[daip] visibility time stamps

[Eric Greisen]

Aaron Cohen wrote:
> Hi Eric and Leonia,
> 
> Thanks for these explanations.  I think I understand things a bit better
> now.  
> 
> Unfortunately the rotation could be a problem for me.  I'm working with
> 74 MHz data, and so the fields are about 12 degrees across.  I'm looking
> into statistics on the relative positions of sources in that field of
> view.  So any rotations will affect this.  
> 
> Is there a way to calculate what the rotation would be for a certain
> time offset?  I assume it would be dependent on RA and DEC, and perhaps
> the altitude and azimuth.  Leonia, when you say that 5 sec. of time
> shift equals ~4E-4 radians of "rotation", I assume you just took the
> fractional rotation equal to 5 sec/ 24 hours?  Would that be the actual
> rotation or just an estimate?

74 MHz is affected with gross position uncertainties due to the 
ionosphere so I doubt that you would be able to see any rotation due to 
erroneous u,v recorded by the on-line system.  The "5 sec" business is a 
red herring due to an error one could have made with UVFIX - but it is 
not relevant if you did not run UVFIX.  Ken Sowinski was willing to 
consider a 10-sec error in the u,v,w - i.e. that the updating of that 
parameter got one cycle off - but he doubted that it would occur.  The 
actual numbers we "measured" were +2 sec at one epoch and about -5 sec 
at another.  The group we were working with seemed unable to follow the 
UVFIX instructions for the longest time and so we got all sorts of 
analysis until they finally did it right.  Then the difference between 
the 2 epochs went away.  But I do not think that 74 MHz can yield 
astrometric position accuracy and that is the level of error here.

If you are really worried - load the data again for one pointing with 
the modern FILLM.  Run UVFIX with UVFIXPRM=0.  Apply your previous flag 
and CL tables and reimage.

Eric Greisen