[Obscore DM update ] Redshift axis and Spectral Axis

[Arnold Rots]

I strongly urge to keep the two separate.
Yes, they have much in common, but they serve very different purposes.
And it is possible to create a 4-D cube where the 4th axis is actually a
spectral axis and the pixels are different lines (rest frequencies).
STC2 will treat the spectral and redshift/Doppler as distinct axes.

I also note that you are missing redshift and that spect.dopplerVeloc makes
no sense: it's either radio or optical (or, in rare cases, so-called
relativistic), and one should not be allowed to gloss that over.

  - Arnold

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Arnold H. Rots                                          Chandra X-ray
Science Center
Smithsonian Astrophysical Observatory                   tel:  +1 617 496
7701
60 Garden Street, MS 67                                      fax:  +1 617
495 7356
Cambridge, MA 02138
arots at cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Mon, Apr 27, 2015 at 2:26 PM, Louys Mireille <mireille.louys at unistra.fr>
wrote:

>  dear DMers,
>
> I am trying to recap on the discussion held since 2013 about the addition
> of a redshift or doppler axis in Obscore.
> there has been various opinions expressed and typically two different ways
> to think about it :
>
> 1. Some dataset contain velocity measurements and are sampled inside a
> velocity range , have velocity resolution , etc .
> this is the case for Marco's Usecase for instance  Then quite naturally we
> could think a new axis is needed .
> this was the first suggestion we had before and at the Banff interop
> meeting.
>
> 2. The doppler shift is always derived from an initial set of spectral
> measurements . Therefore it can be described as a spectral axis , with
> added reference position , rest frequency , etc. , specific unit in km/s ,
> specific ucd.
> if I consider the  Spectral FITS WCS specification,(
> http://www.aanda.org/articles/aa/pdf/2006/05/aa3818-05.pdf)  the
> velocity/doppler axes can have several flavors that are characterized
> precisely. Table 1 and Table 2 of the spec , provide all tags to
> differentiate the various cases.
>
> I also gave a second read to the most recent version of ImageDM published
> in March , and from the description of the Spectral coordinate class ,Fig
> 20. p 55 compared to the RedshiftCoordinate Fig 21, it is clear the two
> subtrees have a lot in common . This means the Redshift coordinate could
> just be a derived class of the spectral coordinate construct, with the
> addition of all the reference frequency, positions , etc. , needed for
> that.
>
> For the needs of the Obscore v1.1 update , I suggest that velocity cubes
> be discovered by using *em_ucd*, with the possible values equal to the
> following , as extracted from the UCD list:
>
> *E | spect.dopplerVeloc | Radial velocity, derived from the shift of some
> spectral feature*
> *E | spect.dopplerVeloc.opt | Radial velocity derived from a wavelength
> shift using the optical convention*
> *E | spect.dopplerVeloc.radio** | Radial velocity derived from a
> frequency shift using the radio convention*
>
> em_dim, em_min, em_max, etc  will describe the dimension and range along
> the velocity axis for the returned datasets.
>
> In a more detailed description , as required for instance for the
> *getmetadata* capability in SIAv2 , then the full WCS axis description
> could be exposed.
>
> Many thanks for comments , Mireille.
>
>
>
> --
> Mireille Louys	, Maître de conférences
> Centre de Données ( CDS)		Icube & Télécom Physique Strasbourg, Pôle API
> Observatoire de Strasbourg 		300, boulevard Sébastien Brant
> 11, Rue de l'Université			CS 10413
> 67000 Strasbourg 				F - 67412 ILLKIRCH Cedexhttp://astro.unistra.fr			http://www.telecom-physique.fr
> tel : 03 68 85 24 34
>
>
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