[daip] shifting/resampling uv/image cubes

[R. Niruj Mohan]

Hello,

I have a special problem and hope you could help me
out with it. I have observed a spectral line source
with two overlapping 50 MHz bands in order to cover
the entire velocity spread, using the VLA. Since the
first channel of 50 MHz bands are quantized, I could
not match them to overlap properly. In fact the second
band is shifted not by integral number of channels
from the 1st, but by almost exactly integral+0.5
channel widths. 

I now need to combine them of course. 
I would prefer to do this in the image plane for
various reasons but could work with doing it in the uv
plane as well. The only task for resampling seems to
be in the uv plans called SPECR. I would rather not
average adjacent channels to get the value in between
since I will change the noise characteristics and
introduce correlations as well.

I dont need to resample -- just to shift by
0.5*channelwidth which is basically putting a phase
gradient in the fourier domain. I suppose there is no
task to do this yet. So I thought I would resample to
double the number and then take the other set of
alternate channels (that is, 2,4,6 etc). The problems
I face are

1. There is ringing in the resultant resampled
spectrum due to the way RESPEC.FOR does things. I can
substantially reduce this Gibbs ringing by subtracting
the first channel from all of them and then adding it
back later. But there is still a small amount of
ringing due to the structure of the spectrum itself
which I cant do anything about. Introducing just a
phase shift instead of padding/truncating may help if
put into a new task. 

2. Once I do manage to resample into 2*n and then take
2,4,6, etc to make the new uv cube, I would need to
change the channel width in the header (PUTHEAD) and
in the FQ table. Is this sufficient or should I modify
the value in other places as well ?

thanks a lot and hope you can help,
Niruj
RRI, India.



	
	
		
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