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<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">Hi Alberto,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">At CASDA we support cutouts of velocity spectral line cubes.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">We currently require the user to provide a BAND or CHANNEL parameter to the SODA service which is translated by the service to the velocity frame of the target cube. This works,
but requiring conversions on both the client and server side isnt ideal. Likewise, in our ObsCore implementation only the em_ucd indicates the cubes are in velocity but it would certainly be useful to have full velocity information for data discovery!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">The extension you outline looks like it would work nicely for the same use case in CASDA. However I think we should add the following to fully describe the velocity axis (as opposed
to its translation into wavelength for the base obscore).<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt;mso-fareast-language:EN-US">v_resolution value of CDELT3 or CDELT4<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><span style="font-size:11.0pt;mso-fareast-language:EN-US">v_unit although this should always be m/s<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US">Another consideration is that a lot of our users tend to use velocity to interact with frequency cubes. So even for these frequency cubes we might end up filling in velocity metadata
if the fields were defined. Tools such as CARTA tend to automatically show the spectral axis as velocity where it can, such that many users would assume the cubes are in velocity not frequency.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Cheers,<o:p></o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:11.0pt;color:#00A9CE">James Dempsey</span></b><span style="font-size:11.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:9.0pt;color:black">Senior Developer </span>
<span style="font-size:9.0pt;color:#00A9CE">|</span><span style="font-size:9.0pt;color:black"> CSIRO <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><a href="mailto:james.dempsey@csiro.au"><span lang="EN-GB" style="font-size:9.0pt;color:#0563C1">james.dempsey@csiro.au</span></a></span><span style="font-size:9.0pt;color:black">
</span><span style="font-size:9.0pt;color:#00A9CE">|</span><span style="font-size:9.0pt;color:black"> 02 6214 2912<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-size:12.0pt;color:black">From:
</span></b><span style="font-size:12.0pt;color:black">dal <dal-bounces@ivoa.net> on behalf of alberto micol <amicol.ivoa@googlemail.com><br>
<b>Date: </b>Thursday, 16 February 2023 at 12:02 am<br>
<b>To: </b>dal@ivoa.net <dal@ivoa.net><br>
<b>Subject: </b>ObsCore for velocity cubes<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><span style="font-size:11.0pt">Dear ObsCorers,<o:p></o:p></span></p>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">I have received with much interest the ObsCore extension for RADIO data, thanks for that very clear document.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">Would it be possible to have a similar extension for cubes with velocity as third axis (the other two being the usual spatial ones)?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Let me call them velocity cubes...<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Here my use case
<o:p></o:p></span></p>
</div>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">At ESO we need to support cutouts of velocity cubes.<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">Up to now our SODA interface supported spectra, images, and wavelength cubes;<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">to provide the user with the SODA input parameters and their possible MIN/MAX values<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt">we query ivoa.ObsCore.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">Now with the introduction of velocity cubes, we would like to get the necessary SODA input params also from ObsCore.<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">We can certainly do so by extending our own ObsCore table (ObsCore allows to extend the table), but I was wondering<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">if others have similar needs, and see if we can agree with a common ObsCore extension.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Velocity cubes should come in a FITS format as described by Greisen et al, 2006, so-called Paper 3.<o:p></o:p></span></p>
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<blockquote style="margin-left:30.0pt;margin-right:0cm">
<div>
<p class="MsoNormal"><span style="font-size:11.0pt">Shortly summarised, the velocity can be either:<o:p></o:p></span></p>
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</blockquote>
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<ul type="disc">
<ul type="circle">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-size:11.0pt">a radio (VRAD), <o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-size:11.0pt">an optical (VOPT), <o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-size:11.0pt">a relativistic velocity (ZOPT), <o:p></o:p></span></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level2 lfo1">
<span style="font-size:11.0pt">a generic apparent velocity (VELO).<o:p></o:p></span></li></ul>
</ul>
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<blockquote style="margin-left:30.0pt;margin-right:0cm">
<div>
<p class="MsoNormal"><span style="font-size:11.0pt">The RESTFRQa (RESTWAVa) keyword conveys the necessary reference frequency (wavelength) to transform velocity into frequency or wavelength. <o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">The velocity unit is m/s.<o:p></o:p></span></p>
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</blockquote>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt"> <o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">SODA cutouts on the velocity axis are usually formulated by passing cuts already expressed in velocity<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">though it must be possible to perform the conversions to and from frequency/wavelength.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">What would then be needed in ObsCore is:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">v_min<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">v_max<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">but also, in case of radio velocity cube:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">f_min<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">f_max (getting these from the RADIO extension)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">f_rest = RESTFRQ<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">and, in case of a optical or relativistic velocity cube<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">em_rest = RESTWAV<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">(we already have em_min, em_max)<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">The em_resolv_power should then be set to:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> lambda freq c<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"> = = <o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt"> delta lambda delta freq 2 * delta_v<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">where delta_v is the CD3_3 in the cube (in the example of a cube with the velocity on the third axis);<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">but I would also likely accept a v_pixel_scale (= delta_v).<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">The UCD for the velocity should then be:<o:p></o:p></span></p>
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<div>
<pre>spect.dopplerVeloc.opt<o:p></o:p></pre>
<div>
<pre>spect.dopplerVeloc.radio<o:p></o:p></pre>
<div>
<p class="MsoNormal"><span style="font-size:11.0pt">unsure what it should be for the relativistic case.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">This would allow us at ESO to implement SODA on top of ObsCore (as already done for all other parameters)<o:p></o:p></span></p>
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<div>
<p class="MsoNormal"><span style="font-size:11.0pt">and would allow astronomer to search the velocity cubes with certain characteristics using ObsCore.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">What do you think? <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Is that doable? I think at ESO we will simply start implementing the above extra ObsCore parameters (unless you suggest different names for that),<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">while waiting to see if there is more interest to build a proper standard.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Eager to know what you think, <o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Cheers,<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">Alberto<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
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