[daip] IMEAN and noise

[Eric Greisen]

Glenn Morrison writes:
 > Hi Eric,

 > 1) Does IMEAN just take all the pixel values in the image and then 
 > constructs a histogram, right?

     The plot portion plots a histogram in NBOXES in pIXRANGE.  The
part that tries to find a "correct" rms does it by using the PIXRMS
and PIXAVG adverbs as inputs and constructs a histogram around pixavg
and then Gaussian fits it for an RMS.

 > 
 > 2) Does it assume that all the pixel are independent, that there exists 
 > no correlation between individual pixels?

    The shape of the histogram is unaffected by independence of
pixels so the fit width is independent of that.  The independent part
comes in when you integrate.

 > 
 > 3) My Boss here at KPNO seems to think that in the radio image we are 
 > looking at (http://www.noao.edu/staff/morrison/home/GOODS/HDFNRADIO.html)
 > --that because the image seem to be smoothly varying (i.e., that the 
 > pixel values do not seem vary much) that the pixel values are 
 > correlated. Does this make sense to you and can you draw this kind of 
 > conclusion by visual inspection?

Visual inspection does imply things and the noise in each pixel is not
independent in an interferometry image.  It has a correlation length
about like the Clean beam even if it has not been Cleaned.

Eric Greisen