The application HISTPEAK examines the pixel values within an image and determines a number of statistical quantities.
The application can be used with the following syntax:
% histpeak in=p2 use=w sfact=4 device=x2w
This leads to the NDF p2 being examined, using the whole image, smoothing the histogram with a filter of radius 4 counts and displaying the histogram on device x2windows. Full details of the parameters may be found in Appendix .
The alternative to using the whole image is to use an ARD file to define the parts of the image to be ignored. In that case the syntax is:
% histpeak in=p2 use=a ardfil=^areas.dat sfact=4 device=x2w
In this example the source image p2 is used together with the ARD file definition areas.dat (note the use of the `^ ' character). The histogram generated to calculate the modal value is smoothed using a Gaussian filter of radius 4 counts. The histogram generated is displayed on device x2w. Other examples are shown in Appendix .
Full details of ARD files may be found in Appendix .
The program output generated is in the the following format:
Filename: p2 Title: Raw Plate Image Shape: 201 x 201 pixels Bounds: x = 1700:1900 y = 600:800 Image size: 40401 pixels HISTPEAK Results: p2 Pixels (used): 40401 Pixels (bad): 0 Lowest count: 4768.000 Highest count: 9388.000 Skewness: 0.516 Kurtosis: 1.795 Mean: 6226.607 Median: 6210.462 Histogram modal values: Unsmoothed: 6179.000 Smoothed: 6176.000 Projected: 6175.306 Interpolated: 6193.840 Absolute dev.: 333.494 Variance: 183890. Standard. dev.: 428.824 Back. st. dev.: 365.752 Smoothing filter radius: Radius request: 4 Radius actual: 4 Contents of the most occupied histogram bin: Unsmoothed: 60.000 Smoothed: 46.204 Interpolated: 39.609
The first section gives the name of the file used, the shape of the image, its title and the co-ordinate range involved. This is output as soon as the file name has been input, thereby allowing you to exit the application at an early stage if the wrong file has been requested.
The later sections are data derived either directly from the pixel values in the file or are determined following the construction of a histogram containing the pixel values. Each of the histogram bins has a default width of 1 count (or larger if the count range present in the image is large). The peak in the histogram is used to determine the modal value by a number of routes. The methods are as follows:
The standard deviation of the pixel values in the image is calculated using the standard equations for a Normal distribution. A value (SIGMA) is also derived, evaluating the standard deviation of the pixel value distribution in the region of the histogram immediately surrounding the modal value. For a pure noise image these two values would be expected to be the same, but the presence of any objects or image flaws acts to skew the distribution, generating `outliers' which quickly causes the standard deviation of the image as a whole to become large compared to the value obtained for parts of the image where no objects are imaged.
A crude estimate of the influence of the outliers may be obtained by considering the ratio of the normal standard deviation to the absolute deviation of the image. Alternatively, the skewness and kurtosis (third and fourth moments of deviation from a Gaussian distribution) may also be considered. It should be noted that the kurtosis value provided has had its base value of three subtracted to allow more digits to be displayed.
As a result of the default bin width used (1), images with a pixel count range less than 3 will not be examined by HISTPEAK. To overcome this, images may be manipulated using KAPPA's CMULT to increase their range by a suitable factor.
ESP --- Extended Surface Photometry