relative fluxes

[Petr Skoda]

> 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

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*  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                                                       *
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