[daip] Calculating flux density error

[Eric Greisen]

Emma Rigby writes:

 > I'm trying to calculate the error on a flux density measured using the 
 > imstat task in AIPS but I'm not getting very far. Do you have any advice 
 > on the best way to do this? Apologies if I've sent this to the wrong 
 > address.
 > 

IMSTAT is a simple verb which does blind summation and rms computations
on all pixels inside a rectangle.  TVSTAT will allow you to include
only the pixels actually having source which should reduce the noise a
little.

The task IMEAN can be pushed to fit the histogram of noise to give a
true rms.  Note that this is an rms in Jy/beam and more or less
assumes that the pixels are independent which they are not (the Clean
beam is usually at least 10 pixels in area if not 20).  If the
uncertainty in each pixel is R then the uncertainty in the sum is
formally sqrt(N) * R where N is the number of pixels over which the
sum is performed.  The actual computation corrects the units so the
FLUX error is sqrt(N) * R / A where A is the beam area in pixels.
In gaussian fitting, the error in total flux is quoted as the area of
the source divided by the area of the beam times R.  This might even
be closer to N * R / A.

I hope this is of some help...

Eric Greisen