[daip] Spectral Index maps

[Amy Mioduszewski]

O.K., you've hit on a major pet peeve of mine.  In order to make a realistic 
spectral index map you should not "taper and convolved the data to the same 
beam" you should put a model for the image for the lower frequency through the 
uv coverage of the higher frequency (and then convolve the beam to the lower 
frequency).  Yes, uv coverage matters and make a big difference.  Simply 
convolving the data to the same beam misses the fact that you have fewer short 
spacings at the higher frequency, therefore you miss flux that might be there 
and *artificially* have a more negative spectral index than is real.  This is 
particularly important for VLBI arrays that are relatively sparsely populated.

Anyway, I can give you detailed instructions on how to do this if you want.

Amy

Mercedes Esteves Filho wrote:
> 
> Hi. Ive data from the VLBA and would like to construct
> spectral index maps. Now Ive tapered and convolved the
> data to the same beam. Image size and increment are the
> same for both images. So added and divided images using COMB
> look ok, but the spectral index maps dont look ok.
> 
> I guess that this will have to do with my choices
> for aparm and bparm...But could you help me
> out with this?
> 
> Mercedes Filho
> 
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> *   Mercedes Esteves Filho, Ph.D.                    *
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