<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Dear ObsCorers,<div class=""><br class=""></div><div class="">I have received with much interest the ObsCore extension for RADIO data, thanks for that very clear document.</div><div class=""><br class=""></div><div class="">Would it be possible to have a similar extension for cubes with velocity as third axis (the other two being the usual spatial ones)?</div><div class="">Let me call them “velocity cubes”...</div><div class=""><br class=""></div><div class="">Here my use case…</div><div class=""><br class=""></div><div class="">At ESO we need to support cutouts of velocity cubes.</div><div class=""><br class=""></div><div class="">Up to now our SODA interface supported spectra, images, and wavelength cubes;</div><div class="">to provide the user with the SODA input parameters and their possible MIN/MAX values</div><div class="">we query ivoa.ObsCore.</div><div class=""><br class=""></div><div class="">Now with the introduction of velocity cubes, we would like to get the necessary SODA input params also from ObsCore.</div><div class="">We can certainly do so by extending our own ObsCore table (ObsCore allows to extend the table), but I was wondering</div><div class="">if others have similar needs, and see if we can agree with a common ObsCore extension.</div><div class=""><br class=""></div><div class="">Velocity cubes should come in a FITS format as described by Greisen et al, 2006, so-called Paper 3.</div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">Shortly summarised, the velocity can be either:</div></blockquote><div class=""><ul class=""><ul class=""><li class="">a radio (VRAD), </li><li class="">an optical (VOPT), </li><li class="">a relativistic velocity (ZOPT), </li><li class="">a generic apparent velocity (VELO).</li></ul></ul></div><blockquote style="margin: 0 0 0 40px; border: none; padding: 0px;" class=""><div class="">The RESTFRQa (RESTWAVa) keyword conveys the necessary reference frequency (wavelength) to transform velocity into frequency or wavelength. </div><div class="">The velocity unit is m/s.</div></blockquote><div class=""> </div><div class="">SODA cutouts on the velocity axis are usually formulated by passing cuts already expressed in velocity</div><div class="">though it must be possible to perform the conversions to and from frequency/wavelength.</div><div class=""><br class=""></div><div class="">What would then be needed in ObsCore is:</div><div class=""><br class=""></div><div class="">v_min</div><div class="">v_max</div><div class=""><br class=""></div><div class="">but also, in case of radio velocity cube:</div><div class="">f_min</div><div class="">f_max (getting these from the RADIO extension)</div><div class="">f_rest = RESTFRQ</div><div class=""><br class=""></div><div class="">and, in case of a optical or relativistic velocity cube</div><div class="">em_rest = RESTWAV</div><div class="">(we already have em_min, em_max)</div><div class=""><br class=""></div><div class="">The em_resolv_power should then be set to:</div><div class=""> lambda freq c</div><div class=""> ——————— = ————— = —————</div><div class=""> delta lambda delta freq 2 * delta_v</div><div class=""><br class=""></div><div class="">where delta_v is the CD3_3 in the cube (in the example of a cube with the velocity on the third axis);</div><div class="">but I would also likely accept a v_pixel_scale (= delta_v).</div><div class=""><br class=""></div><div class="">The UCD for the velocity should then be:</div><div class=""><pre class="">spect.dopplerVeloc.opt</pre><div class=""><pre class="">spect.dopplerVeloc.radio</pre><div class="">unsure what it should be for the relativistic case.</div></div></div><div class=""><br class=""></div><div class="">This would allow us at ESO to implement SODA on top of ObsCore (as already done for all other parameters)</div><div class="">and would allow astronomer to search the velocity cubes with certain characteristics using ObsCore.</div><div class=""><br class=""></div><div class="">What do you think? </div><div class="">Is that doable? I think at ESO we will simply start implementing the above extra ObsCore parameters (unless you suggest different names for that),</div><div class="">while waiting to see if there is more interest to build a proper standard.</div><div class=""><br class=""></div><div class="">Eager to know what you think, </div><div class="">Cheers,</div><div class="">Alberto</div><div class=""><br class=""></div></body></html>