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View this ticket online: <a href="https://help.nrao.edu/app#/t/ticket/41648" title="(#)" style="color: #0065A3; font-family: Helvetica, Arial, sans-serif; font-weight: normal; padding: 0; margin: 0; text-align: left; line-height: 1.3; text-decoration: underline;">https://help.nrao.edu/app#/tickets/t/41648</a>
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Walter Golay <wgolay@cfa.harvard.edu> </td>
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• Walter Golay <walter.golay@cfa.harvard.edu><br>
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Awaiting User
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MAC OS 14 ARM
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Hi Will, I’ll jump in and try to answer some of your other questions. Looking at your plot of amplitude vs frequency with the bandpass calibration applied, the only baseline that worrie...
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<!-- DP_MESSAGE_BEGIN --><div dir="ltr" lang="en" name="dp_message_189885_begin" class="dp_message_189885_begin"><div>Hi Will,</div><div>I’ll jump in and try to answer some of your other questions.</div><div>Looking at your plot of amplitude vs frequency with the bandpass calibration applied, the only baseline that worries me a little is KP-PT. Do all the KP baselines look like that, or is KP-PT particularly bad? It will probably get fixed by amplitude self-calibration (or something like gscale in Difmap), but I would keep an eye on it. I still don’t really understand why amplitude variations like that are so common in VLBA data.</div><div>For VLBAAMP, it is still recommended to use the inner 89% of the IFs for DDC data. However, you can use a smaller percentage if you see more problems at the edges of the IFs in your data.</div><div>For the instrumental delay step, I do not think providing a source model would be very useful. This step is mainly trying to correct for any phase jumps between IFs, and source structure is not going to have any significant impact on that. The instrumental delay calibration should have no impact on the amplitudes. Providing source models for the global fringe fitting may help improve your phases, but most users find that doing phase self-calibration with CALIB produces better results.</div><div>The details of ATMCA are discussed in AIPS Memo 111 (which can be found here: <a href="https://www.aips.nrao.edu/aipsmemo.html)." title="https://www.aips.nrao.edu/aipsmemo.html).">https://www.aips.nrao.edu/aipsmemo.html). </a> In Section 3 of the Memo, the authors say that the secondary calibrator(s) should be observed every 15 minutes. So, you can benefit from using ATMCA even if the calibrators are not observed within the nominal coherence time of the observing frequency. I think the usefulness of ATMCA has more to do with whether or not there are high-quality phase-reference calibrators (bright, compact, with well-known positions) near your target. If you happen to have a superb calibrator (e.g., an ICRF defining source) within 1 degree of your target, I don’t think ATMCA would help much. However, if there are only a few “okay” calibrators within 3-5 degrees of your target, ATMCA could be very helpful. The fact that there are fewer good calibrators at higher frequencies means ATMCA is more likely to be useful for those observations, but it could still useful at lower frequencies (especially if the ionosphere was causing problems during your observation).</div><div>Geodetic blocks are usually used to account for unmodeled components of the troposphere. As such, they are generally more useful at higher frequencies where the troposphere has more impact on the phases. However, the AIPS Memo describing the use of DELZN (AIPS Memo 110) shows that X-band observations can benefit from using geodetic blocks (although this might be mostly due to fixing clock errors rather than tropospheric effects).</div><div>I hope that helps.</div><div>Good luck with your calibration!</div><div>Justin</div></div><!-- DP_MESSAGE_END -->
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