<div dir="ltr"><div>FWIW, this all would have been simpler if the redshift and spectral coordinates has been retained in the coordinate system model.</div><div>Just an observation...<br></div><div><div><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><br><div dir="ltr" style="margin-left:0pt"><table style="border:medium none;border-collapse:collapse"><colgroup><col width="445"><col width="275"></colgroup><tbody><tr style="height:90pt"><td style="border-color:rgb(255,255,255);border-style:solid;border-width:1pt;vertical-align:top;padding:5pt"><p dir="ltr" style="line-height:1.2;margin-top:0pt;margin-bottom:0pt"><span style="font-size:9.5pt;font-family:Arial;color:rgb(34,34,34);background-color:transparent;font-weight:700;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Arnold H Rots</span></p><p dir="ltr" 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style="font-size:9.5pt;font-family:Arial;color:rgb(34,34,34);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"><span><span style="font-size:9.5pt;font-family:Arial;color:rgb(34,34,34);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Email: <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a></span></span></span></p><p dir="ltr" style="line-height:1.2;margin-top:0pt;margin-bottom:0pt"><span style="font-size:9.5pt;font-family:Arial;color:rgb(34,34,34);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Office: +1 617 496 7701 | Cell: +1 617 721 6756</span></p><p dir="ltr" style="line-height:1.2;margin-top:0pt;margin-bottom:0pt"><span 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class="msg-3246293190726509956">
<div dir="ltr">
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
Hi James,</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
that is a good suggestion. The usage of velocity vs. frequency is mostly use case/user driven. The original measurements are in frequency and in some cases the conversion from one to the other is not straight forward, again mostly dependent on the use case,
in which case we end up having 'pure' velocity cubes. Thus encoding both might work for some, but not for others. The other thing to consider is the rest-frame, since quite often these velocity cubes are used for studies of local dynamic effects, like galaxy
rotation curves, in which case the zero point is set to some estimate of the galaxies cosmological velocity (RESTFREQa). Now, obviously all of this is limited to just radial velocities, while in fact we are dealing with velocity vectors. To cover that we are
using proper motion and parallax to describe apparent and intrinsic velocity on the sphere and in addition line-of-sight velocity to capture the intrinsic and apparent radial movement. Taken together, we end up in a set of vectors and probably frames. For
a naive user the proposed description is probably sufficient, for actual scientific use of multiple of such cubes, quite a bit more information is required. Thus probably just a good start....</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
<br>
</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
Cheers,</div>
<div style="font-family:Calibri,Arial,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255)">
Andreas</div>
<div id="m_1877502663250217515appendonsend"></div>
<hr style="display:inline-block;width:98%">
<div id="m_1877502663250217515divRplyFwdMsg" dir="ltr"><font style="font-size:11pt" face="Calibri, sans-serif" color="#000000"><b>From:</b> dal <<a href="mailto:dal-bounces@ivoa.net" target="_blank">dal-bounces@ivoa.net</a>> on behalf of Dempsey, James (IM&T, Black Mountain) <<a href="mailto:James.Dempsey@csiro.au" target="_blank">James.Dempsey@csiro.au</a>><br>
<b>Sent:</b> Thursday, 16 February 2023 7:20 AM<br>
<b>To:</b> Alberto Casa Micol <<a href="mailto:amicol@eso.org" target="_blank">amicol@eso.org</a>>; <a href="mailto:dal@ivoa.net" target="_blank">dal@ivoa.net</a> <<a href="mailto:dal@ivoa.net" target="_blank">dal@ivoa.net</a>><br>
<b>Subject:</b> Re: ObsCore for velocity cubes</font>
<div> </div>
</div>
<div style="overflow-wrap: break-word;" lang="EN-AU">
<div>
<p><span style="font-size:11pt">Hi Alberto,</span></p>
