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View this ticket online: <a href="https://help.nrao.edu/app#/t/ticket/42439" 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/42439</a>
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Lynn Matthews <lmatthew@mit.edu> </td>
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Awaiting Agent
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Unsupported OS
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I have been exploring an ALMA-EHT spectral line VLBI data set in AIPS, using POSSM to examine the raw visibility data. The cross-correlation spectra appear to be "normal" (e.g., attachm...
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<div>I have been exploring an ALMA-EHT spectral line VLBI data set in AIPS, using POSSM to examine the raw visibility data.</div><div><br></div><div>The cross-correlation spectra appear to be "normal" (e.g., attachment 1). The autocorrelation spectra also look fairly normal, but they have visible "spurs" in a few spots produced by the channelization used for the correlation (attachment 2).</div><div><br></div><div>However, when I look at the cross-correlation function (APARM(8)=4) things are very bizarre and ugly (attachment 3). </div><div><br></div><div>IFS 2 & 3, both of which have channelization "seams", show high amplitude ringing. (IF1, which has no seams, appears normal, though it looks like zero because of the auto-scaling).</div><div><br></div><div>Initially I thought I could just flag the spectral channels near these "seams" to get rid of this. But if I then flag the spectral channels surrounding the seams (in IF2 only) the ringing pattern then changes shape and increases in amplitude (attachment 4).</div><div><br></div><div>I am now trying to understand: (a) mathematically, what exactly is causing this given that the cross-correlation spectra look seemingly normal? and (b) is something I can ignore, or is this likely to impact my data in harmful ways?</div><div><br></div><div>Not sure if these are AIPS question or a more general ones, but I thought I would start first by trying to understand what is causing AIPS to generate these strange cross-correlation functions.</div><div><br></div><div>I'd appreciate any insights you can provide.</div><div> </div><div>Thanks,</div><div>Lynn Matthews</div>
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<strong>Attachments</strong>
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• <a href="https://help.nrao.edu/file.php/9955BCBHSMPSXZ99548130067DAT/WHYA_cross.png?access_token=tls83g-suceucrwxg-cc1199e76f26aeb532fe351ac655f27acfa64626" 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;">WHYA_cross.png</a> (62.21 KB)
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• <a href="https://help.nrao.edu/file.php/9955JDNBSKDNZS99548220CE413T/WHYA_auto.png?access_token=tls83e-hljatjmfah-f290330df2f62bbc0819abd676a51a7295e66ca6" 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;">WHYA_auto.png</a> (14.14 KB)
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• <a href="https://help.nrao.edu/file.php/9955KJJBAYDHZR995483429C256T/WHYA_XCF.png?access_token=tls83c-tugjrevmex-c94f1f4390e4bf0b9f3285d488806d7b82af87ce" 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;">WHYA_XCF.png</a> (27.51 KB)
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• <a href="https://help.nrao.edu/file.php/9955XPDDPAGBWT9954882B2A82ET/WHYA_XCF_flagged.png?access_token=tls83a-clsftspoxk-19909b0296078dd81cd9361f20387884bcd597f4" 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;">WHYA_XCF_flagged.png</a> (23.81 KB)
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