[daip] task MOMNT

[Eric Greisen]

Violette Impellizzeri wrote:
> Hello,
>  I have a question concerning the right use of momnt on an absorption 
> map. I tried different options to set the parameters FLUX and ICUT in 
> the task momnt (and xmom).
>  From my understanding FLUX cuts the flux below a certain threshold, 
> whereas ICUT will cut the flux below if > 0, and above if < 0. Since I 
> would like to create a moment map of an absorption I set the flux to be 
> very low (-1 Jy/beam) and I tried setting icut to -0.001 Jy, all this 
> smoothing over 5 pixels. With these parameters all the flux was "cut", 
> which made me think that icut is still cutting the flux below 0.001 Jy, 
> i.e. the absorption I am trying to make a map of.
> When I set icut to something very small (-0.02 Jy), I see the absorption 
> in the moment map, but it is also very noisy, because all the flux below 
> -20 mJy is still being 'cut', but it is still mapping everything above 
> including the noise.
> Is this a bug in the task or am I doing something wrong? I guess there 
> must be a way to set a lower limit, i.e. cut everything above a certain 
> value?
> Thank you,
>    Violette
> 
> 
XMOM and MOMNT are designed for emission lines, not absorption.  The 
best thing to do is to create optical depth images - or, since that is 
really non-linear with giant values of great uncertainty, an image cube 
of continuum - line cube.  I think you can do this with COMB as line 
cube - continuum and then rescale with factor = -1.  That should make a 
cube that would be comprehended by MOMNT.

Eric Greisen