relative fluxes

[Jesus Salgado]

Hi Petr,

We basically agree then. For ABSOLUTE and RELATIVE (as per Spec DM
definition) it is clear what to do in a VO application and the vertical
scaling could be enough. For other kind of spectra, a better description
in the DM is needed to limit the scope of usability of the server data
in VO applications.

In particular, the "RECTIFIED" (divided by a, most of the times unknown,
function) is currently under the uncalibrated tag that also covers
counts, normalized to the continuum, and many things more...

Regards,
Jesus


On Mon, 2009-06-22 at 17:29 +0200, Petr Skoda wrote:
> > This seems like a viable solution - the question is whether this is a problem 
> > that needs solving, its contingent priority, and whether we yet have a 
> > complete description of the problem.
> 
> I think the proper explanation of what is in the spectrum is crucial for 
> all VO science. The VO tools need to understand if the flux is RELATIVE (I 
> suppose the best case is that of jesus - it does not change shape) or 
> ABSOLUTE - you may directly compare with model or NORMALIZED (in sense 
> RECTIFIED) - than you can compare line profiles or measure RV or EW ( by 
> definition you cannot measure EW on unrectified spectrum).
> 
> If you get spectrum absolutley calibrated you have to rectify it 
> (normalize) first to be able to compare with another NORMALIZED spectrum.
> FOr VO tools it  mean they have to support this type of operation spectra 
> obtained by SSA before being able to overplot them.
> 
> > How is this used in the literature?  What fraction of columns/values in VO 
> > DBs/images/catalogs require this?  Is there a need to support an offset as 
> > well as a scaling factor?
> 
> I do not know about Miguel's case but in case of RECTIFIED spectra its 
> tahe 90% of optical ground base spectroscopy (mostly processed by IRAF or 
> MIDAS).
> 
> The absolute flux calibration is quite rare for high resolution spectra 
> and quite common for low resolution (as the people want to estimate SED on 
> resolution like 3000-5000 (SRP to be precise). For higher resolution you 
> are interested in seeing lines in detail - measure EW, RV and see profile 
> changes - for this you need RECTIFICATION.
> Special case are the data for hunting planets - extremely high precision 
> of RV (using I2 cells or dual fibers for simultaneous calibration) - than 
> the cross correlation is applied (again of rectified spectra or on 
> original as come from pipeline).
> 
> In case of space instruments the absolute calibration is quite common but 
> even on STIS people look for line profiles on RECTIFIED.
> 
> > In short, are there domain use cases for this?
> 
> These were just examples from my experience of optical instrument 
> scientist. proper poll might be [erhaps needed in future to get some 
> ratios - but I think the VO should suppose all cases as they represent 
> most of the science everyday done.
> 
> Petr
> 
> *************************************************************************
> *  Petr Skoda                         Phone : +420-323-649201, ext. 361 * 
> *  Stellar Department                         +420-323-620361           *
> *  Astronomical Institute AS CR       Fax   : +420-323-620250           *
> *  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
> *  Czech Republic                                                       *
> *************************************************************************
> 
> 
-- 
Jesus J. SALGADO                       Jesus.Salgado at sciops.esa.int

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