VSOP Data Reduction | HALCA calibration | GRT calibration
VSOP DATA REDUCTION
- Documentation
- Fringe-Fitting and Calibration
- Penticton Correlator Correction Factor
- Imaging
- The VSOP Data Reduction Exploder (VDR)
DOCUMENTATION
AIPS. The newly revised chapter 9 of the AIPS CookBook `Reducing VLBI Data in AIPS' has the relevant details. The AIPS FAQ also contains useful information for those not familiar with this software package.
Difmap. The Difmap Cookbook has recently been updated with information on imaging Space VLBI data with this software (see Section 11 in the Cookbook).
FRINGE-FITTING AND CALIBRATION
Fringe-fitting and calibration should be carried out in AIPS. You will need the most recent version of AIPS (15OCT98 version) in order to successfully reduce your VSOP data. The use of the regularly updated development version of AIPS, which is available at the regional user-support facilities and data-reduction centers, is recommended where possible. HALCA calibration and Ground Radio Telescope (GRT) calibration details are given on separate pages.
PENTICTON CORRELATOR CORRECTION
Recently, the Penticton S2 Correlator group in conjunction with Carl Gwinn from UC Santa Barbara, determined that the equation used in the Van Vleck 4x4 level normalization algorithm used when raw correlator data from the Penticton S2 correlator is exported to FITS format is in error.
The error is such that the output visibilities in **ALL** Penticton FITS data generated prior to February 14th, 1999 are **high** in amplitude by approximately 18%.
The error has been fixed and the FITS export software thoroughly tested. Data exported after February 14th, 1999 from the Penticton correlator will not be in error. In addition, it will include quantizer stream statistics in the PHASE-CAL table. These revisions are identified in the FITS dummy header as "cc2fits Version 1.7".
Previously exported Penticton FITS data did not include quantizer statistics in the PHASE-CAL table so an **exact** correction cannot be performed on existing Penticton FITS data.
However, a correction factor of 0.82 can be applied to all previously exported FITS data to overcome the discrepancy in amplitude. Though this will not correct for low-level fluctuations in individual station quantizer thresholds, it will correct the data to within 2% of the correct amplitude for quantizer thresholds ranging from 0.85 to 1.2 sigma.
An example of an AIPS script for making this correction is available here. (Revised 02 March 1999)
Alternatively, since all raw correlator data from the Penticton correlator are archived, these will be re-exported using the revised FITS export software on request from PI's.
Imaging can be carried out with either AIPS or Difmap both of which have been upgraded to handle space VLBI data. If you choose to use Difmap to image your data, make sure it is version 2.3 or later.
THE VSOP DATA REDUCTION (VDR) E-MAIL EXPLODER
The VDR is intended to provide a forum for people in the mission and PI's who are reducing Space VLBI data. The expertise gathered by everyone in reducing VSOP data will help in providing more complete documentation for AIPS and Difmap.
All activities of VDR is now closed. (Mar 10th, 2019)