[daip] aips

[Eric Greisen]

Cao Hongmin wrote:
> 
> Hi Eric,
> 
> I have some problems not sure,
> 1. aips task imagr
> Suppose the adverb ¡®niter' is set to 15000, and 'dotv' is set to 1,
> if click the 'continue clean' in the tv window, the clean will stop 
> after several iterations of cleaning which is much less than 15000. Then 
> what determines the times of iterations of each (sub-) clean process?
> 2. aips task fring
> if we choose aparm(5) 1, then a combined solution of IFs will be 
> obtained. the combined solution is applied to both polorizations (RR and 
> LL) or the two polorizations will have different solutions?
> 3. VLBI amplitude calibration error (pre-calibration by aips task apcal 
> ) is about 10%, here the 10% mean 1 sigma error or something else?
> 4. a source could be seen as unresolved by VLBi is because the flux 
> recovered by VLBI is similar with that measured by eg. VLA and it looks 
> like a point with no extra structures or just because  it looks like a 
> point?

1. Clean will stop when the peak value in the windows is less than FLUX
also and may also quit if it gets in trouble (it will tell you lots of
messages then).  It always gives a reason for stopping in the message 
window.
The TV menu also has ways to tell it to stop.

2.  APARM(5)=1 will produce solutions which are identical in each IF. 
The solution in the right and left polarizations will be different. 
Note that PRTAB and SNPLT are ways to examine the solutions.

3. The calibration done by APCAL is as accurate as the Tsys values it is
given. If they are measured and calibated correctly and are those that
correctly apply to the source direction, then the amplitudes may be very 
accurate.  Some VLBI telescopes do not make such accurate Tsys 
measurements - due to inaccuracy in the knowledge of the Noise tube 
signal or because the Tsys is measured after the scan and perhaps in a 
somewhat different direction.

4. A VLBI image may look like a point source simply because the VLBI 
observations lack sensitivity to large scale structure.  The most 
reliable test is the one you mention - if the VLA sees the same total 
flux, then it is
more likely to be a point source although the vLA can miss even more 
extended structures that would be seen by e.g. the GBT or Arecibo.  That 
you see a source at all in VLBI of course means that the source does 
contain some point-like
structures even if the source may also contain other structures.

Eric Greisen