[daip] UVLIN & UVLSF questions

[Eric Greisen]

Bryan Butler writes:
 >   FLUX.......Max. residual flux allowed for unity weight. This
 >              should be 6-8 times the rms noise for a single
 >              channel in a 10 second integration.
 > 
 > does it *really* mean in a 10 second integration, or is it

         No it means weight 1 which used to be the weight assigned a
10-sec integration.

 > in the integration time that is in the data (i.e., the rms
 > per channel visibility)?  if it *is* 10 seconds, why?  i
 > can find nothing in the data that scales the residual or
 > the flux cutoff from the true integration time to 10 seconds, 
 > so i suspect that it's the true integration time, but if so,
 > why does it so explicitly say "in a 10 second integration"
 > in the help file?

        The cutoff occurs when
        abs(Resid) > FLUX * sqrt(wt)
where the the data weight is assumed = 1/sigma**2.

 > 
 > now, the UVSLF explain file says:
 > 
 >   FLUX.......If the residual flux in any channel used to fit the
 >              baseline exceeds FLUX, then the spectrum for that time, IF,
 >              and polarization is fully flagged.  <= 0 => 1.0E20
 > 
 > which is more straightforward in a sense, but gives no guideline
 > (similar to the 6-8 times rms in UVLIN) to the user.  but then

              which is no longer really right

 > further down in the explain file for UVLSF it says:
 > 
 >       UVLSF differs from UVLIN in a number of ways. UVLIN does its
 >   flagging only on the peak residual in the channels selected for
 >   fitting and scales the peak by the square root of the
 >   integration time before deciding on flagging.  This is correct
 >   for peak residuals due solely to thermal noise but is an added
 >   confusion to the user.  In any case, you should set the FLUX
 >   cutoff at a conservative level (6 - 8 times the expected sigma
 >   in the usual integration time).  
 > 
 > OK - here is the same recommended factor of 6-8 (why isn't it
 > somewhere above, where folks will actually see it?).  but, is the 
 > "usual" integration time the one that is present in the data, or 
         
        usual here means the usual one in the current data set,
assuming that there will be some short integrations at the end of
scans.  This advice is placed where users needing advice look.  The
fact that FLUX cuts off in a simple way is stated where lazy users who
think they know what they are doing read.

 > 10 seconds (one could argue that 10 seconds is indeed the "usual" 
 > integration time at the VLA anyway)...
 > 
 > 
 > and one final question - are both UVLIN and UVLSF currently
 > supported?

Any task in AIPS is supported if it is distributed.

Eric

BTW - I do not see you volunteering to write Explain files for tasks.
The explain files should not be written by the programmers since they
use the same words and points of view in the help and explain area.
But the last time a scientist contributed an explain file was < 1990.