<p><span style="font-size:11pt"> </span></p>
<p><span style="font-size:11pt">At CASDA we support cutouts of velocity spectral line cubes.</span></p>
<p><span style="font-size:11pt"> </span></p>
<p><span style="font-size:11pt">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 isn’t 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!</span></p>
<p><span style="font-size:11pt"> </span></p>
<p><span style="font-size:11pt">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).</span></p>
<ul style="margin-top:0cm" type="disc">
<li style="margin-left:0cm"><span style="font-size:11pt">v_resolution – value of CDELT3 or CDELT4</span></li><li style="margin-left:0cm"><span style="font-size:11pt">v_unit – although this should always be m/s</span></li></ul>
<p><span style="font-size:11pt"> </span></p>
<p><span style="font-size:11pt">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.</span></p>
<p><span style="font-size:11pt"> </span></p>
<div>
<div>
<p><span style="font-size:11pt">Cheers,</span></p>
<p><b><span style="font-size:11pt;color:rgb(0,169,206)">James Dempsey</span></b><span style="font-size:11pt;color:black"></span></p>
<p><span style="font-size:9pt;color:black">Senior Developer
</span><span style="font-size:9pt;color:rgb(0,169,206)">|</span><span style="font-size:9pt;color:black"> CSIRO </span></p>
<p><span style="font-size:11pt"><a href="mailto:james.dempsey@csiro.au" target="_blank"><span style="font-size:9pt;color:rgb(5,99,193)" lang="EN-GB">james.dempsey@csiro.au</span></a></span><span style="font-size:9pt;color:black">
</span><span style="font-size:9pt;color:rgb(0,169,206)">|</span><span style="font-size:9pt;color:black"> 02 6214 2912</span></p>
</div>
</div>
<p><span style="font-size:11pt"> </span></p>
<p><span style="font-size:11pt"> </span></p>
<div style="border-color:rgb(181,196,223) currentcolor currentcolor;border-style:solid none none;border-width:1pt medium medium;padding:3pt 0cm 0cm">
<p style="margin-bottom:12pt"><b><span style="font-size:12pt;color:black">From:
</span></b><span style="font-size:12pt;color:black">dal <<a href="mailto:dal-bounces@ivoa.net" target="_blank">dal-bounces@ivoa.net</a>> on behalf of alberto micol <<a href="mailto:amicol.ivoa@googlemail.com" target="_blank">amicol.ivoa@googlemail.com</a>><br>
<b>Date: </b>Thursday, 16 February 2023 at 12:02 am<br>
<b>To: </b><a href="mailto:dal@ivoa.net" target="_blank">dal@ivoa.net</a> <<a href="mailto:dal@ivoa.net" target="_blank">dal@ivoa.net</a>><br>
<b>Subject: </b>ObsCore for velocity cubes</span></p>
</div>
<p><span style="font-size:11pt">Dear ObsCorers,</span></p>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">I have received with much interest the ObsCore extension for RADIO data, thanks for that very clear document.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Would it be possible to have a similar extension for cubes with velocity as third axis (the other two being the usual spatial ones)?</span></p>
</div>
<div>
<p><span style="font-size:11pt">Let me call them “velocity cubes”...</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Here my use case…</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">At ESO we need to support cutouts of velocity cubes.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Up to now our SODA interface supported spectra, images, and wavelength cubes;</span></p>
</div>
<div>
<p><span style="font-size:11pt">to provide the user with the SODA input parameters and their possible MIN/MAX values</span></p>
</div>
<div>
<p><span style="font-size:11pt">we query ivoa.ObsCore.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Now with the introduction of velocity cubes, we would like to get the necessary SODA input params also from ObsCore.</span></p>
</div>
<div>
<p><span style="font-size:11pt">We can certainly do so by extending our own ObsCore table (ObsCore allows to extend the table), but I was wondering</span></p>
</div>
<div>
<p><span style="font-size:11pt">if others have similar needs, and see if we can agree with a common ObsCore extension.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Velocity cubes should come in a FITS format as described by Greisen et al, 2006, so-called Paper 3.</span></p>
