[daip] [!6263]: aips - SAD: Questions

[Robert H Gray]

Robert H Gray updated #6263
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SAD: Questions
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           Ticket ID: 6263
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/6263
           Full Name: Robert H Gray
               Email: roberthansengray at gmail.com
             Creator: User
          Department: AIPS Data Processing
       Staff (Owner): Eric Greisen
                Type: Issue
              Status: Open
            Priority: Default
                 SLA: NRAO E2E
      Template Group: Default
             Created: 26 February 2015 08:45 PM
             Updated: 22 March 2015 02:39 PM
      Resolution Due: 06 March 2015 08:45 PM 



Hi Eric,

I have a puzzling result from SAD, and wonder if you can shed some light on what’s going on – I’ve worked on it quite a bit and don’t understand.

SAD reports a 54-sigma peak in channel 6371 of a clean image cube, but I don’t see such a ‘source’ in a single-channel image for that channel (inspecting it with FitsView) although I do see some of the other ‘sources’ SAS reports in this image.   The only clues I see are (1) this Peak is marked by “*” so I’m multiplying it by 1000, but SAD also ranks it as the top ‘source’ in this channel, and (2) these channels are only 15 Hz wide so the noise level is high, but I don’t see why that would make SAD see stuff that is not there.

Below is some of the SAD report, and I’ll attach the image for the channel in question.  Any insight would be much appreciated!  With the exception of this problem, SAD seems to be doing well on my 8192-channel datasets with successive CPARM thresholds like 9, 6, and 3 sigma (I discovered the hard way that using just one threshold like 3 sigma could miss strong sources).  

Best wishes,

Bob Gray 

M33-LSR-CTRC.ICL001.      1     Disk  1     Plane 6371     User    1111
Window BLC   10   106371   1  1  1  1 TRC  502  5026371   1  1  1  1
...
NOTE: Fluxes marked by * have been divided by 1000.
Errors determined by theory from RMS  0.16805
...
#      Peak    Dpeak     Flux    Dflux       RA---SIN      DEC--SIN         
1 *    9.213(  0.157)   24.079(  0.544) 01 33 55.6162  30 24 34.187 ...    
2    763.271(168.049)  443.415(205.774) 01 33 45.5448  30 39 56.489 ...   
3    762.383(168.049)  520.746(227.498) 01 33 44.5035  30 37 01.347 ...   
4    687.991(168.049)  413.247(210.035) 01 34 13.7184  30 37 00.618 ...   
5    666.318(168.049)  430.616(219.764) 01 33 37.7855  30 45 00.595 ...


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