[daip] Question about ZEMAN

[Eric Greisen]

On 12/01/2016 02:41 AM, Bill Cotton wrote:
> Eric,
>
>     I've been trying to run ZEMAN on some maser data and don't
> understand the results, see attached.  The data in question has been
> calibrated using a technique assuming the Stokes V integrated over the
> spectra of all sources in the field is zero.  The Stokes V spectrum in
> this plot is essentially unchanged from that calibration.  My
> understanding of ZEMAN was that it tries on a pixel basis to decompose
> the Stokes V spectrum into a symmetric part ("instrumental
> polarization") and an antisymmetric part ("Zeeman").  My expectation for
> the corrected Stokes V spectrum was a straight line.
>
>     I ran ZEMAN with optype='2SID' and did not need to do Gaussian
> fitting as the masers are all spatially separated with symmetric
> spectra. I ran ZEMAN in batch mode. The field contained a larger number
> of masers but I only gave it the region around this one cluster of
> masers and the spectrum is of the brightest.  I also don't fully
> understand the output images (dooutput=7) as the names are not quite
> what is in the HELP file and looking at them didn't help much. "FIELD1"
> contains only 0s and a few blanks, DFELD1 is mostly largish numbers (few
> 10's) except where there is maser emission where the values are closer
> to 1.  Is there some threshold of significance below which ZEMAN does
> nothing?
>
> Thanks,
>
> Bill
>

The output classes have to be what is said in the help (since the .FOR 
matches) except that uncertainty images are also written (DGAIN, DFELDn).

The operation is only performed on pixels for which 3 consecutive 
channels have brightness > FLUX.

Try running it interactively to start with - this may give you a better 
feel for what it does.  The spectrum you sent looks like it did nothing 
at this pixel.  If the interactive thing does not show this pixel then 
it did not access it.  The ZE table attached to the Vpol image is 
instructive also - use RPARM in PRTAB to select the desired pixel region 
to examine.

Eric