LINMOS and Primary Beam Correction

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julie
Posts: 12
Joined: Tue Mar 22, 2011 2:53 pm

LINMOS and Primary Beam Correction

Post by julie »

I saw on the ATCA forum that in October 2012 there was a change to linmos:

linmos now uses the correct frequency for the primary beam of each image, when combining images at different frequencies.

What does this mean?

I would like to make a wide-band wide-field image. If I apply the primary beam correction to the full cabb band setting bw=2GHz in linmos, the primary beam correction is made to the centre frequency. This is not the best way of doing things. Instead, if I split up the band into 16 subbands and put each subband into linmos with bw=128MHz (size of each 16 subband), will the primary beam be corrected to the central frequency of each subband?

Cheers,
Julie
julie
Posts: 12
Joined: Tue Mar 22, 2011 2:53 pm

Re: LINMOS and Primary Beam Correction

Post by julie »

Just found a similar question posted by Nick on Feb 29, 2012 which is helpful.

http://atcaforum.atnf.csiro.au/viewtopic.php?f=10&t=102

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

Re: LINMOS and Primary Beam Correction

Post by Mark.Wieringa »

Hi Julie,

it means that it uses the mosaic equation to add the images together as Sum I*P/ Sum P*P, where P is the primary beam attenuation (<1), i.e., multiply each image by its primary beam, add them all and then divide by the sum of squared primary beams. For each image I, the correct centre frequency is used to compute the corresponding P image.
The effective frequency in the output image will shift to lower frequencies as you move out from the centre.

> If I apply the primary beam correction to the full cabb band setting bw=2GHz in linmos, the primary beam correction is made to the centre frequency.
Not quite - setting the bw parameter divides the band up into 10 parts, computes the average primary beam correction over those 10 frequencies and applies that instead of the correction for the centre frequency.

If you are splitting up the band into 16 x 128 MHz, you don't need to set the bw parameter, as the change over each 128 MHz band is negligible.

Cheers,

Mark
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