</div>
<blockquote style="margin-left:30pt;margin-right:0cm">
<div>
<p><span style="font-size:11pt">Shortly summarised, the velocity can be either:</span></p>
</div>
</blockquote>
<div>
<ul type="disc">
<ul type="circle">
<li><span style="font-size:11pt">a radio (VRAD), </span></li><li><span style="font-size:11pt">an optical (VOPT), </span></li><li><span style="font-size:11pt">a relativistic velocity (ZOPT), </span></li><li><span style="font-size:11pt">a generic apparent velocity (VELO).</span></li></ul>
</ul>
</div>
<blockquote style="margin-left:30pt;margin-right:0cm">
<div>
<p><span style="font-size:11pt">The RESTFRQa (RESTWAVa) keyword conveys the necessary reference frequency (wavelength) to transform velocity into frequency or wavelength. </span></p>
</div>
<div>
<p><span style="font-size:11pt">The velocity unit is m/s.</span></p>
</div>
</blockquote>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">SODA cutouts on the velocity axis are usually formulated by passing cuts already expressed in velocity</span></p>
</div>
<div>
<p><span style="font-size:11pt">though it must be possible to perform the conversions to and from frequency/wavelength.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">What would then be needed in ObsCore is:</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">v_min</span></p>
</div>
<div>
<p><span style="font-size:11pt">v_max</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">but also, in case of radio velocity cube:</span></p>
</div>
<div>
<p><span style="font-size:11pt">f_min</span></p>
</div>
<div>
<p><span style="font-size:11pt">f_max (getting these from the RADIO extension)</span></p>
</div>
<div>
<p><span style="font-size:11pt">f_rest = RESTFRQ</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">and, in case of a optical or relativistic velocity cube</span></p>
</div>
<div>
<p><span style="font-size:11pt">em_rest = RESTWAV</span></p>
</div>
<div>
<p><span style="font-size:11pt">(we already have em_min, em_max)</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">The em_resolv_power should then be set to:</span></p>
</div>
<div>
<p><span style="font-size:11pt"> lambda freq c</span></p>
</div>
<div>
<p><span style="font-size:11pt"> ——————— = ————— = —————</span></p>
</div>
<div>
<p><span style="font-size:11pt"> delta lambda delta freq 2 * delta_v</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">where delta_v is the CD3_3 in the cube (in the example of a cube with the velocity on the third axis);</span></p>
</div>
<div>
<p><span style="font-size:11pt">but I would also likely accept a v_pixel_scale (= delta_v).</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">The UCD for the velocity should then be:</span></p>
</div>
<div>
<pre>spect.dopplerVeloc.opt</pre>
<div>
<pre>spect.dopplerVeloc.radio</pre>
<div>
<p><span style="font-size:11pt">unsure what it should be for the relativistic case.</span></p>
</div>
</div>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">This would allow us at ESO to implement SODA on top of ObsCore (as already done for all other parameters)</span></p>
</div>
<div>
<p><span style="font-size:11pt">and would allow astronomer to search the velocity cubes with certain characteristics using ObsCore.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">What do you think? </span></p>
</div>
<div>
<p><span style="font-size:11pt">Is that doable? I think at ESO we will simply start implementing the above extra ObsCore parameters (unless you suggest different names for that),</span></p>
</div>
<div>
<p><span style="font-size:11pt">while waiting to see if there is more interest to build a proper standard.</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
<div>
<p><span style="font-size:11pt">Eager to know what you think, </span></p>
</div>
<div>
<p><span style="font-size:11pt">Cheers,</span></p>
</div>
<div>
<p><span style="font-size:11pt">Alberto</span></p>
</div>
<div>
<p><span style="font-size:11pt"> </span></p>
</div>
</div>
</div>
</div>
</div></blockquote></div>