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Walter Golay <wgolay@cfa.harvard.edu> </td>
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Hello, I am working on several simple VLBA experiments involving single-frequency continuum astrometry (phase referencing) and imaging that are likely sensitivity-limited. All my datase...
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<div>Hello,</div><div><br></div><div>I am working on several simple VLBA experiments involving single-frequency continuum astrometry (phase referencing) and imaging that are likely sensitivity-limited. All my datasets use C, X, or K band with the 4Gbps DDC personality using the setups recommended <a href="https://science.nrao.edu/facilities/vlba/docs/manuals/obsvlba/schedfile#autotoc-item-autotoc-16" target="_blank" rel="noreferrer noopener">here</a>. In some cases, I have Effelsberg included as well. Most also include geodetic blocks and multiple phase reference calibrators for using the ATMCA task.</div><div><br></div><div>I have been referring to the calibration guides on the CASA Guides webpages, and I have several questions about the procedure. I am currently running AIPS version 31DEC2025 on my MacBook Pro M1. </div><div><br></div><div>1. When in the reduction should global fringe fitting be done? In what appears to be more recent tutorials <a href="https://casaguides.nrao.edu/index.php?title=VLBA_Basic_Phase-referencing_Calibration_and_Imaging" target="_blank" rel="noreferrer noopener">here</a> and <a href="https://casaguides.nrao.edu/index.php?title=VLBA_AIPS_and_CASA_Walkthrough" target="_blank" rel="noreferrer noopener">here</a> that are demonstrating CASA-analogous calibrations, fringe fitting is done <i>before</i> the bandpass and amplitude (VLBABPSS and VLBAAMP) steps. However, the <a href="https://www.aips.nrao.edu/vlbarun.shtml" target="_blank" rel="noreferrer noopener">VLBARUN</a> pipeline does fringe fitting <i>after</i> these steps. Similarly, the <a href="https://casaguides.nrao.edu/index.php/AIPS-Spectral_Lines_and_Astrometry" target="_blank" rel="noreferrer noopener">AIPS spectral line and astrometry tutorial</a> and the AIPS <a href="https://casaguides.nrao.edu/index.php/AIPS-Simple-Self-Cal" target="_blank" rel="noreferrer noopener">simple self-calibration tutorial</a> also do fringe fitting after the bandpass and amplitude steps. </div><div><br></div><div>I can convince myself that global fringe-fitting could be necessary before bandpass and amplitude calibration if multiple scans are used to derive them. For example, one dataset I am working with has the fringe-finder scans at the beginning and end of an 8-hour observation with EB. The first scan is missing MK, but the final scan is missing EB. What is the recommendation for this situation, as well as more generally? </div><div><br></div><div>2. How concerned should I be about amplitude offsets left over after the bandpass and amplitude calibrations? From the same dataset I mention above, I have attached a plot of the result after all recommended typical calibration steps on the final fringe finder scan (without doing global fringe fitting yet). The bandpass calibration section of <a href="https://casaguides.nrao.edu/index.php?title=VLBA_Basic_Phase-referencing_Calibration_and_Imaging" target="_blank" rel="noreferrer noopener">this guide</a> seem to show in Fig 31 and 32 that this is perhaps expected, but I wanted to verify I understand correctly that this should be corrected with self-calibration (a&p) of the fringe finder or the phase reference calibrator. </div><div><br></div><div>3. I have been using the innner 89% (`ichansel 14 242 1 0` given 256 channels of 0.5 MHz each for each of the 4 IFs) when running VLBAAMP. Is this still the recommended choice for the DDC personality? </div><div><br></div><div>4. I have been having trouble with interactive flagging programs (eg FTFLG) where when I go to apply a flag via the interactive window, I get an error where AIPS is unable to initialize a TV scratch file. This seems to only happen if I have zoomed in the window using "SET WINDOW + LOAD" then "AUTO X Y ZOOM ?" and finally a "LOAD". The log displays the following:<br>"<span style="font-size:1.125rem;">LOCALH> FTFLG1: MINIT: OP WRIT ERROR IN WIN,LX,LY= 1 6 29 71 29</span></div><div>LOCALH> 765</div><div>LOCALH> FTFLG1: FTFLAG: UNABLE TO INIT (W) TV SCRATCH FILE, ERROR 1</div><div>LOCALH> FTFLG1: Purports to die of UNNATURAL causes</div><div>LOCALH> FTFLG1: localhos 31DEC25 TST: Cpu= 59.0 Real= 265 IO= 64249"</div><div><br></div><div>This error doesn't seem to happen if I don't display a sub-range of the data first, however, this makes it more difficult to accurately flag.</div><div><br></div><div>5. Is ATMCA/Multiview still useful to apply if the secondary sources aren't all observed within the phase coherence time (eg 300 seconds at X band)? Also, what is the highest frequency where ATMCA is still typically thought to be useful? </div><div><br></div><div>6. How useful is using a real source/image model for the instrumental phase delay step (VLBAMPCL) and the bandpass step (VLBABPSS)? Is this more or less negated by using an image model during global fringe fitting (VLBAFRGP)? I assume for VLBAMPCL that this wouldn't mitigate the amplitude offsets I mention in my second question since this only solves for phase-related terms, correct? </div><div><br></div><div>7. Similarly, how necessary is using image models vs an assumed point source for determining the multi-band delays from geodetic blocks? </div><div><br></div><div>Thanks in advance for the help. </div><div><br></div><div>Will Golay</div>
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