[daip] [!6804]: AIPS - Calibrating instrumental polarization by using LPCAL

[Eric Greisen]

Eric Greisen updated #6804
--------------------------

       Staff (Owner): Eric Greisen (was: -- Unassigned --)
                 Due: - Cleared - (was: 03 July 2015 01:00 AM)

Calibrating instrumental polarization by using LPCAL
----------------------------------------------------

           Ticket ID: 6804
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/6804



you seem to know a lot more about polarization than I do!

Sadly, the people who wrote LPCAL (and PCAL) no longer in the
AIPS group.   I will try to provide at least a partial answer.  The
code of CLCOR does just what the explain file says it does - add
a parallactic phase or subtract one form the phases already in the
CL table.  In PCAL the choice of SOLTYP determines whether one wants
to add a parallactic angle correction of remove it.  The help file for
LPCAL states that one wants to apply a correction.

LPCAL is really for resolved sources in which one could need partial
CC tables for each part of the source.  Then LPCAL will attempt to
fit different source polarizations to each piece.  As the help says,
lots of data with considerable parallactic angle coverage will be
needed.  If you have an unpolarized point source, PCAL may be
a better choice than LPCAL.  It is used a lot and works well.

Another issue with polarization calibration is the parallel-hand calibration.
Until RR and LL are well calibrated (self-cal in amplitude and phase), any
attempt to apply D terms to them to subtract from RL and LR in ill-advised.

I do not know what is done to fit the data - I can send you the pre-cursor 
comments from the fitting routine in LPCAL:

C   Subroutine to determine instrumental and source polarizations.
C   A total intensity model for the calibrator must be provided
C   in the form of components for which the linear polarization
C   structure is assumed to be a scaled version of the total
C   intensity structure. Solves for complex leakage factors for
C   each station and the polarizations of each submodel.
C   Circular polarization is assumed to be negligible.
C   Only one calibrator source can be used.
C
C   Fits the model:
C
C     RL       = Pn*In        +  DRa * exp(i*2*chia) * LL
C                +         conj(DLb) * exp(i*2*chib) * RR
C     conj(LR) = Pn*conj(In)  +  DRb * exp(i*2*chib) * conj(LL)
C                +         conj(DLa) * exp(i*2*chia) * conj(RR)
C
C     where In is the flux of submodel n, Pn its fractional
C     linear polarization (Qn+iUn)/In, chia, chib are the parallactic
C     angles of antennas a and b, RR, RL, LR, LL are the observed
C     Stokes, and DLx and DRx are the instrumental parameters.
C
C   A standard complex linear least squares solution of the normal
C   equation is done by Cholesky decomposition using LAPACK routines.
C
Eric Greisen




------------------------------------------------------
Staff CP:  https://help.nrao.edu/staff