IVOA - Spectrum Data Model change request

[Petr Skoda]

Hi Mark, Vandana and all DM group,

I would just like to remember once again (see the
https://wiki.ivoa.net/internal/IVOA/IvoaDAL_RunningMeetings/IVOA_DAL_RM11_etherpad.txt 
search PS or
https://wiki.ivoa.net/internal/IVOA/InterOpMay2021DAL/IVOA_May21_DALDM_etherpad.txt
search  "Petr Skoda:")

that we need add two 'order's (we are talking about echelle spectrograph 
as is also shown on Vandana's SPitzer IRS spectra in so called long-high 
(LH) Mode see e.g. 
https://irsa.ipac.caltech.edu/data/SPITZER/docs/irs/irsinstrumenthandbook/IRS_Instrument_Handbook.pdf
page 9  (2.6)




  One is relative - in fact depends how the pipeline on echelle reduction 
names the first extracted order - usually 1 an increasing as you go either 
towards red or blue ...(depends on orientation of the chip and 
cross-disperser .... all combinations are commonly use)

and then the so called absolute order which is defined by construction and 
is stable for given instrument.

It is an order (marked m in https://en.wikipedia.org/wiki/Blazed_grating)
of difraction - given by the number of wavelengths of monochromatic light 
where the diffaction occurs in given geometrical (angles of rays 
incidence, reflection) and construction (spacing of grating grooves) 
conditions.

I would suggest the name "absorder" or "difforder" or something similar in 
updated SDM.

This order is used in reduction (can be used for checking the correct 
wavelength calibration etc) but usually is not transfered in the final 
product.

In fact we still do not have proper standard for expressing echelle 
spectra and many final product attempt to merge many orders in one long 
spectrum (which is in principal bad idea bringing a LOT of problems) in 
order to pretend the echelle spectrum is a simple 1D  vector ....
(e.g. all ESO archives of HARPS, FEROS etc ...)
But as shown on SPITZER and many other properly curated echelle archives, 
there is a need for keeping the so called oder-by-order formats of echelle 
as well. It is important namely for precise RV of exoplanets ....

So IMHO the Vandana's proposal (introducing order - or precisely 2 order 
types) is a hack that would allow at least to properly refer to given part 
of spectra (which allows for quick preview of given line profile on 
multiple spectra or study the proper data at the edges of the order. As 
most orders in properly designed echelle spectrographs overlap, the 
merging into one long  1D enforces some ad-hoc hacks like 
cutting part of each order and average / median the rest of overlapping 
part

etc ...

The question is how to recognize what order in visualizer to use if both 
are stated. IMHO the viewer should allow to switch this labeling or at 
least 
display somewhere that the order 1 is in fact 72 ..
But of course the reference to the given collumn of a flux table will use 
relative orders (1 to ....)





>
> As I understand it, there are 2 main changes:
>  1) add 'order' to the SpectralAxis data content
>      The current model supports a single 1-D spectrum, and has no concept
> of spectral order.
>      So, even if the data were separated into separate Spectrum instances,
> the 'order' information would be lost.
>      This request would add a hook which would allow data containing
> multiple spectral orders to be separated in application.

or aggregated in application to display whole spectral range - if there 
are many single order 1D spectra in multiple files..

(this is e.g. case of Be star database BeSS many files represent separate 
orders of the same echelle exposure)


*************************************************************************
*  Petr Skoda                         Phone : +420-323-649201, ext. 361 *
*  Stellar Department                         +420-323-620361           *
*  Astronomical Institute CAS         Fax   : +420-323-620250           *
*  251 65 Ondrejov                    e-mail: skoda at sunstel.asu.cas.cz  *
*  Czech Republic                             skoda at asu.cas.cz          *
*************************************************************************