VLBA Basic Spectral Line Tutorial DRAFT

This CASA Guide is for Version 6.6.3 of CASA. If you are using a later version of CASA and this is the most recent available guide, then you should be able to use most, if not all, of this tutorial.

Overview

This CASA Guide describes the procedure for calibrating a phase-referenced VLBA observation of two masers in W3OH and W3IRS5. The data were taken specifically for this tutorial. The observation made use of the DDC observing personality, using dual polarization with two spectral windows in each polarization. Each spectral window has a bandwidth of 32 MHz divided into 256 spectral channels. The first spectral window is centered at 4764 MHz for the OH maser line. The second spectral window is centered at 6668 MHz for the methanol maser line.

This tutorial will focus on calibrating the data and creating a continuum (Stokes I) image of the phase reference calibrator and images of the masers in the science targets.

Please note that CASA should be used with caution when calibrating VLBA data. At the current time, CASA should only be used to calibrate relatively simple VLBA observations (basic continuum, simple phase-referenced, etc.). In particular, CASA is currently not recommended for calibrating the following types of VLBA observations:

• Polarimetric — Calibration of resolved polarized sources is not yet available in CASA.
• Astrometric — Earth orientation parameter (EOP) and pulse-cal tone corrections are not yet available in CASA.
• Spectral line projects requiring fringe-rate mapping — Fringe-rate mapping is not yet available in CASA.
• Low Frequency (<4 GHz) — Total electron content (TEC) corrections have not yet been verified to work correctly for VLBI observations in CASA. Also, note that many C-band (4 to 8 GHz) VLBA observations will benefit from TEC corrections.
• VLBA+Y27 or other VLBA+ arrays that require the use of antab files — CASA does not currently support the use of antab files for ingesting calibration data (system temperatures, gain curves, etc.)
• VLBA data correlated prior to December 2009 — CASA cannot load data correlated with the old VLBA hardware correlator. Your data must have been correlated with the DiFX correlator.

If your observation involves any of the above, you should use AIPS to calibrate your data.

A note on VLBA+Y1 observations: It is currently possible to calibrate VLBA+Y1 (VLBA and single VLA antenna) in CASA. Any data taken after 2022 July 06 should work with minimal extra steps (namely, checking that the Y1 antenna diameter is correct). VLBA+Y1 data observed prior to 2022 July 06 may not include the system temperature and/or gain curve values need for calibration. For these data sets, refer to VLBA Scientific Memo 39 for details on how to prepare the data for calibration in either CASA or AIPS.

How to Use This CASA Guide

There are a number of possible ways to run CASA, described in more detail in Getting Started in CASA. In brief, there are at least three different ways to use CASA:

• Interactively examining task inputs. In this mode, one types taskname to load the task, inp to examine the inputs, and go once those inputs have been set to your satisfaction. Allowed inputs are shown in blue and bad inputs are colored red. The input parameters themselves are changed one by one, e.g., selectdata=True. Screenshots of the inputs to various tasks used in the data reduction below are provided, to illustrate which parameters need to be set. More detailed help can be obtained on any task by typing help taskname. Once a task is run, the set of inputs are stored and can be retrieved via tget taskname; subsequent runs will overwrite the previous tget file.

• Pseudo-interactively via task function calls. In this case, all of the desired inputs to a task are provided at once on the CASA command line. This tutorial is made up of such calls, which were developed by looking at the inputs for each task and deciding what needed to be changed from default values. For task function calls, only parameters that you want to be different from their defaults need to be set.

• Non-interactively via a script. A series of task function calls can be combined together into a script, and run from within CASA via execfile('scriptname.py'). This and other CASA Tutorial Guides have been designed to be extracted into a script via the script extractor by using the method described at the Extracting_scripts_from_these_tutorials page. Should you use the script generated by the script extractor for this CASA Guide, be aware that it will require some small amount of interaction related to the plotting, occasionally suggesting that you close the graphics window and hitting return in the terminal to proceed. It is in fact unnecessary to close the graphics windows (it is suggested that you do so purely to keep your desktop uncluttered).

If you are a relative novice or just new to CASA, it is strongly recommended to work through this tutorial by cutting and pasting the task function calls provided below after you have read all the associated explanations. Work at your own pace, look at the inputs to the tasks to see what other options exist, and read the help files. Later, when you are more comfortable, you might try to extract the script, modify it for your purposes, and begin to reduce other data.

NOTE: It is not recommended to use scripts to generate "science-ready" VLBA data products. Nearly all VLBA observations will require some amount of hands-on calibration. Calibration and imaging scripts should only be used as "first pass" attempts at calibration, which can be useful to determine if the observation resulted in a detection and to identify problems in the data.

Obtaining the Data

This Guide is intended to cover the entire process one would follow for calibrating their own VLBA observation. Therefore, we will start with a FITS-IDI file rather than a Measurement Set. The FITS-IDI file for this Guide is: TL016B.idifits (right-click and select "Save Link As..."; file size: 1.8 GB).

Alternatively, you may download the data from the NRAO Archive. In the archive search inputs, enter "TL016" in the Project Code field. Expand the results for TL016B and look for the file VLBA_TL016B_tl016b_BIN0_SRC0_1_220307T210851.idifits. Select the file, click "Download", and follow the instructions. Once you have downloaded the FITS-IDI file from the archive, it will be helpful to change the filename to "TL016B.idifits".

The Observing Log

Before diving into the calibration, it is always a good idea to look over the observing log to check for notes from the operators that can inform us about potential issues with the data (missing stations, bad weather, etc.). These logs are always emailed to the PI's of an observation, but you can also access them later from the NRAO's vlbiobs fileserver. To locate an observing log, first find the directory for observing month and the last two digits of the year (for the observation used in this Guide, that directory is jan24 for January 2024). Once inside the proper month+year directory, look for the project code (in this case, tl016d). Look for a file named <project code>log.vbla (tl016dlog.vlba).

The observing log for this particular observation looked like this:

VERY LONG BASELINE ARRAY OBSERVING LOG
--------------------------------------
Project:        TL016D
Observer:       Linford, Justin
Project type:   TEST
Obs filename:   tl016d.vex
Date/Day:       2024JAN07/007
Ants Scheduled: SC HN NL FD LA PT KP OV BR MK

=UT-Time===Comment===============================================MF#===%AD==AMD=
           Operator: Trevor Seddon                                              
     2359  Begin                                                                
2359-0416 *BR out, FRM stuck in SX position wo-19696             FRM    100  257
2359-0416 *KP out, elevation up limit and brake interlock fault  WEA    100  257
           that are likely caused by ice tripping the switches                  
2359-0112 *HN out to avoid snow                                  WEA    100   73
     0000  Date is 2024JAN08/008.                                               
     0030 %OV windy                                                             
     0228  New Operator: Paul Padilla                                           
     0416  End.                                                                 


Downtime Summary:
	Total downtime : 587 min
	Percentage downtime of observing: 22.8%
	Average downtime per hour : 13.7 min

    Total scheduled observing time (# Antennas): 2570 min (10)

Notes:
    * = Entries where data was affected.
    % = Entries where data may have been affected.
    & = Entries where the site tech was called out.
  WEA = Weather entries.
  MF# = Maintenance form or major downtime category associated with a problem.
  %AD = The percentage of an antenna affected by a problem.
  AMD = Total antenna-minutes downtime for a problem.
 Tsys = System Temperature (TP/SP x Tcal/2)
  ACU = Antenna Control Unit
  FRM = Focus/Rotation Mount
  RFI = Radio Frequency Interference
   CB = Circuit Breaker