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

Thu Feb 26 15:45:59 EST 2015

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): -- Unassigned --
                Type: Issue
              Status: Open
            Priority: Default
                 SLA: NRAO E2E
      Template Group: Default
             Created: 26 February 2015 08:45 PM
             Updated: 26 February 2015 08:45 PM
                 Due: 02 March 2015 08:45 PM (4d 0h 0m)
      Resolution Due: 06 March 2015 08:45 PM (8d 0h 0m)

Hi Eric,

Couple of questions regarding SAD:

1. Is it right that the RMS value is in JY when no units are given (like microJy or milliJy)?  I’m reading this with text processing and need to be sure that’s a valid assumption.

2,  "EXPLAIN SAS" says that “*” preceding a value means multiply by 1,000, and that “!” (in one place) or “#” (in another place) means multiply by 1,000,000.  Is that right?  

3.  I’m confused about the difference between Flux and Peak, and not sure which is the right value to use in a search for radio signals; sometimes they are very different.  It’s my impression that Peak is the largest value in an island, and Flux is some sort of integrated value after fitting... is that right?  Any suggestions on which one to use for this purpose?

4.  I run a PROC specifying, for example, a 5 sigma threshold for sources, but SAD reports “SAD  2: Setting Search limit at 3.0 x RMS of xxx” (I don’t have the value xxx inmy notes, but the RMS from this run is below) which seems to show my specification is not having any effect.  Any idea why that might be?  I’m putting the PROC below, although I understand that you probably don’t care to debug people’s code for them.  And, the resulting list has stuff below the 3 sigma threshold as shown below.  NGAUSS is 100, which might be a factor... (this is from my dataset M31-140-CTR, channel 2 of a 8192-channel cube)...
...
SAD reports...
Sources found down to  0.218483 in JY/BEAM    (which would be 218 mJy)
Errors determined by theory from RMS  0.07283   

For channel 2, it reports a source that seems below that 218 mJy threshold... why would that be? 
    #      Peak    Dpeak     Flux    Dflux
...
    7     66.821( 23.918)    1.706(  5.439)

5.  One last thing... SAD dies on channel 2036 of a 8192-channel cube, and I see from IMEAN that this is a 0 RMS channel, maybe because of RFI.   Is there a way to make SAD keep going?  I put the printout below. 

Thanks for any guidance you can provide.  I should mention that I love SAD, and have gotten good results on tens of thousands of channels so far with hundreds of thousands left to go!  

Bob

Item 4: Proc that fails to have its Sigma value picked up by SAD:

PROC SETUP
scalar  sigma, ch1, ch2 
finish
SETUP
*
*
PROC SRCS( sigma, ch1, ch2 ) 
* Proc to repeatedly run SAD on cubes with user-specified Sigma “CUTOFF”.
* Declare variables used in procedure.
* "channel" not used, to avoid overwriting same name used by IMAGR. 
scalar chan
for chan = ch1 to ch2
*
* Corners set to stay away from in 1024 pixel image (third arg specifies current channel number).
	blc 10 10 chan
	trc 1014 1014 chan	

* Get noise in current channel via IMSTAT, rms returned in "pixstd".
	GO IMSTAT
* Use task SAD to find sources, setting the "cutoff" minimum flux.
* to desired sigma threshold (Sigma is set in command line by user).
	task 'SAD'
	CUTOFF = pixstd*sigma
	NGAUSS=100
	print chan, cutoff, pixstd, sigma
	GO SAD
	wait sad
end
FINISH

Item 5: SAD dies on channel 2036 of a 8192-channel cube... 

AIPS 2: SAD      was not found to be running
AIPS 2: Mean= 8.095E-07 rms= 2.338E-03 JY/BEAM  over     1010025. pixels
AIPS 2: Maximum= 1.3517E-02 at    703    763  2036    1    1    1    1
AIPS 2: Skypos: RA 00 42 10.045  DEC 41 24 28.69
AIPS 2: Skypos: 1617.796 MHz  IPOL
AIPS 2: Minimum=-1.4999E-02 at     63     45  2036    1    1    1    1
AIPS 2: Skypos: RA 00 44 03.335  DEC 41 00 31.29
AIPS 2: Skypos: 1617.796 MHz  IPOL
AIPS 2: Flux density =  4.7865E-02 Jy   Beam area =  17.08 pixels
AIPS 2:   2036             0.0093521     0.002338      4
SAD  2: Task SAD    (release of 31DEC15) begins
SAD  2: ZTXOP2: using translated file name =
SAD  2: ZTXOP2: /lustre/aoc/observers/nm-7005/CUB.SAD
SAD  2: Create M31-1650-CTC.RESID .   1 (MA)  on disk  1  cno  236
SAD  2: Fit to histogram gives RMS = 2.365E-03
SAD  2: Setting Search limit at 3.0 x RMS of 0.0023650
SAD  2: Looking for up to  100 sources in subfield:
SAD  2: BLC:      10.     10.   2036.      1.      1.      1.      1.
SAD  2: TRC:    1014.   1014.   2036.      1.      1.      1.      1.
SAD  2: Using Clean beam  13.48938   4.34139    3.72 from CG table
SAD  2: Searching down to   0.007095 (JY/BEAM )  pass 1
SAD  2: Found    100 Islands in pass 1
SAD  2: Successfully solved    100 islands, all converged
SAD  2:     89 total components had their fits rejected
SAD  2:      7 components rejected for minimum peak
SAD  2:      7 components rejected for minimum flux
SAD  2:     15 components rejected for maximum interior X excursion
SAD  2:     15 components rejected for maximum interior Y excursion
SAD  2:     68 components rejected for maximum residual flux
SAD  2: Resulting number of sources fit is    11
SAD  2: Destroyed residual image as requested
SAD  2: Appears to have ended successfully
SAD  2: nmpost02 31DEC15 TST: Cpu=      0.2  Real=      0  IO=        24
AIPS 2: Resumes
AIPS 2: SAD      was not found to be running
AIPS 2: NO VALID PIXELS FOUND
AIPS 2: USING WHAT?

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