[daip] FRING queustion

[Amy Mioduszewski]

Dear Andrzej,

Eric is away, so I will take a stab at answering your question.

In the first case, where you are using a phase referenced image as a 
model and run FRING a second time, the position should be accurate, 
although, maybe not to astrometric accuracy (i.e., FRING might put in a 
very tiny shift) but for most uses the position will be good.  One thing 
you can try (that might convince your referee) is to apply the solutions 
from the target to the phase reference source and show that it does not 
move.  I.e., phase reference the phase reference calibrator to your 
target.  Do you have any "check" sources, you can do that to those as 
well.  I think your referee is just being pedantic, just because FRING 
could move the source doesn't mean it did.

However in your second case, when using a model from another frequency, 
it is more problematic.  In the gross sense the position will be 
correct, i.e., you won't be 10s of mas off, but since the source might 
have a different distribution of flux at the different frequency, and 
probably does, then FRING will move the source so that the bright points 
are at the same position.  So it is possible that the map shifted.  I 
don't know what your sources looks like so I don't know how much it 
might be shifted, if it is core dominated than my guess is that the 
shift would be a fraction of a beam.  If it had a complicated structure 
it might be more.  It all depends on how accurate a position you need.

Lining sources up at different frequencies is always a problem.  Even if 
you had phase referenced your second frequency to the same source that 
you did your first frequency, you can have shifts from different 
structure in the phase reference sources.  Multiple calibrators is the 
only way to really get around this.

Cheers,

Amy