[Obscore DM update ] Redshift axis and Spectral Axis

[Louys Mireille]

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 Cedex
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tel : 03 68 85 24 34

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