[daip] Re: uvplot vs plots of exported prtuv data

[Eric Greisen]

AIPS en Tuktoyuk writes:

Please use daip at nrao.edu.  I do occasionally go on vacation and others
might be able to help.

 > 
 > I am currently using a IDL program I wrote to plot exported files from AIPS 
 > using PRTUV.  When I compare the plots made using the IDL program to that of 
 > UVPLT there is a difference (both plotted as baseline Vs amplitude). I find that 
 > in the plots from IDL there are visibility's above and below the the amplitudes 
 > shown using UVPLT. For example, in UVPLT the range of amplitudes is 0-4Jy were 
 > in IDL the range is -2-10Jy.  Is there a parameter in PRTUV that could be adding 
 > these points or manipulating the data to cause these??  I have already looked 
 > into the CPARM(10) amplitude multiplication parameter, I set that to 1 so there 
 > is no multiplication.  I also looked into the DPARM parameters but they have no 
 > effect on the exported files.  The last thing I can think of is that PRTUV 
 > scales the weights by a factor of 10, but I cant see that causing these results.

 > Attached is a ps file of the IDL plot I am referring to.  Sorry no ps files of 
 > uvplt, I cant get them to work yet.  They look the same outside of the extra 
 > points above and below the 'normal' region (0-4Jy).


Note that sqrt (Re*Re + Im*Im) cannot be negative.  Therefore the
points plotted as negative cannot be correct.  PRTUV in its header
states whatever scaling might be used for weights.  But UVPLT only
looks at weights to see if they are positive (data are good) or
negative (data are not good).  UVPLT also applies the flag table so
bad data are omitted and can apply calibration.  PRTUV cannot apply
flag tables or calibration (UVPRT does that and would be better for
you to use).  I do not know why you are concerned about weights other
than for flagging - they do not scale the plotted amplitudes.

Making a ps file from aips is very easy.  Run UVPLT with DOTV=-1.
Then run LWPLA with OUTFILE set to the name of the ps file you
desire.

Eric Greisen