1. Find the maximum pixel value – if it is less than 0.5 the loaded image is not suitable
for use as a master flat and so it is rejected with an error message presented to the
user. (No realistic flat field correction image taken with a camera down a
microscope should have a maximum pixel value of <1 assuming grey level readout
is on an integer scale starting at 0.) This condition also prevents the possibility of
divide-by-zero in step 3 below.
2. Find the mean pixel value.
3. Now normalise the image by dividing all pixel values by the mean.
The master flat is now pre-processed and ready to use for flat field divisional correction.
See below for the algorithm.
The ‘
Use corrections mask?
’ option, when selected, uses a user-supplied custom
binary mask image to mask all optional pre-processes. If no user-supplied custom mask
image is provided or if this option is not selected then all optional pre-processes will be
performed on all pixels of the frame (i.e. a full support mask will be internally generated).
You chose your what custom corrections mask image to load with the ‘
Select
’ button
below this check box. This brings up a file chooser dialogue box from which you can
select a file. File formats that are acceptable for use are those with the following
extensions: ‘.ppm’ (a PPM P6 formatted image), ‘,pgm’ (a PGM P5 formatted image),
‘.bmp’ (an MS Windows BMP formatted image of either 8 bpp or 24 bpp).
In order to be accepted as a user-supplied custom mask, the custom chosen file must
also pass the following tests and pre-processing (these take into account and operate on
all pixels in the chosen mask image i.e. the mask image itself is not subject to masking):
The mask image must be the same height and width (in pixels) as the current main image.
If it is not then this chosen mask is rejected with an error message being shown to the
user and the default internal full-support mask is used instead (of course the user has the
option to try again and load a different mask).
OptArc AF51 Camera Page 82 of 99 User Guide v1.02