Scheduling Strategy:
--------------------
The source list for each session is based on the following considerations:

(1) A total of 40 sources are observed per session. 
(2) Roughly equal spacing in R.A. over 0-24h.
(3) Limited number of low declination sources (dec < -10) and circumpolar sources.
(4) No sources within 12 degrees of the Sun.


Observing Strategy 
------------------ 

Our observations are optimized for obtaining suitably high-dynamic
range, full-polarization images on a sample of ~200 sources at
specific intervals based on the angular speed of each invididual source. We have
adopted the following:

(1) One 24-hr VLBA observing session every 4 weeks (optimum time spacing).
(2) Dual-polarization, 4096 Mbps, 4 IFs per polarization hand, 512 MHz total bandwidth.
(3) Scans on a given source are scheduled to have at least 6 antennas
    above 10 degrees elevation.
(4) Typically 10 scans per source, for a total on-source time of ~25 minutes. 


Data retrieval method
---------------------

(1) Run makeSCRIPT.pl <VLBACODE> to create directory and AIPS script files
(2) Transfer NRAO data from archive.nrao.edu in Socorro.
(3) Follow the procedures (cut and pasting into AIPS) in <VLBACODE>.SCRIPT


Data Reduction Method
---------------------

Since several people are carrying out the data reduction, we rely
on standard scripts whenever possible. The major calibration steps are:

 (1) Load data into AIPS, excluding visibilities with weights < 70%.
 (2) Flag bad data according to observation log and elevated system temperatures. 
 (3) Remove correlator bias with ACCOR.
 (4) Apply system temperature and opacity amplitude corrections. 
 (5) Apply parallactic angle correction.
 (6) Apply pulse-cal phase corrections
 (7) Apply fringe fit corrections (excluding any sources < 50 mJy).
 (8) Apply cross-hand phase corrections.
 (9) Apply bandpass amplitude corrections.
(10) Align phases using short-solution interval point-source selfcal.
(11) Average data over 11 second intervals in DIFMAP.
(12) Load clean windows for the source (determined from prior epochs).
(13) Produce initial I clean-component model in DIFMAP, then phase-only selfcal.
(14) Perform global amplitude selfcal (gscale). Adjust individual antenna overall gains in AIPS if necessary.
(15) Refine clean model and perform phase and amplitude selfcals down to 30 sec intervals.
(16) Export time-averaged, edited visibility data and CLEAN model to AIPS and perform amplitude and phase self-cal. 
(17) Use CCEDT to produce sub-models from I models. 
(18) Determine polarization feed solutions from each source using LPCAL.
(19) Apply median LPCAL solutions to data.
(20) Perform another amplitude and phase self-cal in AIPS using I model.
(21) Export final calibrated data to DIFMAP for automated imaging in I,Q,U.
(22) Produce color fractional polarization plots with D. Homan's scripts.


Current Electric Vector Calibration Method
-------------------------------------------------------------

(1) Measure EVPA of individual jet features located downstream from
the core, in multiple sources.

(2) Identify which of these have near-stable EVPAs over time, based on
prior MOJAVE observations. Find the single EVPA correction that
minimizes the difference between the current epoch EVPAs and those of
prior epochs.


Electric Vector Calibration Method (Pre-new VLBA digital back-ends)
-------------------------------------------------------------

(1) Estimate approximate EVPA rotation corrections for each epoch  
from simultaneous UMRAO data on calibrators.

(2) Apply the corrections and check the phases of the dterms for
each IF, Antenna and hand for consistency between epochs.

(3) Throw out any dterm that appears to have
undergone an abrupt change during the period spanned by the epochs. 

(4) Pick one epoch as a "reference" epoch (BL111C was chosen)

(5) Starting with the first epoch (BL111A), find the best rotation correction
that minimizes the phase differences of all dterms between it and the
reference epoch. Repeat for all other epochs. This establishes the
relative rotation correction between epochs. 

(6) Using the UMRAO data, find a single rotation correction to all
epochs that minimizes the EVPA differences between UMRAO and the VLBA
data on calibrators. The establishes the absolute rotation correction to
the reference epoch. 

(7) For subsequent (new) epochs, exploit constancy of DTERM phases at
specific antennas to determine best EVPA rotation for the new epoch. 


Absolute Flux Density Calibration Method
----------------------------------------
Establish amount of arcsecond-scale flux density (not visible to VLBA) for individual MOJAVE sources
using near-simultaneous archival OVRO 40m 15 GHz observations dating back to 2008. 
Estimate the  15 GHz compact flux density by interpolating the OVRO measurements to the
VLBA epoch and subtracting the missing arcsecond scale flux density. Compare these estimates for all 
sources in the VLBA epoch and determine a median flux density correction factor to be applied.


Last updated: Jan 3, 2022