How does linmos calculate effective frequency?

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abutler
Posts: 11
Joined: Fri Feb 06, 2015 1:28 pm

How does linmos calculate effective frequency?

Post by abutler »

Hi Mark, Jamie,

I'm wondering if you can tell me how linmos calculates effective frequency when you set options=frequency. If it helps, I'm making a large 25 deg^2 mosaic at 1-3 GHz (as you might know from my other ATCA forum posts).

And how do I interpret the flux densities in my image mosaic given the frequency mosaic, particularly for large bandwidths? Can the flux density detected at any given pixel in the image mosaic be interpreted as applying at the effective frequency as indicated by the same pixel in the frequency mosaic?

Cheers,
Andrew
Mark.Wieringa
ATCA Expert
Posts: 297
Joined: Mon Feb 08, 2010 1:37 pm

Re: How does linmos calculate effective frequency?

Post by Mark.Wieringa »

Hi Andrew,

the frequency changes across the mosaic because the beam drops below the cutoff at higher frequencies. So for a single field the effective frequency decreases with radius.
The way linmos calculates this is by giving the pixels in each (evaluated) image the value of the frequency and then doing the normal mosaic operation. WIth the normal setting for bw>0, it will do this in 10 planes, so you may see some steps in the response.

>Can the flux density detected at any given pixel in the image mosaic be interpreted as applying at the effective frequency as indicated by the same pixel in the frequency mosaic?
Yes, that's the idea, but the frequency image is meant more as a warning - if there is significant change in frequency in the areas you want to analyse, you may be better off splitting the band in a few sections.

Also, for sources with a significant spectral index, the average flux over a wide band may differ from the flux at the centre frequency. Better accuracy will be obtained using narrower bands or explicit fitting of the spectral variation.

Cheers,

Mark
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