[daip] Smoothing in POSSM.
R. Craig Walker
cwalker at nrao.edu
Tue Mar 19 00:52:05 EDT 2013
Are the attached plots more convincing? They utilize a much stronger
source with much lower fractional noise in the spectra.
The inputs for 3C279_POSSM_SMOOTH_1-64.PS are below and the
variations from that are here.
3C279_POSSM_SMOOTH_1-64.PS smooth=6,6,10; bchan=1; echan=0
3C279_POSSM_SMOOTH_9-56.PS smooth=6,6,10; bchan 9; echan 56
3C279_POSSM_NOSMOOTH_1-64.PS smooth=0; bchan=1; echan=0
As you can see in the plots, there are pronounced phase effects
at the band edges of the channel restricted plot that go to to
phase offsets much greater than the entire scale of the other two
plots. There are also visible amplitude effects. The same
effect occurs with smooth(1)=2. In that case there are other
issues with the amplitudes at the edges which I presume relate to
doing the bandpass calibration after the smoothing.
Note that I picked a couple of the worst baselines, although they
are not unique. On other baselines, the effect can be smaller, or
even not there. And sometimes the sign is opposite. There are
both gain and bandpass calibrations being applied. If I turn off
the bandpass calibration, the effect is still there clearly. If
I turn off the gain calibration the effect is overwhelmed by the
much larger slopes (multiple turns) due to delay offsets.
Cheers,
Craig
>inp possm
AIPS 1: POSSM Task to plot total and cross-power spectra.
AIPS 1: Adverbs Values Comments
AIPS 1: ----------------------------------------------------------------
AIPS 1: USERID 0 User number - ignored
AIPS 1: INNAME 'BW098A_43' Input UV file name (name)
AIPS 1: INCLASS 'MULTI' Input UV file name (class)
AIPS 1: INSEQ 1 Input UV file name (seq. #)
AIPS 1: INDISK 5 Input UV file disk unit #
AIPS 1: SOURCES '3C279' Source list
AIPS 1: *rest ' '
AIPS 1: QUAL -1 Source qualifier -1=>all
AIPS 1: CALCODE ' ' Calibrator code ' '=>all
AIPS 1: SELBAND -1 Bandwidth to select (kHz)
AIPS 1: SELFREQ -1 Frequency to select (MHz)
AIPS 1: FREQID 0 Freq. ID to select.
AIPS 1: UVRANGE 0 0 UV range to be plotted
AIPS 1: TIMERANG 0 14 Time range to be plotted
AIPS 1: 0 45 0 14
AIPS 1: 1 25
AIPS 1: DOCALIB 2 > 0 calibrate data & weights
AIPS 1: GAINUSE 19 CL (or SN) table to apply
AIPS 1: DOPOL -1 If >0 correct polarization.
AIPS 1: PDVER 0 PD table to apply (DOPOL>0)
AIPS 1: BLVER -1 BL table to apply.
AIPS 1: FLAGVER 4 Flag table version
AIPS 1: DOBAND 1 If >0 apply bandpass cal.
AIPS 1: BPVER 3 Bandpass table version
AIPS 1: SMOOTH 6 6 Smoothing function. SEE
AIPS 1: 10 POSSM HELP SPECIAL VALUES
AIPS 1: SHIFT 0 0 Position shift:
AIPS 1: APARM 0 0 Control information:
AIPS 1: 0 0 1: < 0 => scalar average
AIPS 1: 0 0 >= 0 => vector average
AIPS 1: 0 0 2: = 0 => self-scale
AIPS 1: 1 0 > 0 => fixed scale
AIPS 1: CODETYPE ' ' 'A&P ', 'AMP ', 'PHAS',
AIPS 1: POLPLOT ' ' Option to display various
AIPS 1: SOLINT 1 If SOLINT > 0 then it enables
AIPS 1: NPLOTS 6 Number of plots per page
AIPS 1: BPARM *all 0 More control information:
AIPS 1: OUTTEXT ' '
AIPS 1: LTYPE 4 Type of labeling: 1 border,
AIPS 1: FACTOR 0 Scale plus signs by FACTOR
AIPS 1: XYRATIO 0 XY plot ratio: 0 -> 1.636
AIPS 1: BADDISK *all 0 Disks to avoid for scratch
AIPS 1: DOTV 1 > 0 Do plot on the TV, else
AIPS 1: GRCHAN 0 Graphics channel 0 => 1.
> R. Craig Walker wrote:
>> When I try using POSSM on a wideband VLBA data set (8 IF, 32 MHz each
>> full
>> pol with 64 spectral channels per IF), if I use SMOOTH and restrict the
>> channel range (9-56), I get droopy ends to the phases. The ends droop
>> for
>> several channels on each end of every IF. All go in the same direction
>> on
>> a given baseline, but can go the other direction on another baseline.
>> If
>> I open up to all channels, I don't see the effect. If I don't smooth, I
>> don't see any justification in the unsmoothed data for such droops. It
>> happens for both gaussian and box smoothing and for both before and
>> after
>> bandpass calibration. In fact, the effect shows up when bandpass is not
>> applied.
>>
>> See the attached plots. The first shows the effect in a smoothed,
>> channel
>> restricted case. The second shows the full band smoothed and the third
>> shows the full band unsmoothed.
>>
>> Below are my possm inputs followed by an image header. The reference
>> antenna in all tasks that need one is LA.
>>
>> The droops seem to be some artifact of the smoothing. Is there a reason
>> that this behavior is expected, or is there some issue with POSSM?
>
> When I stare at the unsmoothed data I do not see anything that is
> obviously wrong in the smoothed data. Wherever the smoothed droops
> there are a fair number of unsmoothed points in the direction of the
> droop and the smoothed data occupy a smaller range of phases than the
> unsmoothed.
>
> Have you tried the special values of SMOOTH available in POSSM alone?
>
> I worked over this code recently and did a few little things to fix it
> up and then added the post-summation smoothing in POSSM and checked it
> all fairly carefully.
>
> I agree that it looks curious but I am not fully convinced.
>
> Eric
>
---------------------------------------------------------------------
R. Craig Walker Array Operations Center
cwalker at nrao.edu National Radio Astronomy Observatory
Phone 575 835 7247 P. O. Box O
Fax 575 835 7027 Socorro NM 87801 USA
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