[daip] A bug in IMSTAT?

[Eric Greisen]

Enno Middelberg wrote:
>  Hello,
> 
> 
> I include below the complete output of IMSTAT and IMEAN. If I understand the
> IMEAN output correctly, the rms of 6.2875E-05 comes from the fit to the
> histogram (and this value is remarkably close to the IMSTAT output of
> 6.3025E-05, which you say is a "simple" rms), and the rms of 5.7077E-05 comes
> from including all data, as it says. And this value is equal to the rms I
> determine with my Python script.
> 
> So if the IMEAN rms of 6.2875E-05 comes from a fit to the histogram, what is the
> output of IMSTAT?
> 
> 
> Regards,
> 
> Enno
> 
> 
>> tget imean
>> default
>> indisk 3; getn 390; go
> AIPS 2: Got(1)   disk= 3  user=   6   type=MA   S548_lowres.ICL001.1
> IMEAN2: Task IMEAN  (release of 31DEC09) begins
> IMEAN2: Initial guess for PIXSTD taken from ACTNOISE inheader
> AIPS 2: Waiting for returned adverbs
> IMEAN2: Image= S548_lowres .ICL001.   1 3   xywind=    1    1 8192 8192
> IMEAN2: Mean and rms found by fitting peak in histogram:
> IMEAN2: Mean=-7.4585E-08 Rms= 6.2875E-05  **** from histogram
> IMEAN2: Mean and rms found by including all data:
> IMEAN2: Mean=-7.7760E-08 Rms= 5.7077E-05 JY/BEAM  over  67108864 pixels
> IMEAN2: Flux density = -1.2940E-02 Jy.   beam area = 403.28 pixels
> IMEAN2: Minimum=-3.5699E-04 at 2749 8152    1    1
> IMEAN2: Skypos: RA 03 33 35.4629620  DEC -27 45 41.069990
> IMEAN2: Maximum= 3.2447E-04 at   28 2330    1    1
> IMEAN2: Skypos: RA 03 33 35.8729724  DEC -27 45 52.713916
> IMEAN2: Skypos: IPOL  1382.490 MHZ
> IMEAN2: returns adverbs to AIPS
> IMEAN2: Appears to have ended successfully
> IMEAN2: redgum 31DEC09 TST: Cpu=      3.5  Real=      3  IO=       256
> AIPS 2: Resumes
>> imstat
> AIPS 2: Mean=-7.7760E-08 rms= 6.3025E-05 JY/BEAM   over67108864. pixels
> AIPS 2: Maximum= 3.2447E-04 at    28  2330    1    1    1    1    1
> AIPS 2: Skypos: RA 03 33 35.8729724  DEC -27 45 52.713916
> AIPS 2: Skypos: IPOL  1382.490 MHZ
> AIPS 2: Minimum=-3.5699E-04 at 2749 8152    1    1    1    1    1
> AIPS 2: Skypos: RA 03 33 35.4629620  DEC -27 45 41.069990
> AIPS 2: Skypos: IPOL  1382.490 MHZ
> AIPS 2: Flux density = -1.2940E-02 Jy.   Beam area = 403.28 pixels

These are odd results - I cannot make IMSTAT and IMEAN disagree on very 
large images on the simple mean/rms.  Furthermore, it is very odd that 
your fit rms is larger than the simple one.  That is I would think 
mathematically impossible - or nearly so.  I suppose that the fit can 
get wrong answers if the input PIXAVG and PIXSTD are rather wrong but I 
would not think that you would be in that state.  Did you compile AIPS 
locally?  There are a couple of things that a bad compiler could mess 
up.  I will try to fix those to be unambiguous.  IMSTAT is really simple 
minded - IMEAN tries hard but perhaps we should be a robust average 
instead.  Of course with 8K by 8K that requires a lot of dynamic memory.

Eric Greisen