[daip] aips_gripe

[Leonia Kogan]

Hi Lincoln,

Now I start understanding that I has not understood the problem.

So you want XMOM to calculate different threshold for different box 
at each velocity plane. 

YES?

The boxes positions and sizes for each plane should be given by the user. 

YES?

If YES, YES then
   I have understood the problem.
   It looks like this problem solving is a subject of 
   other task before using XMOM. This task should blank the qube
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   by the relevant way
   ^^^^^^^^^^^^^^^^^^^
else
  ???????
  end if

The way to calculate rms is a diferent story (histogram or direct calculation).


Leonia



----- Begin Included Message -----

>From lincoln at play.harvard.edu Wed Nov 14 10:09 MST 2001
Date: Wed, 14 Nov 2001 12:08:59 -0500 (EST)
From: Lincoln Greenhill <lincoln at play.harvard.edu>
To: Leonia Kogan <lkogan at zia.aoc.NRAO.EDU>
Subject: Re: [daip] aips_gripe
In-Reply-To: <200111141637.JAA15123 at bonito.aoc.nrao.edu>
MIME-Version: 1.0
X-Lines: 59
Status: RO


Hi Leonia,


> I use the calculated RMS(I)*COEF as a threshold to exclude all points
> at the plane 'I' which are more than RMS(I)*COEF.
> Actually I substitude the global threshold FLUX by RMS(I)*COEF.

You should exclude points in plane 'I' that are BELOW   RMS(I)*COEF.


>>I think that the best way to compute an RMS is to use the technique
>>applied in JMFIT - to construct a histogram of the source and estimate
>>the width of the counts' number vs flux profile centered around zero.

>You mean IMEAN (not JMFIT). In the case of strong sources at the plane
>you have to find solution for thr source (fit gausian(s)) and then
>to calculate rms either by histogram or by direct calculation.

I do mean JMFIT. The histogram technique seems to me to be the best
way in which to find the RMS in the presence of signal.

>I want to get rid of specifiing the off source area for each plane.
>It can be annoying for the user.

This is good in principle  but in our specific case, we may need to
specify an area because of the noise pattern that remains in the
background of our maps...and which is correlated with the locations
of strong emission.

Each map has a different dynamic range depending on pixel location.
Pixels that are north and south of a strong emission component must be cut
in XMOM at a higher absolute flux level than pixels elsewhere in the map.
This is why I suggest that we need to test the flux in each pixel based on
a local calculation of RMS (say over a 300 x 1000 pixel patch that is
centered on the pixel).

>Then is the cutoff described at the item 3 is what you want?

Yes and no. A CUTOFF in units of RMS is what we need.  A single RMS
that describes each plane is not what we need.


>Suppose we have calculated say 256 RMSs for 256 velocity planes outside
>of XMOM and have an input text file with those 256 RMS.

This does not address the problem of changeable dynamic range, based on
location in each plane.

>Suppose XMOM uses this 256 RMSs to substitude the global threshold
>FLUX and therefore to have individual cutoff for each velocity plane.

No.  See above.

Regards,

Lincoln




----- End Included Message -----