[daip] Fwd: Re: Regarding VLA " First sky survey" catalogue with 946432 sources.

[Eric Greisen]

-------- Forwarded Message --------
Subject: Re: [daip] Regarding VLA " First sky survey" catalogue with 
946432 sources.
Date: Tue, 21 May 2019 08:30:51 -0600
From: Eric Greisen <egreisen at nrao.edu>
Organization: National Radio Astronomy Observatory
To: Prabhakar <prabhak at iitk.ac.in>

On 05/21/2019 02:44 AM, Prabhakar wrote:
> Dear sir,
> 
> I am Msc-PhD student at IIT Kanpur(Indian Institute of Technology, Kanpur)
> (INDIA), in the Department of Physics. I am doing some project work using
> VLA, First sky survey" catalogue with 946432 sources. I have some queries
> regarding this catalogue.Please let me know the reason behind the quires
> that I am mentioning below:
> 
> 1. I saw around 170000 sources with both Bmaj and Bmin zero and around
> 3lakhs sources with Bmaj non zero but still Bmin zero. What it implies!
> 
> 2. Through my work I have noticed when I plotted a histogram of position
> angle(PA), it appeared most of the sources with PA 90 deg. and even more
> sources with PA 0 and 180 deg. What it implies!
> 
> 3.** As for extended sources ratio of peak_flux/flux_inte.  would be less
> than 1 or close to 0.9. But most of the sources with its ratio greater
> than 1. How is it possible? If yes, then What it implies!
> 
> 4** I want very random data, with PA  random distribute. Is there any bias
> or limitations on these data, let me know, please.
> I will be very grateful for your response to these issues.
> Thank you

I know the answer to question 1:

The fitting program tries to deconvolve the fit Gaussian parameters from 
the Clean beam.  This decovolution can fail entirely so Bmaj and Bmin 
are set to zero, or it can fail to find a Bmin (sqrt of negative number) 
while finding a Bmaj.  Then Bmin is called 0.  Do not pay any attention 
to the PAs in these cases.  It is indeterminate.

Eric Greisen