In the same way that it was useful to display images at intermediate stages in the data reduction, it is often useful to be able to calculate and list gross statistics (mean value, standard deviation etc.) for images as they are processed. There are a couple of applications which provide this facility. Proceed as follows.
The following information should be displayed:
Pixel statistics for the NDF structure /acdscratch/sc5/targets/ngc2336_r_2 Title : N2366 R NDF array analysed : DATA Pixel sum : 1.6012529E9 Pixel mean : 1267.44 Standard deviation : 279.5582 Minimum pixel value : 166 At pixel : (1, 547) Co-ordinate : (0.5, 546.5) Maximum pixel value : 50963 At pixel : (1023, 146) Co-ordinate : (1022.5, 145.5) Total number of pixels : 1263376 Number of pixels used : 1263376 (100.0%)
stats ngc2336_r_2 logfile=ngc2336_r_2.log
Text file ngc2336_r_2.log will be created containing a copy of the output.
stats ngc2336_r_2 clip=2
The statistics will again be displayed, but this time the clipping will have been applied before they were computed.
then type histpeak and respond to the prompts:
histpeak ESP HISTPEAK running. IN - Image NDF filename /@ngc2336_r_2/ > ngc2336_r_2 Filename: /acdscratch/sc5/targets/ngc2336_r_2 Title: N2366 R Shape: 1124 x 1124 pixels Bounds: x = 1:1124 y = 1:1124 Image size: 1263376 pixels USE - Use the whole image or an ARD file /'w'/ > SFACT - Smoothing width you wish to use /0/ > DEVICE - Display device code or name /@xwindows/ >
Output similar to the following should be produced:
HISTPEAK Results: /acdscratch/sc5/targets/ngc2336_r_2 Pixels (used): 1263376 Pixels (bad): 0 Lowest count: 166.000 Highest count: 50963.000 Skewness: 63.680 Kurtosis: 7114.975 Mean: 1267.440 Median: 1294.776 Histogram modal values: Unsmoothed: 1298.000 Smoothed: 1298.000 Projected: 1296.511 Interpolated: 1298.696 Absolute dev.: 90.361 Variance: 78153. Standard. dev.: 279.558 Back. st. dev.: 33.353 Smoothing filter radius: Radius request: 0 Radius actual: 0 Contents of the most occupied histogram bin: Unsmoothed: 21104.000 Smoothed: 21104.000 Interpolated: 12126.075
and a plot similar to Figure should appear. For the purpose of determining the sky background level it is probably best to use either histpeak or stats with the clipping option. Also note that the histpeak pre-amble contains the dimensions and bounds of the image.
The 2-D CCD Data Reduction Cookbook