0000329: Baseline dependent gain errors (closure errors)

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Severity

Reproducibility

Date Submitted

Last Update

0000329

[DZB] 0.unknown/unspecified

major

N/A

27-01-11 18:36

23-02-11 18:57

Reporter

Gyula Jozsa

View Status

public

Assigned To

Priority

high

Resolution

open

Status

resolved

Summary

0000329: Baseline dependent gain errors (closure errors)

Description

Report by T. Oosterloo (Ger and first Hans v.S. had already been looking into this):
Hans van S has recently discovered that since a few months there are very large closure errors in WSRT data. Although there never is a reason to question Hans's judgement, I checked some recent data. attached are 2 plots from a line obs done on 2-Jan-11. This field has a strong (1Jy) pointlike source. One image shows the image after several standard selfcal interations (including amplitude in the last few). Rings are still visible which do not disappear, whatever i try in the selfcal. the rings are at the level of 0.1% or so (dyn range Note: 0000300). The other image is after an additional baseline dependent (but time independent) correction (of up to 4%!!!!!). Rings are gone, image is noise limited (dyn range > 3000)

Ger added:
It indeed appears that the closure errors are fairly stable, and are therefore correctable. However I checked the closure errors on the pair of calibrators of yesterdays Abell 773 cluster observation (3C48 = 11100453 and 3C286 = 11100456) and it is clear that their closure errors still differ at the 0.5% level.

This means that a transfer of closure errors will not be producing ideal data. This level of 0.5% is still much higher that what WSRT used to deliver (<0.02% typically in amplitude and no phase closure errors). My last WSRT 8x20, 64x4 WSRT continuum observation was done in late September (3C286 11005333) and was Ok in the sense that the closure errors were at the 0.1% level, a level determined by noise in the 15m observation.

So if your email meant to imply that we have to accept this
I strongly disagree. With possibly 2 more years to go with the IVC/DZB
and a range of 92cm legacy proposal being prepared, I find this very worrying.

Additional Information

Comment by Josh:
Toms report implies baseline dependent gain errors which are, however, constant in time. The amplitude gain is wrong by up to 4% PER BASELINE. The fact that this is a constant calibration error per baseline may be a hint where this is coming from (backend, software?). Tom describes how to solve the problem after it has occurred. However, the method implies the need for a nice calibration source in the observed field that may not be available all the time on top of some experience.

On top of that, Ger reports that the time independence is not valid at a 0.5%-level, which was what George found, experimenting with the long-standing problem with (mostly) RT3 (see below)

Baseline errors occur since a long time, but not at many baselines, only on singular ones (reported in ISSUE 220). However, both phenomena may be related, since it seems that also the baseline amplitude error reported in issue 220 has vanished in Toms experiment (meaning that the error had been constant in time).

nr subbands

nr polarisations

nr channels

1024

bandwidth

10 MHz

multi-subarray

recirculation

Attached Files

with.pdf [^] (315,531 bytes) 27-01-11 18:36

without.pdf [^] (305,330 bytes) 27-01-11 18:37

Relationships

related to

0000220

progressing

Hans van der Marel

Low-level baseline-dependent calibration errors

Notes
(0000329) Yuan Tang 22-02-11 14:01	20110222: The problem has been solved after repairing the power supply of the 180MHz LO.

(0000330) Gyula Jozsa 23-02-11 18:57	The solution of this issue has probably also resolved issue 220.

Issue History
Date Modified	Username	Field	Change
27-01-11 18:36	Gyula Jozsa	New Issue
27-01-11 18:36	Gyula Jozsa	File Added: with.pdf
27-01-11 18:36	Gyula Jozsa	nr subbands	=> 1
27-01-11 18:36	Gyula Jozsa	nr polarisations	=> 2
27-01-11 18:36	Gyula Jozsa	nr channels	=> 1024
27-01-11 18:36	Gyula Jozsa	bandwidth	=> 10 MHz
27-01-11 18:36	Gyula Jozsa	multi-subarray	=> no
27-01-11 18:36	Gyula Jozsa	recirculation	=> no
27-01-11 18:37	Gyula Jozsa	Relationship added	related to 0000220
27-01-11 18:37	Gyula Jozsa	File Added: without.pdf
01-02-11 20:18	Gyula Jozsa	Description Updated
01-02-11 20:18	Gyula Jozsa	Additional Information Updated
22-02-11 14:01	Yuan Tang	Note Added: 0000329
22-02-11 14:01	Yuan Tang	Status	new => resolved
23-02-11 18:57	Gyula Jozsa	Note Added: 0000330