[daip] CNTR (levels)

[Jim Ulvestad]

IMEAN is not as smart as you think.  It doesn't decide where the
source was and subtract anything; it just determines the raw
rms of all the pixels that are inside the box you specify.  
Therefore, you definitely DO NOT want to find the rms by
using IMEAN on a box that includes the source.  Use TVSTAT
to draw an irregular shape around your source, but not
including your source, and that will probably come closest
to what you want.

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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> ::Tel: +44 (0)113 2333872                     ::
> ::email: ip at ast.leeds.ac.uk                   ::
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