Spectral 2.0 - Document update

[Frédéric Paletou]

     dear all,

     i support/agree with petr concerns... i already did mention similar 
issues both at nara (2010) and heidelberg (2013) interops - 
presentations can be found @

  http://filez.irap.omp.eu/27o8xcinv7om

  http://filez.irap.omp.eu/3b9vrzjk9byr


     our main 'product' now is http://polarbase.irap.omp.eu/ and that we 
still wish to be as much VO-compliant as possible!

     best regards,        fpaletou

Le 11/06/2015 20:14, Petr Skoda a écrit :
>
>
> Hi all
>
> I did not want to repeat myself for the 100th time (but it seems its 
> necessary) - so in summary
>
> So far I did not see proper description (and singe working service) 
> using proper SDM elements to describe normalized spectra since the 
> time when I proposed NORMALIZED  in FLUXCALIB (in SSA) and wanted to 
> cover this in SDM (the arith.ratio was there but I have noticed in the 
> meantime that people have different idea about what is normalization 
> to continuum.
> With some hacks we succeed running DaCHS - delivering normalized 
> spectra or even computing them on the fly.  It allowed a lot of 
> interesting science ( more on this as reply to Igor's mail as well).
>
> But so far we had to ignore the fact that the flux axis is simply 
> faked - and even in DaCHS it needed hacks to display some reasonable 
> axis description (as it is internally bound to SDM and so inherently 
> insufficient to describe this properly).
>
> So I was hoping that the work on SDM2.0 and SSAP v2 will correct this 
> historic omission... Thats why I simply asked recently - supposing its 
> naturally included .
>
>
>> Thanks for the questions, I like to get input on use-cases which aren't
>> (perhaps) covered by the current model.  The scope and coverage of the
>> Spectral-2.0 model hasn't changed in some time.
>
> That is what I am worry about !
>
> Already in Heidelberg there was a quite cleanly demostrated by A 
> Pallacios as I remember that Spectropolarimetry na basically all the 
> high resolution spectral analysis thay wanted to do on Narval data was 
> not possible due to the need of clearly describe the continuum 
> normalized spectra - the VOSpec did not expect empty units and it did 
> not work with
> no Dimensional factors ....
>
> In fact in SPLAT there is no proper solution as well.
>
> I think that there is a good time to solve this issues by giving 
> explicit recommendation how to do it properly - e.g as an special 
> appendix of SDM.
>
> The contiuum is explained as a ratio of two spectra - so the problem 
> is how to reference the second one. Which PUBDID should it have - or 
> is it something like the zero point deserving special keywords ?
> How to serialize this in FITS ?
>
> So the problem is
>
> 1) can we freely use EMPTY unit description  in FluxAxis ? What about 
> ucd for this ?
>
> What  Mireille suggests was exactly my good feeling   that it is clear 
> and natural:
>
> Char.FluxAxis.CalibrationStatus='NORMALIZED'
> Char.FluxAxis.unit=NULL  or 'unitless'
> Char.FluxAxis.ucd=phot.flux;arith.ratio;
>
> BTW  will this work as well for ObsCore ucd ?
>
> How will the NULL be expressed in Osuna-Salgado conversions like
> SpectralDataset.fluxSI  ?
>
> And in SDM2.0 Appendix B.1 is only
> phot.flux.density;arith.ratio
>
>  it is of course to discussion if density is required or not .
>
> But the problem is
>
> phot.flux.density;em.wl; spect.continuum
>
> Continuum only
> erg.cm**(-2).s**(-1).angstrom**(-1).arcsec**(-2
>
> It is referring to continuum as a absolute flux-calibrated 
> flux/intensity.
>
> But in optical ground based - namely stellar astronomy it is more 
> useful to reduce spectra to sum of ADUs on "flux axis" - in literature 
> people sometimes write relative intensity. Then follows  a subjective 
> black magic procedure the result of which is typically some polynomial 
> which is divided into the observed (aperture extracted) spectrum.
> The result is then called contiuum normalized (or "rectified") 
> spectrum where the intensity out of spectral lines is put to value of 
> 1.0 .
>
> so
>
> 2) How to describe the function - e.g. polynomial to refer to - it 
> does not have dimension of absolute flux as suggested in ucd above but 
> it is abstract fit of data which are already just in ADUs...
>
> 3) How to serialize and express in FITS keyword ?
>
> Ususally people do the rectification by tweeking some polynomial 
> parameters in IRAF or SPLAT or other tools and produce new spectrum 
> that may have something like original name + some suffix or new name 
> like normxxxx ....
>
> Some resources are implicitly already normalized but the original 
> extracted - ADU  spectra are not available (hidden as a by-product of 
> pipeline) - example it used to be the HARPS or UVES spectra on ESO 
> SSAP resources (working until last year) - part of FEROS and HEROS, 
> NARVAL spectra etc ...
>
> IMHO one possible solution is datalink - we already try to use it to 
> select between normalized and original spectra. In FITS it might be 
> some keyword describing the polynomial coeeficients or second spectrum 
> as a computed points - in another extension or as a second axis using 
> degenerate NAXIS2
>
> But IMHO this is so common (in stellar ground astronomy sure) and so 
> specific case that desires explicit explanation in the documents.
>
>>
>> I'd like to spend some time with you in Sesto to understand the 
>> particulars
>> of what would be needed to describe the continuum.  I think Mireille'
>> suggestion for the UCD and unit sounds about right..
>
> Sure I suppose to discuss this and in fact Markus must be involved as 
> well - either as a DaCHS developer or as a registry head ...
> (what IMHO many people want to do is to search in registry for 
> "contiuum normalized spectra" to compare e.g spectral types ..., 
> estimate emission heights ...
>
>
>> There is this record in the FluxAxis ucd list for ucd, description, unit
>> respectively.
>>    "phot.flux.density;arith.ratio"   "Flux ratio of two spectra"  (empty
>> string)
>
> see above
>
> *************************************************************************
> *  Petr Skoda                         Tel   : (323) 649201, l. 361      *
> *  Stelarni oddeleni                          (323) 620361              *
> *  Astronomicky ustav AVCR, v.v.i     Fax   : (323) 620250              *
> *  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
> *  Ceska republika                                                      *
> *************************************************************************

-- 
Frédéric Paletou, Irap (Cnrs-Umr5277), tél: +33 561332861
Observatoire Midi-Pyrénées, Université de Toulouse
OV-GSO [http://ov-gso.irap.omp.eu] - http://goo.gl/fwYTz5