[daip] Ringing from BPASS

[Ingyin Zaw]

Hello,

I'm running into a problem where the bandpass solution from BPASS 
introduces ringing in spectral line data. Furthermore, the problem is 
excerbated when (8) spectral channels are averaged with AVSPC. Since we 
are looking for narrow water maser lines, this ringing poses a serious 
problem.

The details are as follows:

AIPS version: 31DEC09, 64 bit (problem also seen in 32 bit, 31DEC09)

Computer system:
Linux rglinux9.cfa.harvard.edu 2.6.18-128.1.1.el5 #1 SMP Mon Jan 26 
13:58:24 EST 2009 x86_64 x86_64 x86_64 GNU/Linux

Dataset: BK114A, taken Nov. 28, 2004, 256 Channels, 8 IFs
(NB: similar problem seen in BB242D, taken Jan. 21, 2008, 512 channels, 8IFs)

Details on ringing:
- Ringing in data (amplitude) when viewed with POSSM and BP is applied (doband 1).
   - Mainly in baselines to Ant 12, the phased VLA array.
   - Behavior is different (more severe) on data averaged with AVSPC
     (avoption 'SUBS'; channel 8)
   - Behavior also differs when the file contains only one source or
     when it contains several sources even though BPASS and POSSM are run
     only on one source.
- Ringing is not seen when BP is not applied (POSSM, doband -1)
- Ringing is less noticeable when looking at the BP table itself
   (POSSM, aparm(8)=2)

I've made 3 files available for testing via ftp from:
cfa-ftp.harvard.edu
Name: anonymous
Password: <youremailaddress>
Folder: incoming/lincoln/
Files:
(1) BK114A_2145_256.FITS
(2) BK114A_2145_32.FITS
(3) BK114A_2145FILE.FITS

(1) Contains only the source, 2145+067, on which I'm running BPASS (loaded 
with FITLD with source selection).
(2) Is the file with AVSPC (avoption 'SUBS'; channel 8) ran on file (1).
(3) Contains other sources, in addition to 2145+067, all from one file 
loaded with FITLD.

Each file contains one BP table, output of BPASS with identical inputs. 
The parameters different from default are:

calsour '2145+067'
solint -1
soltype 'L1'
refant 6 (LA)

(docalib -1; doband -1; bpassprm 0)

Please let me know how the ringing can be fixed.

Thanks very much,
Ingyin