relative fluxes

[Frank Valdes]

Hi,

I have not been active in this area since 2006 when I was doing a  
trial implementation of the SSA Data Model of that era.  This was for  
the Indo/US Coude Feed Spectral Library.  In that library the  
instrumentation was not adequate for absolute calibration and so we  
provided spectra that were calibrated to another (lower resolution)  
library of spectra continua.  I debated the question of what flux  
units really mean for the large class of spectra that can't be said  
to be absolutely calibrated.  What I decided to do, and again I am  
probably behind the times and this was before this recent attempt to  
formalize the units syntax, was to use the data model accuracy.   
Below are the data model fields for the flux (in one of the formats -  
text).

# Spectrum.Char.FluxAxis.name="Flux"
# Spectrum.Char.FluxAxis.Unit="erg cm**(-2) s**(-1) Angstrom**(-1)"
# Spectrum.Char.FluxAxis.UCD="phot.flux.density;em.wavelength"
# Spectrum.Char.FluxAxis.Accuracy.Calibration="RELATIVE"

So what I decided was that something needed to define the nature of  
the calibration and the accuracy.calibration seemed appropriate.  I  
was thinking of terms like RELATIVE and CONTINUUM NORMALIZED.  In my  
case since the continuum shape was relatively correct by calibration  
to another lower resolution library, and has numbers which include  
1.0 at the tie point used, the calibration is relative.  Since the  
continuum shape from the other library was supposed to be calibrated  
to standard stars a flux unit seems appropriate.

I don't think the particular debate being conducted is only about  
units but about how to convey the nature of the calibration.  So  
people can take what I considered for whatever it is worth.

Yours,
Frank Valdes, NOAO

P.S. I got as far as implementing the data access protocol but never  
finished the SSA query.  So there is still no VO service to my  
spectral library.

On Jun 22, 2009, at 9:14 AM, Jesus Salgado wrote:

> 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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