[daip] Request to include GMRT primary beam parameters in Aips {External}

[Subhashis Roy]

Hi Eric,

I am going to try Imagr on wideband uGMRT data (300-500 MHz) using the 
wideband option that worked quite well on simulated data.
To fit for spectral indices of sources far away from the primary beam 
centre, one need to apply primary beam correction on the fly during 
imaging via Imagrprm(1).
However, in absence of primary beam parameter for GMRT in Aips, it will 
assume an Ideal antenna (of 45 meter diameter), which is not quite right.

Therefore, I am attaching a summary document from our webpage 
(http://www.gmrt.ncra.tifr.res.in/doc/beam-shape-v1-09sep2022.pdf), which 
provides the uGMRT beamshapes for different frequency bands.

Request to incorporate them in Primeary beam related Aips source files.
I typically use a primary beam cut-off level >0.05.

Thanks,
Subhashis

On Tue, 30 May 2023, egreisen wrote:

> On 2023-05-29 13:23, Subhashis Roy wrote:
>> Hi Eric,
>> 
>> I have been playing around to check how wideband Clean of Aips via
>> IN3NAME works for multi-facet images [of-course setting Imagrprm(8)=0
>> for the time-being].
>> 
>> As I understand, with typical observations, one need to make a image
>> cube from sub-bands and Clean it with BMAJ, BMIN, BPA set to be the
>> same during Clean for the cube (without the 3 parameters set or
>> convolution, SPIXR has trouble to compute spectral index correctly due
>> to differeing beam sizes as a function of frequency) with NFIELD set
>> to N (as needed).
>> 
>>> From the Clean image cube having N facets, one uses FLATN to get an image
>> cube for the whole Field of view. Then run Trans, and Spixr on Trans
>> output to get the spectral index map. Finally, use the Spectral index
>> map generated in the last step as IN3NAME, and set IMAGRPRM(17) (I am
>> setting it as 2).
>> In the next run of Imagr for wideband Clean, set Echan=M, NCHAV=M
>> (where M is the total no. of frequency channels). NFIELD=N set as
>> before.
>> 
>> The above steps seemed to work well with N=3, M=5. Differences of the
>> new Model from UVMOD w.r.t. the previous one are: the 2 point sources
>> were put +/-600" away along North and South from the centre, and
>> spectral index of the central Gaussian was set to -0.2 (earlier was
>> 0.0). I also made Flux (UVMOD) to go down by 4 as compared to the
>> earlier.
>> 
>> The result is attached (gzipped Fits image). You can see that the
>> Clean problem around the compact sources are gone due imaging them at
>> the centre of separate facets.
>
> I have done more tests.  The filtering option does use the spectral index.
> That has been set up and should run correctly. I think the filtering itself
> is not a good thing.  If one  cleans the whole image without Clean boxes
> then it can be good to get rid of components that would never have
> appeared in boxes set properly.
>
> The defects around the northern and southern sources have looked to me like
> 3D issues, but SETFC insists that your original model only needs 1 facet.
> Going to 600 arc sec will use more facets according to SETFC.  I suspect
> that the filtering option should really use a radius of 10 or 20 cells not
> just 3.  I notice when filtering that a bright rim appeared around the
> central Gaussian indicating that the filter simply nibbled away at the edges
> rather than getting "isolated" components.
>
> I guess I should try your more extreme model just for fun.
>
> Note, you are better off running IMAGR with BMAJ 0.  Then run CONVL on the
> output cube with operation GAUS and BMAJ 0.  That will smooth each plane
> including the residuals to the basic (poorest) resolution.  Setting BMAJ in
> IMAGR smooths the Clean components but not the residuals to the desired
> resolution.
>
> Eric Greisen
>
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