is shown in this figure:
(Ronchi Histogram plot)

Finally, the image is normalized such that the peak-to-peak values are -1.0 to 1.0 .

(Slice through a final, calibrated
ronchi image)

Last updated by Cliff Toner on Friday, August 14, 2005, 11:40