[daip] AIPS image scaling effect

[Eric Greisen]

Emil Lenc writes:
 > I have been working on a 90 cm VLBA/WSRT/76m Lovell data set in an  
 > effort to perform a wide-field VLBI survey around two nearby sources  
 > (a phase calibrator J0226+3421 and a gravitational lens 0218+357 1.89  
 > degrees away). The survey targets WENSS sources that exist around  
 > these fields. After initial calibration of each field, I run UVFIX to  
 > shift the data set to each target source, SPLIT the result (averaging  
 > all of the frequency channels together) and create a dirty image.
 > 
    Averaging the channels is not a good idea - you have the channels
in part to provide added accuracy when looking at large position
shifts.


 > In my first attempt to image sources in the field with AIPS 2005 (mid  
 > last year) I noticed that the positional accuracy of my detected  
 > sources appeared to degrade significantly with radial distance from  
 > the phase centre of each field. It appeared as if my image was scaled  
 > down by a factor of 1.29x10^-3 +/- 7x10^-5 which corresponded to an  
 > offset of 53+/-3 frequency channels in our data set (interestingly  
 > this appeared to correspond quite closely to the 50 lower-band  
 > channels that were flagged during editing - though this may just be a  
 > coincidence).

    Not suprising - the average u,v,w for the sample is computed
without regard to which channels actually contributed to the sample -
or at least I would expect that is mant cases.  That you can see a
shift due to N channels means that you should never hjave averaged
them in the first place.  AIPS will grid each in the correct place in
the UV plane and then not make position errors.

 > 
 > At the end of last year I reprocessed the data in AIPS 2006 and this  
 > scaling effect appeared to largely disappear - perhaps as a result of  
 > one of the bug fixes in that release. As I expanded my survey field  
 > of view to just over 2 degrees from the phase centre of each field, I  
 > discovered 4 common sources across both fields. However, when I  
 > imaged these sources and compared the positions from each field there  
 > once again appeared to be a kind of scaling effect occurring (i.e.  
 > the corresponding sources from each field did not exactly coincide).  
 > This time, however, the effect was much smaller ie. 7.3x10^-5 +/-  
 > 1.4x10^-5 or equivalent to an offset of approximately 3 +/- 0.6  
 > frequency channels.
 > 
 > This morning I installed AIPS 2007 as I noticed that there was a fix  
 > in UVFIX, but the offset still continues to be present and by the  
 > same factor.

     The correction in UVFIX was in the code that does position
shifting within UVFIX and applied only to one of the two cases
(compressed and not compressed).

 > 
 > I've attached an image of one of these sources, it is in fact my  
 > phase calibrator (J0226+3421) as seen from the centre of the J0226 
 > +3421 field (green) and as seen 1.89 degrees away from the phase  
 > centre of the 0218+357 field (the red contours are the results from  
 > AIPS 2007 and the blue contours are the results from AIPS 2006 - the  
 > blue contours are partially hidden because they almost exactly match  
 > the AIPS 2007 results). All images have been restored with the same  
 > beam. The three other common sources exhibit offsets of a similar  
 > nature when compared against each other (with offsets of ~440-690 mas).
 > 
 > Would you know what might be causing this effect?
 > 
 > In case this information is useful to you: The observation was  
 > recorded at 320.49 MHz (lower frequency), has a 4 MHz IF, 512  
 > channels, 0.25 s integration time and single polarisation (LL). It  
 > was correlated at the EVN correlator at JIVE, Dwingeloo, the  
 > Netherlands.

Try imaging without averaging up all the channels.  You will get morwe
accurate positions and they should agree.  Also - be sure that you are
using multiple facets as needed at such long wavelengths (task SETFC
can tell you what you should do).

Eric Greisen