The self-correlated image may be used to find flat-fielding faults and faint diffuse objects of a given size.
To reduce the influence of bright objects or cosmic rays; the user may elect to employ a cut out pixel count value where any pixel found to be above that value is ignored. The cutout value is determined by the user inputting a global mode value (usually the sky background count - obtained via HISTPEAK), the background count standard deviation and the number of standard deviations above sky level at which the cutout should occur.
The user is required to enter a value for the size of object(s) of interest (roughly the template size) and also the image pixel size.
The value for each pixel of the output image is determined as follows. An imaginary circle is drawn about the pixel and all pixel pairs within that circle, that lie on opposite sides of the centre from each other, are stored.
Each pair is then considered in turn and the modal count value subtracted from each. The resultant residual pixel count values are then multiplied together. The values found for all the pairs are then summed, the total divided by the number of pixel pairs found and the square root taken. In the event of a negative sum being found the value given is the square root of the magnitude of the self-correlation multiplied by -1.
The resultant value is some measure of the extent to which points within that circle (about the current pixel) are correlated.
The method assumes some sort of symmetry is present in the objects detected but appears to work well on a wide range of image types.
A border is present in the output image which the same width as the radius of the template. All pixels within this border are assigned the value bad.
ESP --- Extended Surface Photometry