[daip] CNTR (levels)

Jim Ulvestad julvesta at aoc.nrao.edu
Tue Jul 30 11:12:31 EDT 2002


After talking to Eric, I realized that my previous note might
have been confusing.  IMEAN outputs two values for the rms,
one that is the raw fit that I mentioned, the other which is
a fit to the histogram, and the value that actually gets
put in image headers and such things.  The fit to the
histogram actually is the better number, provided that most
of the pixels inside your box are noise rather than source.

Best,

Jim Ulvestad

Ian Pattison wrote:
> 
> I wish to set CLEV to be equal to the rms locally in an image (since I
> have ran PBCOR and so the noise varies alot over my wide-field image).
> What is the most accurate way to measure the noise? I have tried 2 methods
> so far:
> 
> 1) using TVWIN and IMSTAT to find the rms in a number of boxes close to
> the source of interest.
> 
> 2) using IMEAN to find the "true" rms over a larger box covering the
> source (source at centre).
> 
> These two methods provide similar, but not equal, results. How "good" is
> IMEAN at calculating the true noise rms (since the rms of the source must
> somehow be subtracted).
> 
> Thankyou.
> 
> Ian Pattison
> 
> ::::::::::::::::::::::::::::::::::::::::::::::::
> ::                                            ::
> ::               Ian Pattison                 ::
> ::                                            ::
> ::Department of Physics & Astronomy           ::
> ::University of Leeds                         ::
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